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Review

Blackwell Publishing Ltd.

Tansley review

Models of shoot apical meristem

function

Fiona Tooke† and Nick Battey

†Department of Plant Sciences, Cambridge University, Cambridge, CB2 3EA, UK; School of Plant

Sciences, The University of Reading, Whiteknights, Reading, RG6 6AS, UK

Contents

Summary 37

I. Introduction 37

II. How things began 38

III. Cytology 39

IV. Morphology 41

V. Developmental genetics 44

VI. Conclusions 49

Acknowledgements 50

References 50

Summary

In this review we describe how concepts of shoot apical meristem function havedeveloped over time. The role of the scientist is emphasized, as proposer, receiverand evaluator of ideas about the shoot apical meristem. Models have becomeincreasingly popular over the last 250 years, and we consider their role. They providevaluable grounding for the development of hypotheses, but in addition they have astrong human element and their uptake relies on various degrees of persuasion. Themost influential models are probably those that most data support, consolidating themas an insight into reality; but they also work by altering how we see meristems, re-directing us to influence the data we collect and the questions we consider meaningful.

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I. Introduction

During embryogenesis certain cells gain the potential to producethe roots and shoots of a plant. These zones of cells havebecome known as the root and shoot apical meristems. Thelatter, often after a period of dormancy in the seed, functions toproduce the entire above-ground structure of the plant. Whatwe know about this process, and how we now perceive the shoot

apical meristem is a consequence of a long history of research,through which has emerged a number of key models, influ-ential to many but traceable back to a dominant few.

The many facets of the shoot apical meristem providetremendous scope for different interpretations. Below, in Fig. 1,are four views of the shoot apical meristem. Notice whataspects each view emphasizes: in the transverse section, the clearphyllotactic arrangement and the organ initiation function

Author for correspondence:

Fiona TookeEmail: ft216@cam.ac.uk

Received:

16 January 2003

Accepted:

3 April 2003

doi: 10.1046/j.1469-8137.2003.00803.x

Key words:

shoot apical meristem, models, flower initiation, leaf initiation, growth, developmental genetics.

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are of apparent; in the longitudinal section, the cellular organ-ization and outgrowth of primordia at the periphery can beseen; by scanning electron microscopy the shoot architectureis visible; and in the living plant, the lateral outgrowths from themeristem and the integration of growth and developmentover time is shown. How would you describe how this systemworks? What or who influences you in your description?

In nearly 250 years of shoot apical meristem research,many would probably agree with the experience of being‘thrilled by the translucent, glistening beauty of the apicalmeristem and surrounding leaf primordia’ described by Sussex(1998). From this perspective, driven by an enthusiasm tofind out how the shoot apical meristem works, it is perhapshard to conceive that very different perceptions of it couldexist. Indeed, it might be assumed that progress to today’sunderstanding of the genetic principles underlying shoot api-cal meristem structure and function would have been quicker,but for technological or resource constraints. Yet this is onlypartly correct. The recognition that the paths we are now onwere not always taken at the first opportunity shows theimportance of individual judgements, and that arrival attoday’s models of genetic determinism was perhaps notinevitable. Goebel (1926) commented that, ‘it is not the factsthereby attained but the conclusions drawn from them thatdetermine the progress of science. These conclusions are influ-enced not only by each particular investigator’s individualitybut by the general posture of science in his day.’

The purpose of this review is to chart the prominent people,from Wolff in the 1700s to Weigel now, who have influencedthe way we view meristems, and to describe their approachesand contributions to current understanding of the shootapical meristem. We have had to be somewhat selective in ourtreatment of the topic; for comprehensive reviews of variousaspects of the shoot apical meristem readers are referred toAdler

et al

. (1997), Jean (1994) and Lyndon (1994, 1998).

II. How things began

In 1759, in his mid 20s, at a time when the microscopewas still relatively underdeveloped and plant anatomy wasfailing to command great attention, Caspar Wolff published

‘Theoria Generationis’ (Wolff, 1896). The work had littleinfluence then and for some time afterwards; its later revival,according to Sachs, resulted from Wolff ’s thoughtfulnessrather than his accuracy (Sachs, 1890). The observationsled Goebel to declare Wolff the ‘true founder of the historyof development’ (Goebel, 1926). For it was Wolff whodiscovered the growing point of the plant, the ‘punctumvegetationis’ as he named it (Fig. 2). Here, he claimed, wasevidence that growth of the plant proceeded by a process ofepigenesis, that is, construction

de novo

, and not by‘preformation’, whereby the mature plant resulted simplyfrom the unfolding of preformed structures. He recognizedtoo that leaf and flower primordia were derived by similarprocesses leading to the suggestion, many years before it wasmade by Goethe, that floral organs were modified leaves(Singer, 1931).

Fig. 1 Views of the shoot apical meristem. (a) Transverse section of the terminal bud of Ligustrum vulgare (1, shoot apex; 2, youngest leaf primordia; 3, older leaves). (b) Longitudinal section of the shoot apical meristem of Zea mays (1, shoot apex; 2, new leaf; 3, previous leaf; 4, primary thickening meristem; 5, procambial strand). (c) Transmission electron microscope image of Linum usitatissimum (1, stoma; 2, bud). (d) Top view of the shoot tip of Aeonium. Reprinted from Bowes (1996), with permission from T. Norman Tait.

Fig. 2 The punctum vegetationis. Diagram by Caspar Wolff (Wolff, 1896), which he described as follows, ‘The tip was peeled and all leaves were removed from the front view, so that it is possible to see the vegetation point. Leaves were not removed from the back to demonstrate their attachment to the vegetation surface. (v) The convex, juicy and translucent vegetation surface. (p) The first leaf to appear, with its concave inner surface adjacent to the vegetation surface. The consistency of this leaf is barely more substantial than a viscous fluid. (a) A different leaf, which is larger and more substantial than the previous. (c) A leaf which has already developed a surrounding edge. (e) Half a leaf. (d) Complete leaf.’

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That Wolff made this proposal, we can surmise, was theresult of a combination of factors: he had a question hewanted answered (epigenesis vs preformation), patience andobservational skill, and suitable equipment (the microscope).In this way the most essential of plant structures was revealed,

together with its mode of action (epigenesis) and, by implica-tion, the ‘open’ form of development of plants in contrastto the ‘closed’ development of animals. The punctum vegetationisdid not, however, become a clear focus of research at once.Thus in 1842, Schleiden could suggest, presumably with asense of originality, that the study of ‘developmental history’as opposed to complete structures, might be the key tomorphology (Goebel, 1926). The approach to phyllotaxisfurther illustrates this reluctance to view plants as dynamicentities. Despite dating back to the time of Pliny, morpholo-gical explanations of phyllotaxis that involved the apex andprimordia were not put forward until the 1860s at the earliest(Adler

et al

., 1997). Before this time studies were made ofthe mature, complete leaf arrangement. Transverse sectionsthrough the shoot apical meristem to reveal the organizationof the primordia were an innovation of the early 20th century(see Church, below).

III. Cytology

The term ‘meristem’ appears not to have entered botanicalvocabulary until 1858. It is attributed to Nageli who in hiswork ‘Beitrage zur wissenschaftichen Botanik’ classifiedtissues as ‘generating’ or ‘permanent’, according to theirmorphology. Parenchymatous generating tissue was termed‘primary meristem’ (Sachs, 1890). The application of thisterm implies a recognition of function because ‘meristem’ isfrom the Greek word ‘merizein’, meaning ‘to divide’. Theearly published views of the shoot apical meristem werelargely observational, aiming to give accurate descriptions ofmeristems under natural, nonexperimental conditions (Sifton,1944).

Nageli’s work centred on the apical cell, which he found inalgae and moss, and its power to divide continuously. At thetime it was assumed that this large, dividing cell was a featureof all plants, and Nageli believed that the sequence of cell divi-sions was of great importance in determining the form of theplant (Reed, 1942). Whilst the apical cell concept remainsimportant today in the study of bryophytes and pterido-phytes, in 1868 Hanstein redirected attention to cell layers,

Fig. 3

Cytological approaches to the shoot apical meristem. (a) Hanstein: histogenic layers. ep, epidermis; ec, subepidermis; cc, central core; d, dermatogen initial layer; pe, periblem initial layer; pl, plerome initial layer. (b) Schmidt: tunica-corpus. t, tunica; c, corpus. (c) Foster: cytohistological zones. zia, apical initial zone; cmc, central mother cells; zp, peripheral zone; mc, central meristem (rib zone). (d) Buvat: méristème d’attente. mm, méristème médullaire; ma, méristème d’attente; pmsp, proméristème sporogène; pmr, proméristème réceptaculaire. (e) Comparison of terminology. Composed from Majumdar (1942) and Buvat (1952). Diagrams reprinted from Annales des Sciences Naturelles (Botanique) 8: Buvat, Structure, évolution et fonctionnement du méristème apical de quelques dicotylédons. pp. 199–300, copyright (1952), with permission from Masson, Paris.

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with his histogenic layer theory of the shoot apical meristem(Fig. 3a,e). On the basis of cell layers he found and studied inangiosperms, Hanstein divided the meristem into dermatogen,periblem and plerome, from which layer initials were derivedthe epidermis, cortex and stele, respectively (Foster, 1939; Reed,1942). In Hanstein’s thinking there is an implicit idea of ‘destiny’,certain cells being predetermined to differentiate in a mannercommensurate with their layer. Echoes of these ‘prospectivevalues’ (Foster, 1939) can be seen in later clonal analysis ofcell layer behaviour. Satina’s colchicine-induced chimeras of

Datura

(Satina

et al

., 1940) provided evidence supportingHanstein’s histogen concept, but some redefinition was required(see Fig. 4a,b,c). In his work on cranberry chimeras however,Dermen (1947) found that there was no reason to supportthese inherent functions of the cell layers (Fig. 4d,e).

Schmidt in 1924 followed a somewhat different approach.Based purely on patterns of growth and cell division, his

model defined the overlying tunica layer(s) by their anticlinaldivisions, with more variable division planes found in thecorpus below (Fig. 3b; Foster, 1939; Steeves & Sussex, 1989).Dermen (1947) considered that the tunica-corpus essentiallylacked meaning but Sifton (1944) believed that adherents ofthe tunica-corpus theory were liberated from the limitationsthat consideration of destiny imposed.

In 1938 Foster made a much broader application of celldivision analysis (Foster, 1938). Like Schmidt’s tunica-corpuslayers, Foster’s was a largely structural model, but it was moreextensive, dividing the meristem into cytohistological zones(apical initial zone, central mother cell zone, transition,peripheral and rib zones) (Fig. 3c). This recognized the arrayof cellular features that Foster found. Foster used a gymno-sperm,

Gingko

, for this original work, but the concept wasfound to be applicable to angiosperms (Majumdar, 1942).Clowes (1961) criticized the imprecision of zonation models

Fig. 4 Chimeras. (a) (b) and (c) Colchicine-induced periclinal chimeras of Datura. Below each diagram the chromosome number of the cell layers is given. Each layer had an independent response to colchicine treatment. The authors recognized a 2-layered tunica, plus corpus. The 3 germ layers proposed by Hanstein were evident, but the constancy of chromosome number of layer 3 and the central core implied that ‘plerome’ in Datura is not an independent tissue but a derivative of L3. Reprinted from American Journal of Botany, 27: Satina et al. Demonstration of the three germ layers in the shoot apex of Datura by means of induced polyploidy in periclinal chimeras. pp. 895–905, copyright (1940), with permission from the Botanical Society of America. (d) Longitudinal section through the shoot apical meristem of a cranberry chimera. The first 3 layers, at least, are diploid and the inner layers, tetraploid. (e) Transverse section of the stem of the plant shown in D. The dark black line marks the boundary between the inner tetraploid area and outer diploid zone. Reprinted from American Journal of Botany, 34: Dermen, Periclinal cytochimeras and histogenesis in cranberry. pp. 32–34, copyright (1947), with permission from the Botanical Society of America.

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saying, ‘… it depends so much on the pattern which catchesthe eye, and the pattern is made of many different visualimpressions’. Yet the terms derived from Foster’s work –central zone (CZ), peripheral zone (PZ) and rib zone (RZ), arenow in regular usage as locational reference points in themeristem. What is the basis of that revival today? Clark (1996)suggests that you could ‘superimpose’ these regions with thoseof the central undifferentiated cell pool and its surroundingcells heading for differentiation and organ formation. Hence,although Clark admits to a lack of evidence for this interpre-tation, a functional significance fortifies a histological model.Furthermore, it takes on a molecular significance with thesuggestion that the genes

WUSCHEL

(

WUS

)

CLAVATA

(

CLV

),

SHOOTMERISTEMLESS

(

STM

) are involved indelimiting/maintaining the CZ and PZ regions (see Develop-mental genetics, below; Clark, 1996; Weigel & Clark, 1996).

With the exception of Hanstein, concepts of the shoot api-cal meristem based on cell division patterns were until the1940s generally structurally based. Few attempts were madeto ascertain the functional significance of cell division rates orpatterns. This makes the méristème d’attente theory of Buvat(1952) something of a turning point. To Buvat, the infre-quency of cell divisions in the central region of the meristemwas of great significance. It was this area he named theméristème d’attente, a zone of cells he envisaged waiting, inac-tively, until the initiation of floral activity (Fig. 3d,e; Buvat,1952; Steeves & Sussex, 1989). Until flowering, the focus ofvegetative development of the plant was the peripheral region,the ‘anneau initial’.

In arriving at this model, Buvat defined the ‘French School’(Fig. 5; Wardlaw, 1957; Cutter, 1959). The theories ofMajumdar (1942) and Plantefol (1946) were acknowledgedby the attention given to the peripheral regions of the meris-tem. Majumdar had seen the apex as a ‘self-perpetuatinggroup of central initial cells, surrounded by a cylinder of moreactive flank meristem from which the primordia originate’(Majumdar, 1942). This idea was reinforced in Plantefol’sphyllotaxis theory (see Morphology below) in which leaves

arise from the ‘anneau initial’ incorporated into Buvat’stheory. There would appear also to be some influence ofGrègoire’s ‘radical viewpoint’ (Foster, 1939) which strayed farfrom the classical interpretation of the flower, by suggestingthat vegetative and floral meristems bore no relation to eachother. To Grègoire, whilst tunica and corpus were recogniza-ble in vegetative meristems, the floral meristem was defined asa ‘manchon meristematique’, a cloak or mantle of meristem-atic tissue overlying the ‘massif parenchymateux’, a region ofhighly vacuolated infrequently dividing cells (Foster, 1939;Philipson, 1949; Buvat, 1952). This perception of the floralmeristem as independent, rather than a derivative of the veg-etative meristem, is reflected in Buvat’s delimitation of vege-tative and floral zones in the meristem.

Beyond the French School, criticized by Wardlaw (1957)for observations ‘somewhat selective in character’, theméristème d’attente theory was a view not shared by many.Nevertheless, the theory did galvanise enquiry into cell behav-iour in the meristem. This was in spite of differences of inter-pretation and crucially, the limited evidence for the inactivityrequired of the waiting cells in the meristem summit (Wardlaw,1957; Cutter, 1959; Steeves & Sussex, 1989).

IV. Morphology

Morphological approaches to the shoot apical meristem areessentially concerned with how and where the meristemestablishes growth. Featuring strongly in this aspect of shootapical meristem research were the Snows who recognized thatmodels of phyllotaxis until that time (1931) fell into twogroups – those that proposed pattern to be determined by anunknown property of the stem itself, and those that advocatedleaf arrangement to be dictated by contact with existingprimordia. The Snows set about testing the validity of themodels – assuming that if those in the second group werecorrect, manipulating primordia as they arose could influencethe positions of those that formed later. Where others hadobserved and calculated (for review see Schwabe, 1984), theSnows became experimentally involved using a ‘cataract knife’and a dissecting microscope to perform microsurgery on theshoot apical meristem. The Snows observed that, under certaincircumstances isolating a primordium or its site caused thephyllotactic spiral to be reversed with a primordium and itssuccessors forming at opposite sides to those expected, hadnormal phyllotaxy been retained (Fig. 6a,b). This led theSnows to propose a concept of ‘first available space.’ Existingprimordia determined where a new primordium formed –and this was in the widest gap, furthest from the growing apex(Snow & Snow, 1931).

The Snows also pioneered the study of how a leaf forms.They found that application of auxin to the shoot apical mer-istem of

Lupinus albus

and

Epilobium hirsutum

gave rise toenlarged, united primordia in an altered arrangement andinterpreted these results to support their model of phyllotaxisFig. 5 The French School.

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(Snow & Snow, 1937). To an extent this work foreshadowscontemporary morphological approaches (see below).

The 1940s brought two influential models of phyllotaxis.The first was Plantefol’s ‘multiple foliar helices’ model whichis rarely considered now but was a very significant feature ofthe French School (Fig. 7; Wardlaw, 1957; Cutter, 1959). Inthis model, leaves were arranged in helices of variable numbersdepending on the plant. Helices terminated in leaf-generatingcentres in the anneau initial and these were under the controlof an organizer (Plantefol, 1946; Cutter, 1959). This theoryhas not gained wide acceptance because it is not clear how todetermine which foliar helices have most ‘biological reality.’Neither has it been given rigorous mathematical testing.

In the second of the post-Snow models, we return to theirsurgical approach, the knife this time wielded by Wardlaw. Hefavoured the large meristems and widely spaced primordia ofthe fern

Dryopteris

, and using a similar technique arrived atsimilar results to those of the Snows. He did not reject theSnows’ ‘first available space’ hypothesis but instead set about

interpreting ‘space’ along the lines of Schoute in 1913, drawinginhibitory fields across the meristem (Fig. 6c,d). Wardlawargued that space itself does not ‘do’ anything and is not a‘causal factor’ in morphogenesis. A new primordium emergedat the position of weakest inhibition with the most recentlyformed primordium the inhibitor source (Wardlaw, 1949).The view that phyllotactic patterns can be explained by inhibi-tory fields is an enduring one.

Richards (1951) sought a phyllotactic description thatcompletely defined the arrangement of leaves and, most sig-nificantly, was independent of any theory of the origin ofphyllotactic pattern. He achieved this using the divergenceangle and the plastochron ratio (the radial distances of succes-sively initiated primordia from the centre of the system).Richards clearly saw the importance (and limitations) of treat-ments by Schimper and Braun more than 100 years earlier, inwhich phyllotaxis was partly defined by the divergence betweentwo successive leaves in a tangential direction; Church’s clas-sification by pairs of orthogonally intersecting contact

Fig. 6 The Snows and Wardlaw – first available space/inhibitory fields. (a) and (b) Diagrams of transverse sections of the shoot meristem of Lupinus albus from Snow & Snow (1931). The straight line across (a), a normal bud, marks the position of the incision made to isolate the area of tissue (I1 region), about to initiate the next primordium. The phyllotactic alteration that this causes is shown in (b). Note the different positions of I4 and I5 after incision (b). P1, P2, P3, etc. = existing primordia, in order of increasing age. I1, I2, etc. = incipient primordia, invisible at the time of operation, in order of appearance. Reprinted from Philosophical Transactions of the Royal Society, London, Series B 221: Snow M, Snow R. Experiments on phyllotaxis. I. The effect of isolating a primordium. pp. 1–43, copyright (1931), with permission from The Royal Society. (c) and (d) Representations of the shoot apex of Dryopteris showing, in (c), the position of leaf primordia and incipient primordia, and in (d), the possible ‘fields’ associated with these ‘growth centres’. ac, apical cell; b, bud rudiments; m-m1, lower limits of apical meristem. The dashed line indicates the approximate base of the apical cone. I1, I2, etc., as above. Numbers = existing primordia (increasing age). Reprinted from Growth (supplement) 13: Wardlaw, Experiments on organogenesis in ferns. pp. 93–131, copyright (1949), with permission from Growth Publishing Co. Inc.

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parastichies (Church, 1904, 1920); and van Iterson’s studiesof packing of spheres on cylinders.

Although Richards’ treatment of phyllotaxis did notdepend on any particular mechanism, it was consistent withthe inhibitor theory adopted by Wardlaw. Later mathematicalmodels simulating the growing shoot apex by, for example,Thornley (1975a,b), and Veen & Lindenmayer (1977) usedthe idea that primordia (and the bare apex) are sources of aninhibitor of primordium initiation (see Schwabe, 1984; Jean,1994 for reviews).

These models tend to be concerned with the mechanism bywhich the shoot apical meristem continues to function. Amore contemporary morphologist, Green, was, however, notso concerned with maintenance of existing pattern at the

shoot apical meristem. He wanted to explain the origin of pat-tern as well as its further propagation (Green

et al

., 1996), andhe recognized that initiation and patterning mechanisms werenot necessarily divorced from each other (Green, 1992). Per-haps most significantly, Green suggested that concepts of pat-tern formation need not be anticipatory (i.e. that no regionaldifferences need precede and direct development), a contrast-ing view to that which predominates in developmental genet-ics today. Whilst Meyerowitz has advocated that ‘globalpatterning’ arises out of the coordination of local controlmechanisms (Meyerowitz, 1996), Green considered thisapproach to be anticipatory and favoured a view of develop-ment which was reflective of global pattern; so for example, insome cases, ‘a tissue-level physical process appears to beupstream of organ-specific expression’ (Green, 1997).

Green proposed a biophysical solution to shoot apical mer-istem pattern formation, similar to that which gives ‘wrinklesin wet skin’ (Hernandez & Green, 1993); this was the physicalbuckling theory. Patterning in this model arose from instabi-lity, in turn arising from the ‘need to expand’ of a uniformsheet tied to a uniform underlying layer which gave upwardpressure. These attributes were provided by the tunica andcorpus of the meristem (Fig. 8). To redress this unbalancedstate, physical buckling would occur and generate bulges onthe meristem surface (Green, 1992; Hernandez & Green,

Fig. 7 Plantefol: multiple foliar helices. (a) Stem of Lilium candidum showing three foliar helices. (b) The classical view of leaf generation and the multiple foliar helices model for a stem with three foliar helices. Reprinted from Annales des Sciences Naturelles (Botanique) 7: Plantefol, Fondements d’une théorie phyllotaxique nouvelle. pp. 1–77, copyright (1946), with permission from Masson, Paris.

Fig. 8 Green: undulations. (a) Wild-type floral meristem of Antirrhinum. Stamens arising in whorl 3. (b) Floral meristem of a deficiens mutant of Antirrhinum, which makes sepals in the first two whorls of the flower, followed by carpels. (c) and (d) Mechanical simulations of undulations in a fixed circular region. It is proposed that in the meristem, undulations depend on the ratio of corpus (flexible) to tunica (rigid). A shift from out-of-plane undulations, giving stamens, to in-plane undulations will give rise to carpels, mimicking the effects of the deficiens mutant. s, sepals; p, petals; st, stamens; c, carpels. Reprinted from American Journal of Botany 86: Green, Expression of pattern in plants: combining molecular and calculus-based biophysical paradigms. pp.1059–1076, copyright (1999), with permission from the Botanical Society of America.

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1993; Green

et al

., 1996). These bumps could then provide anew set of physical parameters for continued buckling topropagate pattern (Green, 1992). A nice example of the pro-cess of leaf initiation and the subsequent organization of ashoot apical meristem is provided by the work of Selker &Lyndon (1996) with axillary explants of watercress (

Nastur-tium officinale

). Roughly triangular surfaces, showing none ofthe characteristic structural organization of a meristem, wereable to initiate new buds. The phyllotactic pattern of theseexplant buds was sometimes different to that of the parentplant. The authors interpret their observations as showingthat the pattern is related to the shape of the bud-formingspace and mechanical stresses within it (Selker & Lyndon,1996; Lyndon, 1998).

Green perceived that in the shoot apical meristem patternwas generated from ‘nothing’ (Hernandez & Green, 1993)and he saw this model as preferable to the more commonlyapplied positional information and reaction-diffusion genre,both of which require an anticipatory prepattern for which hefound very little evidence (Green, 1992; Hernandez & Green,1993). Nor under the physical buckling mechanism was therea need for inhibitory fields to account for phyllotaxis. Greensuggested that there was no reason to believe that organ initi-ation would be spontaneous unless inhibited (Green, 1992).In Green’s model ‘inhibition’ derives from the ‘intrinsic reluc-tance’ of the surface to undergo a sharp change in shape(Green

et al

., 1996).To a degree Green’s work was carried out on inanimate

models, simulations which gave phenocopies of plantmutants and fused organs (Green, 1996). Applying the prin-ciples

in vivo

, growing sunflower heads in constraints resultednot only in predictable alterations in pattern but also inaltered organ identity (Hernandez & Green, 1993). Greeninterpreted this as ‘abnormal buckling’, that is, a physicalprocess having an influence on gene expression (Hernandez &Green, 1993; Green, 1994).

Overall, Green’s approach to meristems was strikingly orig-inal; this is apparent not least in the vocabulary he used todescribe them. He placed emphasis on shape and form, writ-ing of ‘tissue undulations’, ‘an initial bump pattern’ which is‘roughly sinusoidal’ and primordia ‘delimited by parallel creasesof equivalent clarity and depth’ (Hernandez & Green, 1993).Green’s work spanned a time when morphology was, to someextent, giving way to the emerging field of developmentalgenetics. Perhaps partly because of this, his work on patterninitiation was founded on almost uniquely held principles.

Modern-day morphologists have much in common withthe Snows in attempting to discover the workings of the shootapical meristem through its manipulation. Reinhardt applieda similar technique to the Snows – the application of syntheticindole-3-acetic acid (IAA), but in combination with plantmaterial in which auxin transport had been inhibited, chem-ically (with

N

-1-naphthylphthalamic acid (NPA) in tomatoshoot apical meristems) or genetically (the

pin-formed 1-1

(

pin1-1

) mutant of

Arabidopsis

) (Reinhardt

et al

., 2000).Tomato shoot apical meristems on NPA media failed to pro-duce leaves, but leaf initiation could be restored by auxin.Similarly auxin application recovered flower formation in

pin 1-1

mutants. The finding that leaves always appeared atthe precise point of auxin application in the radial axis ofthe shoot apical meristem but at a constant distance from themeristem tip led Reinhardt to propose a model of shoot apicalmeristem function in which the apical-basal axis, Foster’s CZ/PZ, is maintained by genes such as

CLV

,

WUS

,

STM

(seeDevelopmental Genetics, below) but that radial patterning isauxin dependent. Organogenesis of leaves and flowers requiresauxin, and, in line with the Snows, correct positioning ofleaves (but not their initiation) requires existing primordia(Reinhardt

et al

., 2000; Kuhlemeier & Reinhardt, 2001).Recently a more comprehensive model, linking transcription

factors, hormones and primordium outgrowth has been elab-orated. A new leaf differentiates where there is gibberellin anda high concentration of auxin. These conditions arise through anauxin gradient in the meristem with existing primordia actingas auxin sinks. Gibberellin levels are affected when homeodo-main transcription factors such as

KNAT1

and

STM

, whichnormally inhibit gibberellin biosynthesis are down-regulated.In this way transcription factors can regulate meristematic cellsthrough growth hormones (Vogler & Kuhlemeier, 2003).

At a biophysical level, expansins are proposed to operate incell wall expansion during cell extension. Reinhardt suggestedthat genes such as the tomato expansin

LeEXP18

could beactivated to induce bulging of the meristem in the final ele-ment of a phyllotactic mechanism (Reinhardt

et al

., 1998).This role of expansin in leaf development is supported by thework of Fleming

et al

. (1997) and Pien

et al

. (2001). In bothcases expansin in localized areas induced leaf primordia andreversed phyllotaxis. Pien

et al

. suggested that this was ‘evi-dence of cell-division independent mechanisms controllingmorphogenesis’ (Pien

et al

., 2001).

V. Developmental genetics

Although there is little to suggest that cytological andmorphological considerations of the shoot apical meristem arenow implausible, the developmental genetics era of the last15 years is largely independent and disconnected from theseearlier phases – phases which may now sometimes appearirrelevant. Models are a significant component of this researchphase, being typically the end result and the starting point forfurther work. In the current climate of genetic determinismthere is a sense that, in molecular biology, we have the answer;that this is the time when we will define the root causes ofmeristem functions.

The molecular models do, however, elaborate on earlierstructural models, being particularly concerned with the bal-ance between self-perpetuating and differentiating cells thatunderlies meristem maintenance. Attention is also given to

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how meristems form, initiate the organs characteristic ofvegetative or floral phases, and terminate.

Barton has considered the origin of leaves and of the shootapical meristem itself. These are processes that require tissuedifferentiation, to distinguish the shoot apical meristem fromother embryonic tissues, and, later to define leaf founder cellsfrom amongst the undifferentiated cells of the shoot apicalmeristem. Barton’s research addresses patterning arising in theembryo and ‘genetic circuitry that distinguishes meristemfrom leaf ’ (Barton, 2001). She has studied these differentia-tions using the homeodomain transcription factor STM inArabidopsis. stm mutants fail to make a functional shoot apicalmeristem. The gene is expressed in the late globular stage ofthe embryo and then in the shoot apical meristem throughoutthe life of the plant, suggesting a role in the formation andmaintenance of the meristem. STM is down-regulated in cellsdestined to become leaves (Long et al., 1996), although this isnot the first sign of leaf identity as the expression of PIN-HEAD/ZWILLE (PNH ) precedes the down-regulation ofSTM (Lynn et al., 1999). Barton revived an earlier term,‘promeristem’ to encompass the initials and their recent undif-ferentiated derivatives in the shoot apical meristem (Barton,1998). STM and PNH are effectively markers for the outcomeof decisions facing promeristem cells – to continue in a mer-istematic role or to become leaf.

Barton proposes a model of cyclic action. In this model theshoot apical meristem initiates leaves, then the adaxial side ofthese leaves induces shoot apical meristem formation in theleaf axil – so shoot apical meristems make leaves of which makemeristems (McConnell & Barton, 1998). In arriving at this model,Barton considered the correlation that exists between adaxialleaf tissue and the formation of meristems. This relationshipis seen in phabulosa mutants of Arabidopsis, in which leaves areadaxialized, in overexpression studies with the maize homeo-domain KNOX transcription factors and also in earlier workby the Snows, in which surgery resulting in leaf abaxializationwas accompanied by a loss of axillary meristems. A significantconcern of this model is whether shoot meristem tissue arisesonly once, as the original embryonic assignment, or manytimes over in the life of the plant. Does the new axillary mer-istem arising in association with the adaxial side of the leafderive from ‘detached meristem’, that is, a few meristematiccells that remain undifferentiated? Or is it possible for cells todifferentiate but then to respond to a localized signal thatre-establishes their meristematic properties? (McConnell &Barton, 1998).

These considerations of the distinction between the leafand the meristem are reminiscent of the thoughts of Arber(1950). Arber observed the self-similarity of the system: ‘eachbranch shoot echoes the character of the parent shoot’ andconsolidated the earlier botanical interpretations on thistheme in her view of the leaf as a ‘partial-shoot’, adding that‘the partial-shoot has an urge towards the development ofwhole shoot characters’ (Arber, 1950).

Laux and Clark have focused on how the meristem sustainsits two main functions, providing cells to an undifferentiatedpool, and differentiating cells into organs. Laux primarilyconsiders the Arabidopsis homeodomain transcription factor,WUS (Mayer et al., 1998) and Clark investigates the role ofthe signal transduction pathway encoded by the CLAVATAgenes. CLAVATA1 encodes a receptor kinase, CLAVATA2 areceptor-like protein and CLAVATA3 a peptide ligand (Clarket al., 1997; Jeong et al., 1999; Trotochaud et al., 2000). Theinvolvement of these genes in meristem maintenance can beinferred from their mutant phenotypes. The meristems of wusmutants terminate prematurely (Laux et al., 1996), whilstthose of clavata mutants proliferate (Clark et al., 1993,reviewed in Clark, 1996). The model of meristem functionwhich emerges from mutant, expression and overexpressionanalyses uses the CZ/PZ terminology of Foster to provide gridreferences, within which gene expression patterns and cellfunctions can be defined (Fig. 9). Laux’s group picture themeristem as including an organizing centre defined by WUSexpression. This organizing centre lets overlying cells knowthat they are stem cells by inducing CLV3 expression, whichmaintains the cells in an undifferentiated state. To becomeorgans, cells need to be beyond the range of WUS, thus losingtheir stem cell identity and becoming available to be assignedorgan fate (Schoof et al., 2000). Although WUS inducesCLV3 expression, CLV3 is secreted from the outer layers to thelower layers of the meristem where the CLV1 and 2 genes areresponsible for curtailing the expression of WUS. It is thisWUS–CLV negative feedback interaction which is proposedto maintain the balance of cells in the meristem (Schoof et al.,2000).

Revealingly, Laux’s group have expressed WUS under thecontrol of the promoter of the AINTEGUMENTA (ANT )gene, thereby directing its expression to organ primordia. Theresulting misexpression of WUS extends CLV3 inductionand undifferentiated cell fate to a tissue at an early stage

Fig. 9 The molecular basis of shoot apical meristem maintenance. WUS expression in a subset of cells in the central region of the meristem establishes an ‘organizing centre’ and activates CLV3 expression in the overlying outer cell layers of the meristem. CLV3 negatively regulates the expression of WUS via its interaction with CLV1. These interactions between WUS and the CLV genes, regulate the proliferation of stem cells in the meristem. STM is expressed throughout the meristem where it represses cell differentiation. It is down-regulated in organ primordia (see text for details). CZ, central zone; PZ, peripheral zone.

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of differentiation (Schoof et al., 2000). Thus, WUS can apparentlyrespecify relatively undifferentiated tissue, such as organ pri-mordia, and initiate stem cell identity there. More differenti-ated cells, however, require the action of both STM and WUSto reverse their fate (Lenhard et al., 2002). These genes appearto function independently, regulating different downstreamtargets. Their common task is to prevent differentiation,either through its suppression (STM ) or by the specificationof stem cells (WUS ). Lenhard et al. (2002) found that coex-pression of STM and WUS led to an extension of small CLV3-expressing cells from the apex onto the lamina of cotyledons.Gallois et al. (2002) induced WUS expressing sectors in seed-lings and independently activated STM, with the result thatoutgrowths occurred from both abaxial and adaxial sides ofthe cotyledons. These outgrowths expressed CLV1 transientlybut then developed as leaf-like organs, suggesting that theyhad a meristem-like phase but ultimately were not self-maintaining structures. Thus, the details of WUS / CLV /STMaction still need to be worked out, particularly when thegenes are expressed in pre-existing tissues in different stages ofdifferentiation.

In the era of developmental genetics the key question ofhow leaf primordia are positioned has received relatively littleattention. Hake has described two mutants of maize whichprovide possible support for Wardlaw’s inhibitory field model.In the terminal ear 1 (te1 ) mutant alternate phyllotaxis is dis-rupted to an irregularly opposite or spiral arrangement (Veitet al., 1998); in the abphyl1 mutant it becomes decussate(Jackson & Hake, 1999). The expression of TE1, a putativeRNA-binding protein, was found in ‘horseshoe’ formationsalternating from side to side around the stem. New leaf pri-mordia arose at the discontinuity of expression, leading Veitet al. to suggest that TE1 inhibited cells from ‘acting as organ-izers of leaf development’ and Scanlon to wonder whetherTE1 might ‘divulge the first molecular evidence for the inhi-bitory field theory of plant phyllotaxy’ (Scanlon, 1998). Cru-cial to the interpretation of abphyl1 is the observation of theenlargement of the embryonic shoot apical meristem, andthat the novel phyllotactic pattern is established in theembryo. This implies that, depending on the precise timing ofleaf position determination, phyllotactic patterning may fol-low the change in meristem size and not vice-versa ( Jackson &Hake, 1999), a finding consistent with the model proposed byRichards (1951). Jackson and Hake envisage that the enlargedmeristem affects fields – all inhibitory – but either chemical orbiophysical.

Up to this point we have considered how differentiated andundifferentiated tissues of the meristem are specified andmaintained. This is a constant underlying feature of the mer-istem, even when alterations to meristem function occur inaccordance with the growth phase of the plant. Mechanismsthat elaborate on this basic pattern during the floral phasehave been the subject of many models, which will now beconsidered.

Coen’s view of the shoot apical meristem is unitary (i.e. themeristem is divisible into a set of simple, self-containedregions and functions), reductionist and highly conceptual-ized. He suggests that meristems have an ‘identity’ – vegeta-tive, inflorescence or floral. This identity is controlled by‘meristem identity genes’ and can be described and classifiedin terms of four main features, phyllotaxy, organ identity,determinacy and internode length (Coen, 1991; Coen &Carpenter, 1993). Consequently, in interpreting the floricaula( flo) mutant of Antirrhinum majus, Coen proposes that theloss of FLO gene expression results in a failure of the transitionof the meristem from inflorescence to floral identity (Fig. 10a;Coen et al., 1990). Hence FLO is a meristem identity geneorchestrating a switch in the crucial meristem properties thatcompose ‘identity’.

The conceptual nature of this picture is illustrated by thework of Huala & Sussex (1993). Drawing largely on pre-developmental genetics work, they described cases in whichmeristems may be considered determined but their derivativesmay not. For example, a maize meristem may be reproductivelydetermined yet still initiate primordia which develop as vege-tative shoots (Huala & Sussex, 1993). In such cases assess-ment of meristem identity must rely on characteristics ofphyllotaxy, determinacy and internode length (Huala &Sussex, 1993).

Fig. 10 Coen: FLORICAULA and partitioning. (a) Longitudinal section of an inflorescence of Antirrhinum majus, viewed in dark field. The section shows FLO expression in the meristem detected using in situ hybridization with digoxigenin-labelled probes (1990). Reprinted from Cell 63, Coen et al. FLORICAULA: a homeotic gene required for flower development in Antirrhinum majus. pp. 1311–1322, copyright (1990), with permission from Elsevier. (b) Top view of a wild type meristem of Antirrhinum majus, showing a regular initiation pattern of discrete primordia. The outline of a double helix is marked. (c) flo mutant of Antirrhinum majus lacking discrete primordia. Reprinted from Plant Cell 7, Carpenter et al. Control of flower development and phyllotaxy by meristem identity genes in Antirrhinum. pp. 2001–11, copyright (1995), with permission from the American Society of Plant Biologists.

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For Coen a unifying factor underlying a number of thesemeristem properties is the idea of partitioning. Cells becomepartitioned off from the shoot apical meristem to becomeorgan primordia or secondary meristems, and which of thesetwo types of partitioning is favoured dictates determinacy.Conforming to a phyllotactic arrangement requires a precisepattern of partitioning (Coen & Carpenter, 1993). This con-cept was elaborated further in one of the first molecularapproaches to phyllotaxis: the analysis of phyllotaxis of themeristem identity gene mutants of FLORICAULA (FLO) andSQUAMOSA (SQUA) (Carpenter et al., 1995). In bothmutants phyllotaxis is altered from that of the wild type,implicating FLO and SQUA in promoting the transition fromspiral to whorled phyllotaxis. This change is considered to becontrolled by these genes in a two-stage process. Areas of themeristem are set aside for ‘potential primordium initiation’ instage 1, and then partitioning of these areas in stage 2 resultsin primordia. Rarely, in flo mutants, this second stage fails andplants form double spirals, which led Carpenter et al. to ques-tion whether the primary initiation state is of discrete primor-dia (which unite in flo mutants) or of spirals which requirepartitioning. This second proposal is recognized as identifyingwith the ideas of Plantefol (Fig. 10b,c).

Coen proposes that the nature of the organs which a mer-istem of floral identity initiates is controlled by ‘organ identitygenes’ (Coen & Carpenter, 1993). The model of organ iden-tity gene action, the ABC model, devised by Coen and Meye-rowitz (1991, see below) is a current, highly influentialconcept of flower development. Coen’s interest in this areaarose from a PhD on Drosophila, during which he becameinterested in studying development and evolution fromgenetic and molecular perspectives (Coen, 1996). By analyzingAntirrhinum flower structure mutants, Coen began to con-struct a genetic model, with first two (independent), thenthree gene functions to explain flower structure (Carpenter &Coen, 1990; Coen, 1991). The resulting ABC model proposesthat organ identity genes operating in three overlappingdomains, A, B and C control the identity of organs in theflower. a function expressed alone in whorl 1 gives rise tosepals; a and b in whorl 2 to petals; b and c in whorl 3 to sta-mens; c in whorl 4 to carpels. a and c are antagonistic and can-not be expressed in the same domain (Fig. 11a,b; Coen &Meyerowitz, 1991; Coen, 1991; Coen & Carpenter, 1993).

For Huala & Sussex (1993), floral homeotic mutants dem-onstrate that determination of the floral meristem and its pri-mordia are separable events (see above). It is also evident that,whilst mutation of organ identity genes alters identity, otherfloral properties (e.g. phyllotaxy, lack of internodes) persist.This narrow role for organ identity genes was apparent toLord, who proposed the importance of relative timing in mer-istem functions, and emphasized the heterochronic as well ashomeotic nature of floral mutants. One of the conclusions ofher work was that whilst organ identity was altered, the char-acteristic cell division patterns that gave rise to the primordia

in each whorl remained as wild type (Lord et al., 1994; Hill &Lord, 1989; Crone & Lord, 1994). Weigel has also consideredthe mechanisms which underlie the correct positions of ABCexpression patterns, concluding that meristems have pattern-ing systems common to vegetative and floral meristems,which involve the genes UNUSUAL FLORAL ORGANS andWUSCHEL (Parcy et al., 1998; Lohmann et al., 2001).

Coen introduced a second dimension to complete the‘polar coordinate model’ for the control of primordium fate(Coen, 1991). As well as the ‘r’ or radial axis, along whichorgan identity varies in the flower, there is the ‘y’ axis. Alongthis second axis, the expression of the gene CYCLOIDEA isproposed to vary – and control the asymmetry (where rele-vant) of the flower. It achieves this, it is proposed, by slowingthe growth rate and reducing the organ number in the area ofits expression (Luo et al., 1995). According to this model, thefate of a primordium is the reflection of its coordinates on theidentity (r) and symmetry (y) axes (Coen & Meyerowitz,1991; Coen, 1991). What is striking about these models ofmeristem function is that they rely on the ideas of identity andsymmetry as specific end points to which genetic cascades aredirected.

As Meyerowitz, coproposer of the ABC model, points out,there are few, if any, reasons why a model of flower develop-ment along these lines could not have been arrived at100 years earlier. Detailed descriptions of monstrous and nor-mal flowers have been made for centuries, but, with some

Fig. 11 Coen and Meyerowitz: the ABC model. (a) The ABC model of floral organ identity. In this diagrammatic representation the whorls of the flower are shown as concentric rings. Superimposed on this are the domains A, B and C. See text for details. Redrawn from Coen & Meyerowitz (1991). How this is proposed to translate to the meristem is shown in (b), a diagram of the meristem in longitudinal section, showing patterns of ABC expression in present or presumptive whorls 1–4. Reprinted from Plant Cell 5, Coen and Carpenter, The metamorphosis of flowers. pp. 1175–1181, copyright (1993), with permission from the American Society of Plant Biologists.

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exceptions, it was not until the late 1980s that the study offlower development capitalized on the availability of flowerabnormalities (Meyerowitz et al., 1989; Meyerowitz, 1995).The aim of these approaches was to ‘understand the flow ofinformation from the genome that led to the 3-dimensionalstructure of the flower’ (Meyerowitz, 1995). For Meyerowitzthis began with the description of recessive homeotic mutantsof Arabidopsis thaliana. He established that the mutations inquestion were often flower-specific, the organs developed ona time course linked to their whorl rather than their identity,that replacement organs in the mutants were often mosaic innature, and there were few constraints on the identity oforgans in any whorl. This last discovery was evidence againstthe few earlier mechanistic models there had been, which pro-posed sequential means of organ specification, and were atodds with the data on abnormalities (Bowman et al., 1989;Meyerowitz et al., 1989). Consideration of the mutant phe-notypes led Meyerowitz to the proposal that the flower pri-mordium could be ‘divided into fields or compartments, eachconsisting of adjacent whorls’ with the genes acting in the‘delineation of concentric-ring shaped compartments, eachwith a different fate’ (Bowman et al., 1989). The preliminarymodel proposed three classes of homeotic gene capable of pro-ducing a flower, with the leaf as a ‘ground state organ’ (Coen& Meyerowitz, 1991; Meyerowitz et al., 1991). Later tests ofthe model revealed the necessity for a further function, pro-vided by the MADS-box SEPALLATA genes, to convertleaves to floral organs (Honma & Goto, 2001). Theissen(2001) considered this grounds to re-evaluate the ‘outmodedABC model’, by then amounting to the ABCDE model, ques-tioning whether ‘floral homeotic functions are still a usefulconcept at all’ since the concept ‘no longer provides a usefulsimplification’ (Theissen, 2001).

In Coen’s opinion, we need now to think of the basis ofprimordium fate in genetic terms and consider, ‘how are thedomains of whorl identity genes established?’ (Coen, 1991).Seen in relation to the cytological and morphologicalapproaches outlined in earlier sections, this indicates a hugeshift in perspective. The view of the meristem here is abstract,a recurrent feature of developmental genetics-based meristemmodels. What are the relative merits of this direction in mer-istem research and how have these new ideas come about? TheABC model is probably the most influential model of meris-tem function that there has been. It is simple and neat – twodesirable features in any model. It is possible that the ABCmodel is so successful because it presents the closest explana-tion to the biological reality of how a flower forms. Certainlyit is reassuring that, almost independently, and working onseparate systems, Coen and Meyerowitz arrived at similarmodels. But it could be argued that this has more to do withtheir common perspective than with reality. The massive sup-port found for this model shows that many choose to interprettheir data in the light of it, and that they share the same per-spective as Coen and Meyerowitz.

A molecular model is necessarily abstract because, as Coenhas pointed out, what has recently been revealed is ‘howorganisms contain a whole series of regional differences thatwere previously hidden from view’ (Coen, 1996). But, whilstwe can perceive the organization of floral organs, and in situhybridization allows us to visualize gene expression, in thesemodels we need to envisage partitioning, domains, ‘molecularterritories’ (Coen, 1996) and boundaries (Vincent et al.,1995). This can be challenging, although similar demandshave been made in the past. In Wardlaw’s proposal of phyllo-tactic determinants we had to visualize inhibitory fields radi-ating from growth centres. Interestingly, both models providestrong visual images, and it seems legitimate to ask how farthey colour our perceptions. Ask yourself: ‘would I havethought like this from experience? How much is this a prod-uct of my imagination?’ The answer is that the model deter-mines what you see, the questions you can ask; it defines a wayof thought.

How did we start to see plants in such unitary terms? Outof the beginnings of mutant analyses arose ‘animal-plant’ thought.Initially this was merely a recognition that the initiative to usemutants for the study of pattern formation followed a prece-dent set in animal developmental studies. Drawing from ear-lier work on Drosophila, Meyerowitz concluded that findingthe genes underlying flower development was the next step(Meyerowitz et al., 1989). When the first of these genes wasuncovered, the DEFICIENS A (DEF A ) gene of Antirrhinummajus (Sommer et al., 1990), and the functional significanceof its homology to transcription factors was considered, it waspossible to envisage a regulatory network determining mor-phogenesis at the meristem (Sommer et al., 1990).

It was, however, the report of the second transcriptionfactor to be cloned, AGAMOUS (AG ) from Arabidopsis(Yanofsky et al., 1990), that seemed to signal a turning point inperceptions of how the meristem might function. Discoverythat this second gene had extensive homology to the DEF Agene prompted many thoughts in an animal-plant direction.The possibility that a family of transcription factors was at theheart of pattern formation of flowers as well as flies grew evermore likely. Meyerowitz proposed that we might considersome sort of ‘floreo’ box, similar to the homeobox of Dro-sophila homeotic selector genes (Meyerowitz et al., 1991) andthe AG report concluded that, analogous to animal develop-ment, floral organ development might be regulated by a set ofclosely related genes (Yanofsky et al., 1990). Furthermore,with DEF and AG MADS box transcription factors (as theybecame known, Schwarz-Sommer et al., 1990) from differentplant species, there was the possibility of an evolutionarilyconserved mechanism.

Thus the domains and compartments of meristems in today’sdevelopmental biology are traceable to concepts of animaldevelopment. From a similarity of approach (analysis of devel-opmental mutants) to a discovery of gene families, we havereached a similarity of perception and mechanism. Sablowski

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& Meyerowitz (1998) commented that ‘homeotic mutationshave provided evidence for modularity in the development ofboth animals and plants; the identity of discrete body partscan be transformed by loss of function or ectopic expressionof homeotic genes’. This concept of modularity extendsbeyond the meristem consumed with flowering, and also backto a pre-developmental genetics era when vegetative modulesbecame known as phytomers.

More recently Meinhardt (1996) has proposed a segmenta-tion model of plant development (Fig. 12). Disappointed inthe inability of morphological spacing models to explain howleaves develop distinct abaxial and adaxial leaf surfaces withcorrect orientation to the shoot apical meristem, Meinhardtproposed that segments M1, M2 and M3 form a repeatingsequence to construct a vegetative plant. An activator-inhibitormechanism is involved in determining the generation ofleaves but, crucially, leaves only form at the junction of seg-ments. The junction position ensures that leaves are com-posed of two tissue types, M1 and M2 (abaxial-adaxialsurface) and are correctly orientated. Thus the polar characterof the boundary ensures the polar structure of the leaf. Mein-hardt sets this model amongst others of biological patternformation (tropical sea shells, leg, wing initiation … ) andproposes a ‘common mechanism’ in plant and animal devel-opment. Further endorsements of such analogies come fromthe comparisons of leaf and wing development (Waites &Hudson, 1995; Scanlon et al., 1996).

The end of the shoot apical meristem is most frequently con-sidered as accompanying flower formation. This determinacy

dimension to flowering has been prominent in the thinking ofdevelopmental geneticists and is associated with the C func-tion gene AG (Yanofsky et al., 1990). ag mutants fail to makethe reproductive organs of the flower and produce an indeter-minate proliferation of sepals and petals (Bowman et al., 1989),whilst ectopic AG expression can convert an indeterminateinflorescence meristem to a determinate floral meristem(Mizukami & Ma, 1997). The termination of growth associatedwith flowering results from an interaction between AG andWUS (Lenhard et al., 2001; Lohmann et al., 2001). As describedabove, WUS is involved in assigning stem cell fate via CLV3. Inconjunction with the meristem identify gene LEAFY it alsoup-regulates AG expression but, later in development, AG actsto repress WUS, thus halting the supply of undifferentiatedcells and preventing further growth. Although the C functionmay lie at the heart of meristem determinacy, developmentcontinues after the carpels it specifies. The D function genesFBP7 and FBP11 evoke ovule and placental column develop-ment from the central meristem of Petunia (Angenent &Colombo, 1996). In some cases determinacy is only incom-pletely maintained (Fig. 13).

When and where growth termination occurs dictates thearchitecture of the plant (Battey, 2003). There are times whenflowering does not stop a meristem (floral reversion), andother times when meristems cease growth without flowering.One such example is found in Arabidopsis itself: the inducedapical meristem is an inflorescence meristem, and althoughgrowth at this apical point is indeterminate, it isn’t infinite.Another example is found in inflorescence meristems ofBougainvillea, which under long day conditions fail to developand become lignified thorns (Hackett & Sachs, 1968). In calabrese,apical abortion or ‘blindness’, an environmentally induceddisorder, results in cells losing their meristematic ability anddifferentiating as parenchyma. The tunica-corpus distinctionsdisappear and the meristem stops without death or flowering,a case of ‘terminal differentiation’ (Forsyth et al., 1999).

VI. Conclusions

Describing the study of botany in the early 1800s, Sachsobserved that, ‘a curious misconception crept in among thephytotomists at this time; they believed that more correct andtrustworthy figures would be obtained if the observer andwriter did not himself make them, but employed other eyesand other hands for that purpose; they imagined that in thisway every kind of prejudice, of preconceived opinion wouldbe eliminated from the drawings’ (Sachs, 1890). Today wemight feel that in an age of photography, sequencing,imaging, we have machines of objectivity that place usbeyond that problem, yet we still find only what we look for;what we see depends on how we interpret the informationavailable. Through recent history the shoot apical meristemhas been a constant but its scientific representation has beenever-changing. A model of shoot meristem function summarizes

Fig. 12 Meinhardt: segmentation. Schematic longitudinal section of a plant. M indicates an ‘elementary module’ composed of node (N) and internode (I), and producing a leaf (L) and bud (B). Each module is made of at least 3 subunits M1, M2, M3, which ensure polarity. A, shoot apical meristem; P, primordia. Stem shown, ‘unrolled’ in 2-dimensions. Cells produced by the shoot apical meristem aid stem elongation and differentiate into subunits. Note leaf initiation on M1, M2, border. Reprinted from International Journal of Developmental Biology 40, Meinhardt, Models of biological pattern formation: common mechanism in plant and animal development. pp. 123–134, copyright (1996), with permission from the University of the Basque Country Press.

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available knowledge, directs future research, and promotes apoint of view.

Acknowledgements

FT would like to thank Lucy Cavendish College, Cambridgeand the Stanley Smith (UK) Horticultural Trust. NHBacknowledges research funding from BBSRC, DEFRA andThe University of Reading Research Endowment Trust Fund.The legend to Fig. 2 was kindly translated from the Germanby Alex Angenendt (The University of Reading).

References

Adler I, Barabe D, Jean RV. 1997. A history of the study of phyllotaxis. Annals of Botany 80: 231–244.

Angenent GC, Colombo L. 1996. Molecular control of ovule development. Trends in Plant Science 1: 228–232.

Arber A. 1950. The natural philosophy of plant form. Cambridge, UK: Cambridge University Press.

Barton MK. 1998. Cell type specification and self renewal in the vegetative shoot apical meristem. Current Opinion in Plant Biology 1: 37–42.

Barton MK. 2001. Leaving the meristem behind: regulation of KNOX genes. Genome Biology 2: 1–3.

Battey NH. 2003. Plant Culture: thirteen seasonal pieces. February – constructing a corymb. Journal of Experimental Botany 54: 605–608.

Bowes BG. 1996. A colour atlas of plant structure. London, UK: Manson Publishing.

Bowman JL, Smyth DR, Meyerowitz EM. 1989. Genes directing flower development in Arabidopsis. Plant Cell 1: 37–52.

Buvat R. 1952. Structure, évolution et fonctionnement du méristème apical de quelques dicotylédones. Annales des Sciences Naturelles (11th Series, Botany) 8: 199–300.

Carpenter R, Coen ES. 1990. Floral homeotic mutations produced by transposon-mutagenesis in Antirrhinum majus. Genes and Development 4: 1483–1493.

Carpenter R, Copsey L, Vincent C, Doyle S, Magrath R, Coen E. 1995. Control of flower development and phyllotaxy by meristem identity genes in Antirrhinum. Plant Cell 7: 2001–2011.

Church AH. 1904. The principles of phyllotaxis. Annals of Botany 18: 227–243.

Church AH. 1920. On the interpretation of phenomena of phyllotaxis. New York, USA: Hafner.

Clark SE. 1996. The shoot meristem as a site of continuous organogenesis. Seminars in Cell and Developmental Biology 7: 873–880.

Clark SE, Running MP, Meyerowitz EM. 1993. CLAVATA1, a regulator of meristem and flower development in Arabidopsis. Development 119: 397–418.

Clark SE, Williams RW, Meyerowitz EM. 1997. The CLAVATA1 gene encodes a putative receptor kinase that controls shoot and floral meristem size in Arabidopsis. Cell 89: 575–585.

Clowes FAL. 1961. Apical meristems. Oxford, UK: Blackwell Scientific Publications.

Coen ES. 1991. The role of homeotic genes in flower development and evolution. Annual Review of Plant Physiology and Plant Molecular Biology 42: 241–279.

Coen ES. 1996. Floral symmetry. EMBO Journal 15: 6777–6788.Coen ES, Carpenter R. 1993. The metamorphosis of flowers. Plant Cell 5:

1175–1181.Coen ES, Meyerowitz EM. 1991. The war of the whorls: genetic interactions

controlling flower development. Nature 353: 31–37.Coen ES, Romero JM, Doyle S, Elliott R, Murphy G, Carpenter R. 1990.

FLORICAULA: a homeotic gene required for flower development in Antirrhinum majus. Cell 63: 1311–1322.

Crone W, Lord EM. 1994. Floral organ initiation and development in wild-type Arabidopsis thaliana (Brassicaceae) and in the organ identity mutants apetala 2–1 and agamous-1. Canadian Journal of Botany 72: 384–401.

Cutter EG. 1959. On a theory of phyllotaxis and histogenesis. Biological Reviews 34: 243–263.

Dermen H. 1947. Periclinal cytochimeras and histogenesis in cranberry. American Journal of Botany 34: 32–43.

Fleming AJ, McQueen-Mason S, Mandel T, Kuhlemeier C. 1997. Induction of leaf primordia by the cell wall protein expansin. Science 276: 1415–1418.

Fig. 13 An indeterminate flower of Impatiens balsamina. Proliferations of petal-like organs arise on the placental column and burst out from within the seed pod. Photograph courtesy of Jason Pole.

Tansley review

© New Phytologist (2003) 159: 37–52 www.newphytologist.com

Review 51

Forsyth JL, Pearson S, Hadley P, Barnett JR. 1999. Apical abortion in calabrese is induced by periods of low temperature and results in premature differentiation of apical meristem cells. Journal of Experimental Botany 50: 861–868.

Foster AS. 1938. Structure and growth of the shoot apex in Gingko biloba. Bulletin of the Torrey Botanical Club 65: 531–556.

Foster AS. 1939. Problems of structure, growth and evolution in the shoot apex of seed plants. Botanical Review 5: 454–470.

Gallois J-L, Woodward C, Reddy GV, Sablowski R. 2002. Combined SHOOTMERISTEMLESS and WUSCHEL trigger ectopic organogenesis in Arabidopsis. Development 129: 3207–3217.

Goebel K. 1926. Wilhelm Hofmeister. The life and work of a nineteenth century botanist. London, UK: The Ray Society.

Green PB. 1992. Pattern formation in shoots: a likely role for minimal energy configurations of the tunica. International Journal of Plant Sciences 153: 59–75.

Green PB. 1994. Connecting gene and hormone action to form, pattern and organogenesis: biophysical transductions. Journal of Experimental Botany 45: 1775–1788.

Green PB. 1996. Expression of form and pattern in plants – a role for biophysical fields. Seminars in Cell and Developmental Biology 7: 903–911.

Green PB. 1997. Expansin and morphology: a role for biophysics. Trends in Plant Science 2: 365–366.

Green PB. 1999. Expression of pattern in plants: combining molecular and calculus-based biophysical paradigms. American Journal of Botany 86: 1059–1076.

Green PB, Steele CS, Rennich SC. 1996. Phyllotactic patterns: a biophysical mechanism for their origin. Annals of Botany 77: 515–527.

Hackett WP, Sachs RM. 1968. Experimental separation of inflorescence development from initiation in Bougainvillea. Proceedings of the American Society for Horticultural Science 92: 615–621.

Hernandez LF, Green PB. 1993. Transductions for the expression of structural pattern: analysis in sunflower. Plant Cell 5: 1725–1738.

Hill JP, Lord EM. 1989. Floral development in Arabidopsis thaliana : a comparison of the wild type and the homeotic pistillata mutant. Canadian Journal of Botany 67: 2922–2936.

Honma T, Goto K. 2001. Complexes of MADS-box proteins are sufficient to convert leaves into floral organs. Nature 409: 525–529.

Huala E, Sussex IM. 1993. Determination and cell interactions in reproductive meristems. Plant Cell 5: 1157–1165.

Jackson D, Hake S. 1999. Control of phyllotaxy in maize by the abphyl1 gene. Development 126: 315–323.

Jean RV. 1994. Phyllotaxis: a systematic study in plant morphogenesis. Cambridge, UK: Cambridge University Press.

Jeong S, Trotochaud AE, Clark SE. 1999. The Arabidopsis CLAVATA2 gene encodes a receptor-like protein required for the stability of the CLAVATA1 receptor-like kinase. Plant Cell 11: 1925–1933.

Kuhlemeier C, Reinhardt D. 2001. Auxin and phyllotaxis. Trends in Plant Science 6: 187–189.

Laux T, Mayer KFX, Berger J, Jürgens G. 1996. The WUSCHEL gene is required for shoot and floral meristem integrity in Arabidopsis. Development 122: 87–96.

Lenhard M, Bohnert A, Jürgens G, Laux T. 2001. Termination of stem cell maintenance in Arabidopsis floral meristems by interactions between WUSCHEL and AGAMOUS. Cell 105: 805–814.

Lenhard M, Jürgens G, Laux T. 2002. The WUSCHEL and SHOOTMERISTEMLESS genes fulfil complementary roles in Arabidopsis shoot meristem regulation. Development 129: 3195–3206.

Lohmann JU, Hong RL, Hobe M, Busch MA, Parcy F, Simon R, Weigel D. 2001. A molecular link between stem cell regulation and floral patterning in Arabidopsis. Cell 105: 793–803.

Long JA, Moan EI, Medford JI, Barton MK. 1996. A member of the KNOTTED class of homeodomain proteins encoded by the SHOOTMERISTEMLESS gene of Arabidopsis. Nature 379: 66–69.

Lord EM, Crone W, Hill JP. 1994. Timing of events during flower organogenesis: Arabidopsis as a model system. Current Topics in Developmental Biology 29: 325–356.

Luo D, Carpenter R, Vincent C, Copsey L, Coen E. 1995. Origin of floral asymmetry in Antirrhinum. Nature 383: 794–799.

Lyndon RF. 1994. Control of organogenesis at the shoot apex. New Phytologist 128: 1–18.

Lyndon RF. 1998. The shoot apical meristem: its growth and development. Cambridge, UK: Cambridge University Press.

Lynn K, Fernandez A, Aida M, Sedbrook J, Tasaka M, Masson P, Barton MK. 1999. The PINHEAD/ZWILLE gene acts pleiotropically in Arabidopsis development and has overlapping functions with the ARGONAUTE1 gene. Development 126: 469–481.

Majumdar GP. 1942. The organization of the shoot in Heracleum in the light of development. Annals of Botany 6: 49–82.

Mayer KFX, Schoof H, Haecker A, Lenhard M, Jürgens G, Laux T. 1998. Role of WUSCHEL in regulating stem cell fate in the Arabidopsis shoot meristem. Cell 95: 805–815.

McConnell JR, Barton MK. 1998. Leaf polarity and meristem formation in Arabidopsis. Development 125: 2935–2942.

Meinhardt H. 1996. Models of biological pattern formation: common mechanism in plant and animal development. International Journal of Developmental Biology 40: 123–134.

Meyerowitz EM. 1995. The molecular genetics of pattern formation in flower development: a perspective after ten years of the flowering newsletter. Flowering Newsletter 20: 4–12.

Meyerowitz EM. 1996. Plant development: local control, global patterning. Current Opinion in Genetics and Development 6: 475–479.

Meyerowitz EM, Bowman JL, Brockman LL, Drews GN, Jack T, Sieburth LE, Weigel D. 1991. A genetic and molecular model for flower development in Arabidopsis thaliana. Development 112: 157–168.

Meyerowitz EM, Smyth DR, Bowman JL. 1989. Abnormal flowers and pattern formation in floral development. Development 106: 209–217.

Mizukami Y, Ma H. 1997. Determination of Arabidopsis floral meristem identity by AGAMOUS. Plant Cell 9: 393–408.

Parcy F, Nilsson O, Busch MA, Lee I, Weigel D. 1998. A genetic framework for floral patterning. Nature 395: 561–566.

Philipson WR. 1949. The ontogeny of the shoot apex in dicotyledons. Biological Reviews 24: 21–50.

Pien S, Wyrzykowska J, McQueen-Mason S, Smart C, Fleming A. 2001. Local expression of expansin induces the entire process of leaf development and modifies leaf shape. Proceedings of the National Academy of Sciences, USA 98: 11812–11817.

Plantefol L. 1946. Fondements d’une théorie phyllotaxique nouvelle. Annales des Sciences Naturelles (11th series, botany) 7: 1–77.

Reed HS. 1942. A short history of the plant sciences. Waltham, MA, USA: Chronica Botanica Company Publication.

Reinhardt D, Mandel T, Kuhlemeier C. 2000. Auxin regulates the initiation and radial position of plant lateral organs. Plant Cell 12: 507–518.

Reinhardt D, Wittwer F, Mandel T, Kuhlemeier C. 1998. Localized upregulation of a new expansin gene predicts the site of leaf formation in the tomato meristem. Plant Cell 10: 1427–1437.

Richards FJ. 1951. Phyllotaxis: its quantitative expression and relation to growth in the apex. Philosophical Transactions of the Royal Society of London B 235: 509–564.

Sablowski RWM, Meyerowitz EM. 1998. A homolog of NO APICAL MERISTEM is an immediate target of the floral homeotic genes APETALA3/PISTILLATA. Cell 92: 93–103.

Sachs J. 1890. History of botany. Oxford, UK: Clarendon Press.Satina S, Blakeslee AF, Avery AG. 1940. Demonstration of the three germ

layers in the shoot apex of Datura by means of induced polyploidy in periclinal chimeras. American Journal of Botany 27: 895–905.

Scanlon MJ. 1998. Force fields and phyllotaxy: an old model comes of age. Trends in Plant Science 3: 413–414.

Tansley review

www.newphytologist.com © New Phytologist (2003) 159: 37–52

Review52

Scanlon MJ, Schneeberger RG, Freeling M. 1996. The maize mutant narrow sheath fails to establish leaf margin identity in a meristematic domain. Development 122: 1683–1691.

Schoof H, Lenhard M, Haecker A, Mayer KFX, Jürgens G, Laux T. 2000. The stem cell population of Arabidopsis shoot meristems is maintained by a regulatory loop between the CLAVATA and WUSCHEL genes. Cell 100: 635–644.

Schwabe WW. 1984. Phyllotaxis. In: Barlow PW, Carr DJ, eds. Positional controls in plant development. Cambridge, UK: Cambridge University Press, 403–440.

Schwarz-Sommer Z, Huijser P, Nacken W, Saedler H, Sommer H. 1990. Genetic control of flower development by homeotic genes in Antirrhinum majus. Science 250: 931–936.

Selker JML, Lyndon RF. 1996. Leaf initiation and de novo pattern formation in the absence of an apical meristem and pre-existing patterned leaves in watercress (Nasturtium officinale) axillary explants. Canadian Journal of Botany 74: 625–641.

Sifton HB. 1944. Developmental morphology of vascular plants. New Phytologist 43: 87–129.

Singer C. 1931. A short history of biology. Oxford, UK: Clarendon Press.Snow M, Snow R. 1931. Experiments on phyllotaxis. I. The effect of

isolating a primordium. Philosophical Transactions of the Royal Society, London, Series B 221: 1–43.

Snow M, Snow R. 1937. Auxin and leaf formation. New Phytologist 36: 1–18.Sommer H, Beltrán J-P, Huijser P, Pape H, Lönnig W-E, Saedler H,

Schwarz-Sommer Z. 1990. DEFICIENS, a homeotic gene involved in the control of flower morphogenesis in Antirrhinum majus: the protein shows homology to transcription factors. EMBO Journal 9: 605–613.

Steeves TA, Sussex IM. 1989. Patterns in plant development, 2nd edn. Cambridge, UK: Cambridge University Press.

Sussex I. 1998. Themes in plant development. Annual Review of Plant Physiology and Plant Molecular Biology 49: 8–12.

Theissen G. 2001. Genetics of identity. Nature 414: 491.Thornley JHM. 1975a. Phyllotaxis. I. A mechanistic model. Annals of Botany

39: 491–507.Thornley JHM. 1975b. Phyllotaxis. II. A description in terms of intersecting

logarithmic spirals. Annals of Botany 39: 511–526.Trotochaud AE, Jeong S, Clark SE. 2000. CLAVATA3, a multimeric ligand

for the CLAVATA1 receptor-kinase. Science 289: 613–617.Veen AH, Lindenmayer A. 1977. Diffusion mechanism for phyllotaxis:

theoretical physico-chemical and computer study. Plant Physiology 60: 127–139.

Veit B, Briggs SP, Schmidt RJ, Yanofsky MF, Hake S. 1998. Regulation of leaf initiation by the terminal ear 1 gene of maize. Nature 393: 166–168.

Vincent CA, Carpenter R, Coen ES. 1995. Cell lineage patterns and homeotic gene activity during Antirrhinum flower development. Current Biology 5: 1449–1458.

Vogler H, Kuhlemeier C. 2003. Simple hormones but complex signalling. Current Opinion in Plant Biology 6: 51–56.

Waites R, Hudson A. 1995. PHANTASTICA: a gene required for dorsoventrality of leaves in Antirrhinum majus. Development 121: 2143–2154.

Wardlaw CW. 1949. Experiments on organogenesis in ferns. Growth Supplement 13: 93–131.

Wardlaw CW. 1957. The reactivity of the apical meristem as ascertained by cytological and other techniques. New Phytologist 56: 221–229.

Weigel D, Clark SE. 1996. Sizing up the floral meristem. Plant Physiology 112: 5–10.

Wolff CF. 1896. Theoria generationis. Leipzig, Germany: Verlag von Wilhelm Engelmann.

Yanofsky MF, Ma H, Bowman JL, Drews GN, Feldmann KA, Meyerowitz EM. 1990. The protein encoded by the Arabidopsis homeotic gene AGAMOUS resembles transcription factors. Nature 346: 35–39.