TRENDS in Genetics Vol.18 No.7 July 2002

http://tig.trends.com 0168-9525/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved.

345News&Comment

Leaves of higher plants are determinatestructures derived from a population oforganogenic cells at the flanks of the shootapical meristem (SAM). Current evidencesuggests that repression of KNOTTED1-likehomeobox (KNOX) transcription factors iscentral to the acquisition of leaf identity in awide spectrum of plant species. In plantswith simple leaves, transcripts for KNOXgenes are expressed within the SAM butexcluded from developing leaf primordia.Ectopic expression of KNOX genes resultsin aberrant leaf development. Recent workin both monocot plants (such as maize) and eudicot plants (snapdragon andArabidopsis) has uncovered a geneticpathway required for repression of KNOXgenes in leaves. This pathway is defined bythe ROUGH SHEATH2/ PHANTASTICA/ASYMETRIC LEAVES1 proteins. A recent

paper [1] by Mike Scanlon’s group suggeststhat at least one more such pathway mayexist in leaves of monocot plants.

Scanlon et al. describe mutations in theSEMAPHORE (SEM) locus of maize. Loss-of-function of SEM results in ectopic expressionof two KNOX genes in leaf primordia.Consequently, sem plants demonstrate aseries of phenotypes commonly associatedwith ectopic KNOX expression in maize,including proximodistal transformations,ectopic tissue outgrowths and vasculardisorganisation. Disruption of transport ofthe plant hormone auxin is also associatedwith these phenotypes. Intriguinglyhowever, sem also conditions a class ofphenotypes not commonly associated withKNOX misexpression in maize. Theseinclude severe defects in embryonicpatterning, aberrant endosperm and

impaired root development. This fact,together with the demonstration that roughsheath2 and sem mutations show anadditive interaction, lead Scanlon et al.to suggest that SEM and RS2 function in two genetically separable pathways, both of which are required to correctlycompartmentalise KNOX expression in themaize shoot. Cloning of the SEM gene willhelp to reveal the precise nature of this novelpathway required for repression of KNOXexpression in maize leaves.

1 Scanlon MJ, Henderson DC, Bernstein B. (2002)SEMAPHORE1 functions during the regulationof ancestrally duplicated knox genes and polarauxin transport in maize. Development.129, 2663–2673

Miltiandes Tsiantis

miltos.tsiantis@plant-sciences.oxford.ac.uk

Maize shoot development: beyond KNOX genes

Recently, several genes that control earlydevelopmental patterning have also beenfound to regulate stem cell maintenance and renewal in the adult organism. Fewexamples of this are more striking than thatof the Hox genes, a family of closely relatedhomeodomain-containing transcriptionfactors that pattern the early embryonic axis.Various different Hox genes are mentionedfrequently in the vast literature concerningblood development, and in particular theproliferation and differentiation ofhaematopoietic stem cells (HSCs).

Numerous studies have examined thefunction and expression of Hox genesduring the differentiation of these cells, with intriguing results. For example, Hoxexpression seems to follow a temporal anddevelopmental order in HSC maturationthat closely reflects that seen in embryonic

development. Hence, those Hox genes thatare expressed earliest in the embryo(e.g. Hoxb4), are also expressed in the leastmature haematopoietic cells, and Hoxgenes expressed at later developmentalstages are expressed in maturing HSCs.Furthermore, and closely reflectingexperiments in early embryos, themis-expression of Hox genes can interferewith HSC differentiation.

Despite the extensive insight into themolecular basis of haematopoiesis, anumber of practical difficulties in culturingand expanding HSC populations in vitroremain that impose major limitations on the surgical application of HSCtransplantation. In particular, it makestransplants from very small amounts ofblood, for example umbilical cord, ratherunreliable. However, Antonchuk et al. [1]

now show that HSCs derived from mousebone marrow, which are usually lost duringin vitro culture, can be greatly enriched(×1000) if they are transduced with Hoxb4.Furthermore, these cells retain their abilityto differentiate into lymphoid and myeloidlineages, an ability that is absent in HSCsthat have been immortalized by theexpression of an oncogene. In a relatedstudy, Kyba et al. [2] show thatHoxb4-transduced HSCs can repopulate the bone marrow of irradiated adult mice,and establish multilineage haematopoiesis.They also show that Hoxb4 can actuallyinduce primitive embryonic progenitorcells, for example those derived from the yolk sac, to adopt a definitive HSC phenotype.

These intriguing results point to apossible method for culturing large

Young blood: Hox genes and haematopoietic stem cells

solely to the lack of Smac/DIABLO, as theaddition of recombinant Smac/DIABLO to thecell lysates restored procaspase-3 cleavage.In contrast to the cell lysate results, however,the procaspase-3 processing was unchangedin Smac/DIABLO-deficient MEFs followingUV-irradiation.

The in vivo results obtained from theSmac/DIABLO-deleted mice contrast withprevious biochemical evidence that

Smac/DIABLO is crucially involved in theregulation of IAPs, and hence apoptosis.The lack of phenotype in these micesuggests that there are redundant orcompensatory molecules (such as otherIAP-binding proteins) and pathways. Theidentification of these pathways will nodoubt further our understanding of theregulation of apoptosis. On a cautionarynote, this paper illustrates that the

biochemical data (such as the cell lysateexperiments) are not necessarily a truereflection of in vivo biological mechanisms.

1 Okada, H. et al. (2002) Generation andcharacterization of Smac/DIABLO-deficientmice. Mol. Cell. Biol. 22, 3509–3517

Martin Holcik

martin@mgcheo.med.uottawa.ca

How to keep duplicate genesGene duplication has played a vital role inthe rise and diversification of the vertebratelineage, and a recent study in zebrafish haspointed to a likely mechanism for retainingthese duplicates in the genome. VictoriaPrince and her colleagues looked at thefunction and regulation of two forms of thezebrafish Hoxb1 gene: Hoxb1a and Hoxb1b.These genes arose from the fourfoldduplication of an ancestral Hox gene cluster.Many of the duplicated Hox genes inzebrafish have been lost altogether, or havebecome pseudogenes, owing to theaccumulation of mutations in one of theredundant copies. However, both copies ofHoxb1 have survived. The authors suggestthat this is due to the early loss of differentregulatory elements in the Hoxb1a andHoxb1b enhancers, resulting in partiallycomplementary expression patterns.Hence, as their functions are no longerwholly redundant, both genes have been‘protected’ from random genetic drift.(McClintock, J.M. et al. [2002] Development129, 2339–2354) RM

Sequencing genomes – fastand cheapA scientific team led by Andrew Patersonfrom the University of Georgia has developeda new method of genome sequencing bycombining modern cloning technologies andold-fashioned Cot analysis. Cot analysis isan old nucleic acid hybridization techniquethat was used for separating genomic DNAthat is repetitive from sequences that arenon-repetitive or unique. The authors of thisstudy used this method to create genomiclibraries from Sorghum bicolor. Thegenomic DNA was first fractionated byhydroxyapatite chromatography into highlyrepetitive, moderately repetitive andsingle/low copy sequence components andcloned into plasmids to make genomiclibraries. Filter hybridization and sequence

analysis confirmed that the libraries derivedfrom the highly repetitive components hada high copy number and those derived fromnon-repetitive sequences had a low copynumber and represented true genes. Thus,this method can be used for selectiveisolation and sequencing of the regions ofthe genome that are gene-rich. The authorsestimate that the number of clones thatgenerally need to be sequenced using thistechnique will be only about a tenth thatrequired by shotgun sequencing, resultingin significant savings for large sequencingprojects. (Peterson, D.G. et al. [2002]Genome Res. 12, 795–807) AP

Hidden genetic variationA recent study about environmental stress inplants could provide an alternative to geneticmodification of crops. Researchers haveplaced the actions of a particular molecule,the heat stress protein Hsp90, at the interfaceof environment and genetics, and potentiallyprovide an explanation for a long-standingevolutionary puzzle: how do multiple geneticchanges requiring the synchronous alterationof several features occur during evolution?The scientists (Whitehead Institute inCambridge, MA, USA) knew that Hsp90 helpsother proteins to fold properly. Now, they findthat it acts like a buffer for naturally occurringsubtle mutations, allowing them toaccumulate without visible effect. Then whenstress compromises the ability of Hsp90 to fold its critical target proteins, thesevariations are revealed – in other words, theylie dormant until a stress event, and then areexposed all at once. Most of these changeswill be bad. But a few unusual combinationscould produce valuable new traits.

This mechanism might be harnessed tocreate better crop plants by conventionalbreeding methods, without the need fortransgenic plants, said group leaderSusan Lindquist, senior author of the study.Manipulating the Hsp90 buffer couldidentify useful naturally occurring variants,

such as plants that grow better in poor soilor have increased pest resistance. “Our datasuggests that there may be a wealth ofgenetic variation out there right now that wecan’t see because the plants haven’t beengrown in the right conditions,” Lindquistexplained. (Queitsch, C. et al. [2002] NatureDOI:10.1038/nature749) PL

Sox and gutsThe functions of some genes involved inearly development are highly conservedthroughout the vertebrate lineage, and arecent study has revealed that the HMG/Sryclass transcription factor Sox17 is aparticularly notable example of this. Sox17 isrequired for the specification of endodermalcells in both the frog and zebrafish, but untilrecently a similar function had not beenshown in mammals. Now Yoshihiro Hayshiand his colleagues found that thedevelopment of the endodermal lineage isseverely perturbed in Sox17 –/– mice.Initially, the main endodermal derivatives,such as the gut, develop normally. Thingsstart to go wrong at the late neural platestage though, when the foregut is lost dueto apoptosis and the rest of the gut fails toexpand. These specific endodermal defectsindicate that the function of Sox17 hasindeed been highly conserved in thevertebrates. (Kanai-Azuma, M. et al. [2002]Development 129, 2367–237) RM

Unto the third generationWhat are the true genetic risks of radiation?If exposed parents acquire germlinegenomic instability, their offspring couldhave higher mutation rates. Barber et al.present the first experimental evidence for such a transgenerational effect inmammals. They exposed three inbred linesof mice to either fission neutrons (highenergy) or X-rays (low energy). In bothtreatments, the two subsequentgenerations of mice had increased germline

TRENDS in Genetics Vol.18 No.7 July 2002

http://tig.trends.com 0168-9525/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved.

346 News&Comment

In Brief

numbers of HSCs in vitro fortransplantation, starting with only a verysmall progenitor population. They alsoraise a number of new questions about the role of Hoxb4 in haematopoiesis. Inparticular, future studies might address the issue of whether Hoxb4 is required

for in vivo HSC determination andexpansion, and how it interacts with other regulatory genes involved in thehaematopoietic pathway.

1 Antonchuk, J. et al. (2002) HOXB4-inducedexpansion of adult hematopoietic stem cellsex vivo. Cell 109, 39–45

2 Kyba, M. et al. (2002) HoxB4 confers definitivelymphoid-myeloid engraftment potential onembryonic stem cell and yolk sachematopoietic progenitors. Cell 109, 29–37

Richard Morgan

rmorgan@sghms.ac.uk