Arbuscular mycorrhiza symbiosis in viticulture: a review · 2017-08-28 · REVIEWARTICLE Arbuscular...
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Arbuscular mycorrhiza symbiosis in viticulture: a review
Sophie Trouvelot1 & Laurent Bonneau1 & Dirk Redecker1 & Diederik van Tuinen2 &Marielle Adrian1 & Daniel Wipf1
Accepted: 17 July 2015 /Published online: 1 September 2015# INRA and Springer-Verlag France 2015
Abstract Viticulture is a major worldwide economic sectorwith a vine area of 7.52 million ha, wine production of288 Mhl, and wine exports of 26 billion euros. Nevertheless,viticulture has to adapt to new challenges of pest management,such as pesticide reduction, and climate change, such as in-creasing droughts. Viticulture adaptation can benefit fromarbuscular mycorrhiza, a plant–fungus symbiosis. Here, wereview the ecosystemic services of arbuscular mycorrhizafor grapevine production. The major points are the following:(1) arbuscular mycorrhiza fungi increase grapevine growthand nutrition by a better access to soil nutrients and by acti-vating the regulation of plant transport proteins for phospho-rus (P), nitrogen (N), and other elements. (2) Arbuscular my-corrhiza fungi increase the tolerance to abiotic stresses such aswater stress, soil salinity, iron chlorosis, and heavy metal tox-icity. (3) Arbuscular mycorrhiza fungi protect against bioticstresses such as root diseases. (4) Arbuscular mycorrhiza fungiproduce glycoproteins and a dense hyphal network that in-creases soil stability and save soil nutrients up to 14 % ofthe grape production income. (5) P fertilisation reducesmycorhization. (6) Using herbaceous plants as cover cropsfavors arbuscular mycorrhiza fungi.
Keywords Arbuscular mycorrhiza . Grapevine . Vineyard
Contents1. Introduction2. Potential importance of mycorrhizal diversity3. Enhancement of growth and nutrition
3.1 AM symbiosis reduce the need of fertilizers3.2 AM symbiosis enhance P uptake3.3 AM symbiosis enhance N uptake3.4 Impact of AM symbiosis on the uptake of other
nutrients3.5 How does grapevine cope with AM symbiosis car-
bon cost?4. Increase of tolerance to abiotic stresses
4.1 AM symbiosis alleviate grapevine water stress4.2 AM symbiosis help grapevine to cope with calcar-
eous soil and iron deficiency4.3 AM symbiosis increase grapevine tolerance to soil
salinity4.4 AM symbiosis increase grapevine tolerance to
heavy metals5. Protection against biotic stresses6. Increase of soil stability7. Impact of vineyard agricultural practices on grapevinemycorrhization7.1 Impact of soil management practices7.2 Impact of high fertilizer inputs7.3 Impact of weed / cover crops7.4 Impact of pest management practices
8. Conclusion9. References
Sophie Trouvelot and Laurent Bonneau contributed equally to the work.
* Daniel [email protected]
1 Université de Bourgogne, UMR1347 Agroécologie, BP 86510,F-21000 Dijon, France
2 INRA, UMR1347 Agroécologie, BP 86510, F-21000 Dijon, France
Agron. Sustain. Dev. (2015) 35:1449–1467DOI 10.1007/s13593-015-0329-7
Grapevine is a perennial crop grown in various areas aroundthe world. It is highly responsive to local environmental con-ditions and viticultural practices, hence leading to highly spe-cific wines. This feature gradually brought about the « terroir »concept, officially defined by the International Organizationof Vine and Wine (OIV) as “a concept which refers to an areain which collective knowledge of the interactions between theidentifiable physical and biological environments and appliedvitivinicultural practices develops, providing distinctive char-acteristics for the products originating from this area” (Reso-lution OIV/Viti 333/2010). Climate and soil characteristicsassociated to terroir have now been largely studied for a longtime. In comparison, interest for the « microbial terroir » ofvines (Gilbert et al. 2014) is recent and remains lessinvestigated. Valero et al. (2007) and Setati et al. (2012) wereamong the first to study the yeast microbiome in vineyards.More recently, on the basis of the analysis of the wine–grapesurface microbiome, Bokulich et al. (2014) proposed the ex-istence of a non-random microbial terroir as a determiningfactor of regional variation among vines. The soil microbiomeis undoubtedly also of importance but still remains lessstudied.
Among soil microorganisms, arbuscular mycorrhizal fungi(AMF) are able to establish symbiotic associations with vineroots (Fig. 1). Arbuscular mycorrhizal symbiosis is probablythe most widespread beneficial interaction between plants andmicroorganisms (Smith and Read 2008; Parniske 2008). Thisassociation between plant and fungi from the Glomeromycotaphylum dates back to the first appearance of land plants (about400 million years ago) (Redecker 2000). Nowadays, the vastmajority (80 %) of land plant species form AM symbiosis,
living in a wide range of terrestrial environments (Wang andQiu 2006). This mutualistic interaction is based on biotrophicnutrient exchanges between the plant and the fungal partner:The host plant supplies the biotrophic partner with carbon (C)while the AMF enhances the ability of the plant to get waterand nutrients from the soil (Smith and Read 2008). Nutrientsare mainly transferred to the host at the biotrophic interface incortical cells within which complex fungal structures, namelyarbuscules, are developed (Fig. 2). As they constitute a directsoil–vine link, investigating their occurrence and impact invineyard conditions will allow the assessment of their contri-bution to the « terroir » that is undoubtedly of particular highimportance in viticulture.
This is nevertheless complex regarding the high diversityof vine material compared to other cropping/agricultural sys-tems (rootstocks, cultivars, and clones), pedoclimatic
Fig. 1 Belowground, vine rootscan form, with soil fungi, asymbiotic association (thearbuscular mycorrhiza) withmutual benefits. Fine fungalhyphae greatly increase thevolume of soil explored, giving soa better access to soil nutrientsand water. (Drawing by Lacaralleand OhPonyBoy)
Fig. 2 Trypan blue stained root cell of Vitis vinifera, showing anarbuscule, intracellular specialized structure formed by branching offungal hyphae that invaginate during penetration of cortical root cells.The arbuscule is the main site of reciprocal nutrient exchanges betweenthe two partners
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conditions, and viticultural practices. Additionally, there isincreasing interest from winegrowers, technical institutes,and the scientific community for a better knowledge of thepossible ecosystemic services AMF could provide with re-spect to adaptation to climate evolution and development to-wards sustainable viticulture. Obtaining an overview of theoccurrence, functioning and benefits of AFM in the vineyardtherefore still remain a true issue.
This paper thus presents the state-of-the-art knowl-edge about AMF, their impact on grapevine, and theinfluence of viticultural practices on these populations.It does not aim at presenting a complete review butrather highlighting the main aspects of the thematic,including recent papers.
2 Potential importance of mycorrhizal diversity
Biodiversity has long been recognized as one of the elementsof functional ecosystems (Balvanera et al. 2006; Hector andBagchi 2007), but most studies targeted aboveground organ-isms (van der Heijden et al. 1998). Recent studies show thatkey ecosystem processes are affected by a loss in soil biodi-versity (Wagg et al. 2014) and that land usage has a greatimpact on it (de Vries et al. 2013). AMF are an importantfunctional group in soil ecosystems, and a reduction of theirbiodiversity will impact plant functionality and more general-ly global ecosystems (Jeffries et al. 2003). All the fungi able toestablish an arbuscular mycorrhizal symbiosis are grouped inthe Glomeromycota phylum (Schüßler et al. 2001), but rootcolonization strategies differ among the different families. Forexample, fungi from the Glomerales family primarily colonize
root systems starting from a hyphal fragment, while root col-onization by Diversisporales starts from spores (Hart andReader 2002a). At the family level, this implies that fungifrom the Diversisporales are slower colonizers than membersof the Glomerales (Hart and Reader 2002b). Differences ingrowth strategies between these fungi imply that soil manage-ment can greatly impact the diversity of AMF and as a conse-quence their ecosystemic services (Fig. 3). Indeed, in the vine-yard, AMF communities are highly influenced by the soilcharacteristics but also to a smaller extent by the host plantdevelopment stage (Schreiner and Mihara 2009; Balestriniet al. 2010). Moreover, although AMF from grape roots main-ly belong to the Glomerales group, members of theDiversisporales are mainly found in sandy vineyard soils(Balestrini et al. 2010). As a consequence, this biologicalfunctional community has to be better taken into account(Jeffries et al. 2003).
Although AMF are not host specific, their effect on plantscan vary among species and isolates. When grown in the pres-ence of heavy metals, Glomus versiforme improved mineralnutrient uptake of Sorghum bicolor and Zea mays to a greaterextent than Glomus mosseae (Guo et al. 2013). On the otherhand, G. mosseae was more efficient in bioprotecting tomato(Pozo et al. 1999) and petunia (Hayek et al. 2012) plants thanGlomus intraradices. As discussed above, AMF develop anextensivemycelium in the soil, thereby extending the nutrient-prospecting zone of the plant. The extent of the myceliumvaries among species and also among isolates within a samespecies (Munkvold et al. 2004; Avio et al. 2006), implyingthat the beneficial effect of this fungal network against soilerosion or for improving plant nutrition varies depending onfungal diversity.
Fig. 3 Ecosystem services provided by arbuscular mycorrhiza
Arbuscular mycorrhiza symbiosis in viticulture 1451
The beneficial effect on plant nutrient content has also beenshown to be dependent on fungal diversity for various cropssuch as sweet potato (Farmer et al. 2007) or pepper plant(Gogoi and Singh 2011). When a mycorrhizal inoculum isused, the positive effect of mycorrhizal fungi on plant growthstrongly depends on its composition, and a combination ofmycorrhizal fungi is more effective than a monospecific inoc-ulum (Gustafson and Casper 2005; Jansa et al. 2008; Sharmaet al. 2009; Gogoi and Singh 2011). For example, in Vitisvinifera, the use of a mixed inoculum was more efficient thana monospecific inoculum regarding increases in leaf numberor in leaf surface (Krishna et al. 2005). Inoculation with AMFeven increased total phenolic components in leaves by up to800 % (Krishna et al. 2005).
The question as to which AMF are associated with grape-vine has been addressed in several studies, initially using sur-veys based on spore morphology (e.g., Schubert and Cravero1985; Oehl et al. 2005). These studies highlighted the domi-nance of species of the former genus Glomus s.l., now classi-fied in genera such as Glomus s.str., Rhizophagus,Funneliformis, Claroideoglomus, and Paraglomus.
Recent studies have used DNA-based detection andidentification methods to analyze AMF communities incolonized roots. Schreiner and Mihara (2009) mainly detectedsequences of phylotypes assigned to the Glomeraceae familyin a set of ten vineyards in Oregon (USA). Among them wasonly one known species, nowadays classified as Rhizophagusirregularis, as well as very few Claroideoglomeraceae andGigasporaceae sequences. The family Acaulosporaceae wasnot found in roots, although species were present as spores
and their ability to colonize grapevine roots was demonstratedin a separate experiment using sterilized soil. The majority ofgrapevine roots across vineyards were dominated by the samethree or four phylotypes (out of a total of 6–11), with subtledifferences across different soil types and vineyard ages.However, no seasonal effect was found. In addition,Paraglomeraceae and Archaeosporaceaewere detected usingprimers specifically targeted to these families. Interestingly,there was a strong discrepancy between these results and thespecies detected in the same sites. Scutellospora calosporawas the most frequently encountered species based on spores(Fig. 4), but it was rarely found in roots. Funneliformismosseae, although abundant as spores, was not detected atall within roots.
These findings are largely in agreement with a study byBalestrini et al. (2010) conducted in two vineyards in Pied-mont (Italy). Glomeraceae equally dominated the AMF com-munities detected by these authors in roots using DNAmarkers, whereas Acaulosporaceae were completely absent.Some phylotypes were in common with the Oregon study,such as certain Glomeraceae, and rare S. calospora. Commu-nities in the two different sites differed strongly, which wereattributed to the different soils. Again, spore and sequence-based approaches were in strong disagreement.
Lumini et al. (2010) used a pyrosequencing approach invineyards in Sardinia to characterize local AMF communities.This method can be expected to deliver a much more exhaus-tive analysis of AMF diversity. These authors defined opera-tional taxonomic units based on their sequences but limitedtheir conclusions to Glomeromycotan orders. Overall, the
Fig. 4 Spores of different speciesof arbuscular mycorrhizal fungi(Picture by Louis Mercy)
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order Glomerales was again dominant. Two differently man-aged types of vineyards, analyzed as part of a land-use gradi-ent, harbored slightly different levels of AMF diversity. Usinga similar sequencing technology, Holland et al. (2014) ad-dressed the question whether communities in grapevine rootswere different from those detected in the accompanying inter-row vegetation in vineyards in British Columbia (Canada).Although both habitats were similar in alpha diversity andqualitative composition of AMF communities, significant dif-ferences were found in terms of sequence abundance aftercasting aside possible effects of soil chemistry. The authorsinterpreted these findings as an effect of host identity andconcluded that the presence of inter-row vegetation in vine-yard had an effect on the overall vineyard’s AMF community.However, we should keep in mind that all inter-row plantswere herbaceous and thus strongly differed in root morpholo-gy and lignin content from the woody roots of vine. The taxaidentified in this study belonged to the genera Rhizophagus,Funneliformis, and Claroideoglomus, together with many un-identified Glomeraceae. Notably, soil chemistry did not havea significant influence in this study.
In summary, there is currently not much conclusive evi-dence to set AMF communities in vineyards apart from com-munities in similar habitats. A dominance of Glomeraceae incolonized roots has also been demonstrated for many otherecosystems. Differing molecular marker systems and sam-pling efforts make it difficult to compare the different studiesto obtain a general picture of the diversity of AMF communi-ties in vineyards. Table 1 summarizes the taxonomy of theAMF species cited in this review at the time of these publica-tions and the present consensus taxonomy. To avoid confu-sion, in the text, we have used the classification at the time of
publication except in cases where DNA sequences allow un-equivocal assignment to currently recognized taxa.
3 Enhancement of growth and nutrition
3.1 AM symbiosis reduce the need of fertilizers
Vine nutrition is essential as it determines vine functioningand wine quality. Fertilization is complex and highly depen-dent on plant material, soil characteristics, and the type ofproduction (table grapes, vines, etc.). Most of the vineyardsoils are poor in nutrients, and annual exports (pruned canesand clusters) have to be considered for that perennial cropplanted for several decades.
It is well established that AM colonization occurs mainlyunder low levels of either plant nutrient status or low levels ofsoil fertility (mainly P) (Smith and Read 2008). Indeed, ingrapevine, high levels of soil fertility (N or P) as well as ahigh plant nutrient status reduce arbuscular mycorrhiza colo-nization (reviewed by Schreiner 2005b). Moreover, plants’dependency on arbuscular mycorrhiza for nutrients could de-pend on root architecture and mainly root hairs, according tothe Baylis hypothesis developped for magnaloid (Baylis1975). Grapevine root architecture was described as havinga low density and large diameter fine roots (Richard 1983;Smart et al. 2006), albeit depending on rootstock and culturalpractices (Smart et al. 2006; Southey and Archer 2015). Fol-lowing Baylis hypothesis (Baylis 1975), AM could thereforebe highly beneficial by extending the volume of the exploredsoil allowing an adequate uptake of water and nutrients suchas phosphate (Schreiner 2005b). Once the plants colonized,nutrients can be absorbed via two pathways that could operateindependently, the root uptake pathway, and the arbuscularmycorrhiza pathway involving the extraradicular fungal my-celium. Fungal mycelium increases the exploitable soil vol-ume and since individual hyphae have diameters comparableto root hairs, they allow access to soil pores that would havebeen otherwise unexplorable (Smith and Read 2008).
3.2 AM symbiosis enhances P uptake
Grapevine phosphorus deficiency is usually rarely observed,not only mainly because of limited phosphorus requirement,but also because of sufficient phosphorus richness in the ma-jority of vineyard soils and remobilization from bark, wood,and roots during periods of high P demand (Jackson, 2014).Nevertheless, P deficiencies have been described in vineyardsin Australia (Tulloch and Harris 1970), France (Champagnol1978), Germany (Gärtel 1965), and USA (Cook et al. 1983).Phosphorus deficiency symptoms correspond to stunted shootgrowth, decrease in dry matter, and berry clusters (Skinner andMatthews 1989). Conversely, it cannot be concluded that high
Table 1 Glomeromycota taxonomic name correspondences betweenpresent and previous classification
Arbuscular mycorrhizal fungiclassification at the time of publications
Gigaspora margarita Unchanged
Glomus constrictum Septoglomus constrictum
Glomus deserticola Septoglomus deserticola
Glomus etunicatum Claroideoglomus etunicatum
Glomus intraradicesb Rhizophagus intraradices,Rhizophagus irregularis
Glomus mosseae Funneliformis mosseae
Glomus versiforme Diversispora epigaea
Scutellospora calospora Unchanged
a Following classification by Redecker et al (2013)b In the majority of cases, the name G. intraradices was in the pastmisapplied to a species nowadays known as R. irregularis, and not toR. intraradices which has a very limited distribution (Stockinger et al.2009).
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soil phosphorus concentrations lead to a better wine quality(Reynolds 2010). P deficiency mainly occurs in acidic soilsthat are favorable for fixing phosphorus as well as in higherrainfall areas or following fumigation of low phosphorus soils(Menge et al. 1983). In such acidic and/or low phosphorussoils, applying fertilizers containing phosphorus is needed toavoid deficiency symptoms (Conradie 1988). It is wellestablished that AMF can make a significant contribution toP uptake (Smith et al. 2011). The main form of P absorbed byplants is the soluble Pi, (HPO4
2−), resulting from the slowbreakdown of organic forms transformed by soil microorgan-isms. Pi availability is generally low because of its very slowdiffusion through soil solution. Indeed, availability can bereduced by fixation in soils with high levels of calcium orby bounding to organic matter or soil particles. Moreover,once absorbed by roots, a depletion zone might appear aroundthem, thus impairing plant nutrition and growth (Nye andTinker 1977). In mycorrhizal roots, the extraradical myceliumforms an extensive network beyond the root depletion zone,thus shortening the distance for the diffusion of Pi in soil(Harrison 1999). Indeed, an increase in P levels in either rootsor leaves was observed in almost all studied grapevines in thepresence of AMF (for review, see Schreiner 2005a; Khalil2013). It must be kept in mind that although P uptake doesnot depend on soil pH or texture, AM colonization of grape-vine, and in turn P uptake, could be reduced for pH valuesranging from 5 to 5.5 (reviewed by Schreiner 2005b). More-over, it has been reported that total P levels in grapevine coulddepend on the AMF species (Biricolti et al. 1997) or isolates(Biricolti et al. 1997; Mortimer et al. 2004; Schreiner, 2005b),but the growth effect cannot be systematically linked to anincrease in P concentrations, suggesting that plant growthstimulation is not linked with a greater absorption. Since ithas been reported that cytokinin-like compounds are producedby G. mosseae (Barea and Azcon-Aguilar 1982), it can behypothesized that changes in the hormonal balance followingarbuscular mycorrhiza inoculation could control root growthand the root:shoot ratio (Fusconi 2014). However, the involve-ment of AMF in the hormonal regulation of grape develop-ment still has to be investigated.
Although not yet reported for grapevine, it is wellestablished that arbuscular mycorrhiza colonization leads tothe specific induction of Pi transporters in root cortical cells(Javot et al. 2007). The arbuscular mycorrhiza pathway canplay a major role in P uptake by arbuscular mycorrhiza-colonized plants as all of the P were delivered via the mycor-rhizal pathway whenG. intraradiceswas the fungal symbiontof three different plant partners (Smith 2003), indicating a lossof function of the direct uptake pathway in roots colonized byAMF. Themechanism of such a shift in arbuscular mycorrhizaroots is still unclear. Interestingly, a proteome analysis ofgrapevine SO4 rootstock upon colonization by two separatefungal species revealed that AMF colonization results in a
more drastic reprogramming of host genes than in herbaceousplants (Cangahuala-Inocente et al. 2011). Indeed, several pro-teins involved in plant adaptation to P deficiency, and previ-ously reported as induced upon P starvation were downregu-lated, strongly suggesting that grapevine reprioritizes its Piuptake strategies to take advantage of P transfer via the fungalpartner (Cangahuala-Inocente et al. 2011).
3.3 AM symbiosis enhance N uptake
Although nitrogen input is usually minimized to limit thegrowth of grapevine aerial parts and susceptibility to diseases,N capture from the soil is crucial for vine metabolism andbuilding up of wood reserves (Wermelinger et al. 1991). Fur-thermore, among soil elements picked up by the roots, nitro-gen is the one that influences the most both vine developmentand grape composition (Reynolds, 2010). For example, lownitrogen availability is not favorable to white wine(sauvignon) quality (Choné et al. 2006). Adequate N suppliesare usually needed to allow rapid development of young re-plant grapevines and to ensure rapid shoot growth in thespring. Since nitrogen uptake depends on soil type and organicmatter turnover and thus can be considered as a terroir char-acteristic, a better understanding of N uptake would thereforehelp optimizing N supply and limiting the use of N fertilizers.Within this frame, it is now established that arbuscular mycor-rhiza can also play a major role in N uptake (Smith et al.2010), although the role of N in AM symbiosis is less clearthan that of P. AM fungi are able to take up both NO3
+, as well as organic N. However, a preference for NH4+
has been reported since NH4+ supply reduces NO3
− uptake(Johansen et al. 1992; Johansen et al. 1996; Toussaint et al.2004; Gachomo et al. 2009). This can partly be explained bythe energy cost needed for reducing NO3
− to NH4+ before its
incorporation into organic compounds (Marzluf 1997). In-deed, mycorrhizal plants may display better growth with Nsupplied as NO3
−. In grapevine, mycorrhizal N uptake fromthe soil can also enhance plant biomass (Karagiannidis et al.2007). The form of N fertilization can influence the dry weightof mycorrhized vines. Indeed, mycorrhized vines had a greaterdry weight with NO3
− (the best N source being Ca(NO3)2)compared to other N forms, while urea significantly reducedboth AMF root colonization and sporulation. Interestingly,grapevine mycorrhizal dependency was the same whateverthe N source (Karagiannidis et al. 2007).
Besides inorganic N uptake, it is well established that AMFcan draw substantial amounts of N from decomposing organicmaterials (Hodge and Fitter 2010). Organic nitrogen repre-sents a large proportion of total soil N. Indeed, in organicvineyards, incorporation of legume crop residue is used as aprimary mean of modifying soil fertility (Cheng et al. 2008).In the absence of N fertilization only, total N uptake from theresidue was significantly enhanced by the fungus: A
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significant increase in the percentage of N derived from theresidue was noted in grapevine leaves in response to hyphalproliferation (Cheng et al. 2008). However, although the abil-ity of AMF to take up organic N in the form of arginine hasbeen recently reported (Fellbaum et al. 2012), direct uptake oforganic N might not be the main N uptake pathway, and or-ganic N could be mineralized before it is absorbed by hyphae(Hawkins et al. 2000; Smith et al. 2011).
3.4 Impact of AM symbiosis on the uptake of othernutrients
Among the other chemical elements essential for grapevinegrowth, potassium (K), magnesium (Mg), zinc (Zn), and bo-ron (B) are important for fertilization. Potassium deficiencyoccurs in high calcium (Ca) or Mg containing soils, as well asin high rainfall regions. Low-containing K vines are moredrought-prone and cold-sensitive (Reynolds 2010). Zinc defi-ciency, due to reduced solubility in alkaline soils, leads to poorfruit set (Reynolds 2010). B deficiency occurs in sandy soilswhere excessive leaching occurs following rainfall or irriga-tion (Jackson 2014). Other chemical elements, iron (Fe), man-ganese (Mn), and sulfur (S), although highly necessary forharmonious growth and development, are rarely deficient invineyard soils (Christensen 2000). However, certain culturalpractices or soil properties can impair nutrient uptake. Indeed,it has been reported that nitrogen fertilization depressed theuptake of some elements such as B and Mn. Ca deficiencysymptoms are uncommon but can occur in strongly acidicsoils, below pH 4, while Fe deficiency symptoms (lime-induced chlorosis) are normally found in calcareous soils forsensitive rootstocks (Jackson 2014).
Data relying on the role of AMF in the uptake of other soilnutrients by grapevine are scarce. Macro- and micro-nutrientuptake could depend partly not only on the fungal partner, butalso on the host plant. In studies on the interaction betweenarbuscular mycorrhiza infection and soil lime content,arbuscular mycorrhiza inoculation of Kober 5BB rooted cut-tings with three different Glomus species (G. constrictum,G. deserticola, and G. mosseae) resulted in an increase in leafcopper (Cu) concentrations and in a decrease in B,Mn, and Znconcentrations, Mn and Zn depending on the fungus (Biricoltiet al. 1997). In another set of experiments, Ca and K concen-trations increased in mycorrhizal Victoria grapevine graftedonto rootstocks 3309C or 110R (Nikolaou et al. 2002). Simi-larly, inoculation of different grapevine rootstocks with thefungus Gigaspora margarita showed that inoculation tendedto increase K concentrations in all rootstocks but decreased Caand Mg concentrations in tetraploid rootstocks only. Surpris-ingly, the place of the inoculumwas reported to modulate bothnutrient uptake and grapevine growth (Petgen et al. 1998).Indeed, leaf blade Zn and P concentrations and the shoot dryweight significantly increased when the inoculum was placed
in the top soil while only Zn and Cu concentrations increasedwhen the inoculumwas placed in 36–45 cm soil depth (Petgenet al. 1998). This suggests that colonization levels would be adetermining factor in modulating AM-induced nutrient uptakeand plant growth. It has also been reported that the content ofmost nutrients in grapevine cuttings (Pinot noir) was signifi-cantly increased depending on the soil type (Schreiner,2005b). Indeed, P, K, Ca, Mg, Fe, and B contents were in-creased only in a low P soil. This effect would be linked to theprimary stimulation of P uptake and resulting growth stimu-lation. However, since accumulation of S, Zn, and Cu alsooccurred in high P soil, uptake mechanisms independent ofP-induced growth effects might exist.
Interestingly, when two G. mosseae isolates from two dif-ferent soils were used to inoculate grapevine cuttings, only thenative one from the high P soil enhanced Cu and S uptake(Schreiner, 2005a). These results suggest that the selection ofappropriate inocula for specific functions would be the beststrategy for inocula producers.
Although AM-induced increases in chemical elementscould be linked to the fungus hyphae, decreases in ionic con-centrations would involve several mechanisms. Decreasedionic concentrations could result from a dilution effect follow-ing increased growth in response to P uptake (Smith and Read2008). However, since the concentrations of some ions in-creased following arbuscular mycorrhiza inoculation in someof the experiments described above, the “ion decrease” effectis unlikely to occur and other mechanisms probably operate.Indeed, experiments with maize showed that AMF could af-fect the populations of Fe- and Mn-reducing microorganismsin the rhizosphere. As a result, Mn-reducing potential or theamount of reduced Mn in the soil, and hence Mn availabilityfor plants, decreased (Kothari et al. 1991). Evidence for suchinteractions between microorganisms regarding nutrient up-take in the grapevine mycorrhizosphere is currently missing.Interestingly, inoculation of grapevine with G. mosseae inreplant soil significantly stimulated the growth of vegetativeaerial parts, while the number of fluorescent pseudomonadsdropped (Waschkies et al. 1994). However, no changes in leafmineral concentrations (P, K, Ca, Fe, or Mn) were observedfollowing arbuscular mycorrhiza inoculation, suggesting thatplant growth stimulation mainly resulted from the arbuscularmycorrhiza-induced reduction of pseudomonads present inreplant soil, rather than in nutrition enhancement (see in themycorrhiza-induced resistance part).
3.5 How does grapevine cope with AM symbiosis carboncost?
The C costs of the fungal partner can be considerable inarbuscular mycorrhiza symbiosis since the fungus can receive4 to 20 % of the plant’s photosynthetically fixed C (Smith andRead 2008). Grapevine, as a deciduous plant, mainly relies on
Arbuscular mycorrhiza symbiosis in viticulture 1455
stored C for growth of new tissues, mainly in spring (Buttrose1966). Since AMF growth and nutrient acquisition also relyon host plant reserves, C reserve mobilization and plant de-velopment could be significantly influenced by the AMF(reviewed by Smith et al. 2010). Therefore, when arbuscularmycorrhiza costs exceed the nutritional benefit to the host, anegative growth response can occur (Smith et al. 2010). Ingrapevine, a detailed analysis revealed that a negative growthresponse might occur during the first 2 months of colonizationwith Glomus etunicatum (Mortimer et al. 2005). The C takenup by the AMF was devoted to the growth and developmentof new fungal structures and spores. This C drain during rapidroot colonization had a negative impact on host stem growthbefore colonization was maximum. Once the plateau ofarbuscular mycorrhiza development was reached, C drainfrom stem tissues stopped, and then the stem reserves wererefilled. Once symbiosis is functional, more C can be used fornutrient acquisition (Mortimer et al. 2005; Gilbert et al. 2014).Such a negative response following the first 2 months ofarbuscular mycorrhiza colonization is thought to be transientin grapevine as a significant increase in growth was recorded112 days after inoculation of seven rootstocks and twoungrafted cultivars with four arbuscular mycorrhizal fungi(Lindermann and Davis 2001). Growth increase varied withindividual rootstocks and cultivars and ranged between 64 and220 %. The largest growth increase was observed withG. intraradices.
Rapid growth response following inoculation could alsodepend on the fungus. Shoot biomass was enhanced after5 weeks in grapevines inoculated with either G. intraradicesor G. mosseae (Cangahuala-Inocente et al. 2011). Beyondnutrient uptake stimulation, arbuscular mycorrhiza coloniza-tion most probably increases photosynthetic rates in grapevine(Mortimer et al. 2005) as in other plant systems such asTrifolium (Wright et al. 1998; Schreiner 2005b; Smith andRead 2008). Unfortunately, data relying on arbuscular mycor-rhiza and photosynthetic activity in grapevine are scarce(Bavaresco et al. 2003) and puzzling. Surprisingly, althoughvines exhibited greater shoot and root growth compared to thecontrol, leaf chlorophyll contents remained either reduced orunchanged by AMF, depending on the fungus (Biricolti et al.1997; Smith and Read 2008). However, arbuscular mycorrhi-za applications could increase plant soluble sugars contents(Cetin et al. 2014) that do not only act on the metabolic activ-ities of the host but also as a signal regulators in processesrelated plant growth and development (Jang and Sheen 1997;Loreti 2001). Moreover, AMF may induce changes in hor-monal balance, leading to an increase in cytokinin contentresponsible for greater vegetative development (Biricoltiet al. 1997; Harrison 1999).
Studies on the physiological effects of inoculation of grape-vines with AMF reveal that symbiosis enhances plant growthin almost all cases (Motosugi et al. 2002; Schreiner 2005b;
Ridgway et al. 2006; Karagiannidis et al. 2007; Cangahuala-Inocente et al. 2011). Nevertheless, the arbuscularmycorrhiza-dependent enhancement of grapevine growth asa whole, or of grapevine roots only, varies with rootstock andfungus (Lindermann and Davis 2001; Smith and Read 2008;Nogales et al. 2009; Cangahuala-Inocente et al. 2011). More-over, in field conditions, the physical, chemical, and biologi-cal composition of the replant soil (mycorrhizal propagules,pathogenic fungi) might be crucial in controlling the effects ofarbuscular mycorrhiza inoculation (reviewed by Schreiner2005b; Nogales et al. 2009). Although plant growth was en-hanced at the end of the first year after planting for two dif-ferent arbuscular mycorrhiza-inoculated rootstocks, no moredifferences were reported after the second year for one of therootstocks (Smith and Read 2008; Nogales et al. 2009). Plantscould achieve optimal growth after being colonized by nativefungal propagules and thus no longer benefit from artificialinoculation. Therefore, a preliminary study of the native inoc-ulum present in the replant soil is essential before consideringthe suitability of arbuscular mycorrhiza inoculation.
4 Increase of tolerance to abiotic stresses
Abiotic stresses cause extensive losses to agricultural produc-tivity. Grapevine is no exception to the rule and faces severalabiotic stresses throughout its lifespan. Drought, salinity, orheavy metals are serious problems in many parts of the world.The potential of AMF to enhance plant tolerance to abioticstress conditions has long been known (Smith and Read2008), and their use in sustainable agricultural systems willbe of tremendous importance for soil quality and crop produc-tivity under severe edapho-climatic conditions (Lal 2009).
4.1 AM symbiosis alleviate grapevine water stress
Soil water status and plant water use play a key role in grape-vine growth and yield and in grape quality. However, sincevineyards are mainly located in areas of dry climate during thegrowing season, water deficit frequently occurs. Moreover,according to the global climate models (IPPC 2014), in thenear future, water deficit may become a limiting factor forgrapevine development and wine quality. Indeed, climatechange was already shown to influence on grape phenology(Jones and Davis 2000; Webb et al. 2007). Whereas moderatewater stress during maturation has a positive effect on grapequality (Chaves et al. 2010), prolonged water stress may havea strong negative effect on photosynthesis and yield. There-fore, in some vineyards, vines require additional irrigation inkey periods to limit water deficit stress. Vineyard managementstrategies thus play a determining role in the capability of soilsto retain nutrients and water. AMF are known to improvewater uptake by plants and nutrient uptake at the same time
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(Kohler et al. 2008; Adesemoye et al. 2008). An effectivedevelopment of external mycelium and/or arbuscules can becrucial in dry situations. In irrigated vineyards, regulated def-icit irrigation (RDI) is commonly applied in order to controlvegetative development and improve fruit quality. When anadditional water deficit was applied to RDI, the frequency ofarbuscules was greater than in the standard RDI plot althoughfine root production was reduced, indicating that grapevinescompensated for a lower density of fine roots by increasingarbuscular colonization (Schreiner et al. 2007). As RDI treat-ments did not have a significant effect on aboveground vinegrowth, yield, or fruit quality, it appeared possible to producehigh-quality wine grapes with less irrigation or in arid cli-mates. However, the mechanisms of improved water uptakeare as yet unclear, no arbuscular mycorrhiza-specific orarbuscular mycorrhiza-inducible aquaporin gene has beenidentified so far in microarray studies. However, althoughthere is no current evidence of direct water transfer to plantsvia arbuscular mycorrhiza fungal hyphae, increased water useefficiency in arbuscular mycorrhiza plants has been reportedin several studies including studies on grapevine.
In coarse-rooted plants such as grapevine, the beneficialeffect of arbuscular mycorrhiza symbiosis is crucial and reliesmore on arbuscular mycorrhiza symbiosis than for fine-rootedplants (Bolan 1991; Eissenstat 1992). For example, in a Hun-garian vineyard, Donkó et al. (2014) reported that the degreeof mycorrhizal colonization is higher in drier soil areas.Arbuscular mycorrhiza-colonized grapevine roots can exhibitmore efficient water uptake and allow grapevine to cope withwater stress. During drought stress, although arbuscular my-corrhiza colonization declined, the biomass and the prolinecontent were higher in arbuscular mycorrhiza plants than innon-arbuscular mycorrhiza plants (Valentine et al. 2006). Sur-prisingly, in these conditions, similar photosynthetic rateswere observed despite lower stomatal conductance andsubstomatal CO2 concentrations in arbuscular mycorrhizagrapevines, clearly indicating that under drought stress, wateruse efficiencywas higher in arbuscularmycorrhiza plants. Thephotosynthetic response of arbuscular mycorrhiza plantsmight be associated with higher Rubisco activity and electrontransport rates. In two woody European species (Oleaeuropaea and Rhamnus lycioides), the effects of arbuscularmycorrhiza inoculation on water use efficiency have beenreported to depend not only on the plant species but also onthe composition of the inoculum (Querejeta et al. 2006). Theseresults highlight a “functional diversity” in arbuscular mycor-rhiza symbioses that should be taken into account when opti-mizing arbuscular mycorrhiza colonization protocols, espe-cially in the field. Surprisingly, the significant increase of thewater flux in arbuscular mycorrhiza plants appears to be inde-pendent of changes in the nutrient status of the host plant(Read 1992; Sylvia and Williams 1992; Koide and Dickie2002). The same situation was reported for young transplanted
grapevines. In 1-year-old arbuscular mycorrhiza-colonizedgrapevines, the water flux (stomatal conductance, transpira-tion rate, and xylem conductance) and photosynthesis rateincreased compared to non-arbuscular mycorrhiza grapevines,although no increase in biomass or mineral nutrition was re-corded (van Rooyen et al. 2004). These results indicate thatarbuscular mycorrhiza inoculation can enhance the plant wa-ter relations and the photosynthesis performance oftransplanted grapevine rootstocks, thus potentially alleviatingthe transplanting shock and increasing potential survival. Thisimproved drought tolerance is of special importance fortransplanted plants in which root systems are less extendedand more superficial during the first years of growth.
Although AM can enhance water uptake, thus improvingdrought tolerance, the expected benefit of the symbiosiswould depend on the rootstock. It is established that whilemost roots are produced in spring when soil temperatures arefavorable, root development is rootstock-dependent sincesome do produce roots during the warm dry season, at deeperdepths, thus improving water foraging and transport (Smartet al. 2006). Such grapevines are likely less AM-dependent forwater uptake. In dry soils, the root extension rate is lowerbecause of reduced turgor and increased soil strength, as wellas reduced nutrient availability (Viets 1972). In these situa-tions, the development of extraradicular mycelium can be cru-cial for optimizing nutrient uptake. Improved drought toler-ance is closely linked to improved nutrition (especially P) ofarbuscular mycorrhiza plants. Arbuscular mycorrhiza plantsindeed grew better under drought stress than NMplants (Augé2001). In sorghum, regardless of the plant water status,arbuscular mycorrhiza roots were able to absorb P more effi-ciently from dry soil than NM roots (Neumann and George2004). Because hydraulic conductivity of root cortical cells islower under P deficiency (Radin and Eidenbock 1984;Andersen et al. 1988; Reinbott and Blevins 1999; Radin andMatthews 2004), maintenance of P supply by arbuscular my-corrhiza colonization may also play a role by increasing roothydraulic conductivity and facilitating water uptake.
4.2 AM symbiosis help grapevine to cope with calcareoussoil and iron deficiency
Iron is in relatively high abundance in the earth’s cultivatedsoils, but plant Fe acquisition is often impaired that resulted insevere crop losses. Among the soil properties which impair Fenutrition, calcium carbonate (CaCO3) plays a major role(Loeppert et al. 1994). High concentrations of bicarbonateions, called active carbonate or active lime (Drouineau1942), in calcareous soils reduce iron availability, with subse-quent chlorosis that can seriously impair grapevine yield andquality (Bavaresco et al. 2006). The best way to avoid chlo-rosis risks in these soils is to select lime-tolerant rootstocks(Tagliavini and Rombolà 2001). A factorial experiment aimed
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at determining the influence of AMF on the tolerance of therootstock Kober 5BB to lime-based soils. Interestingly, it re-vealed that mycorrhization always increased vegetativegrowth independently of the lime concentration compared tothe control, but did not reduce the grapevine susceptibility tochlorosis (Biricolti et al. 1997). The greater growth of mycor-rhizal plants was not linked to increased photosynthesis sincethe chlorophyll content was higher in control plants evenwhen the soil did not contain lime. Leaves of inoculated plantshad higher levels of visual chlorosis whatever the lime con-centration, except for one Glomus species (Glomusdeserticola).
In contrast, in susceptible rootstocks, arbuscular mycorrhi-za inoculation may be another way to alleviate chlorosissymptoms on calcareous soils by controlling the chlorophyllcontent (Bavaresco et al. 2006). Chlorophyll content and ironuptake increased in the lime-susceptible rootstock 101-14 in-oculated with G. mosseae and then grown alone or graftedwith Chardonnnay clone R8 (Bavaresco et al. 2010). Further-more, inoculation of the susceptible graft combinations Pinotblanc/ 101-14 or Pinot blanc/3009C also brought about in-creased chlorophyll and Fe contents and reduced chlorosissymptoms (Bavaresco et al. 2010). Inoculating grapevine withAM fungi to alleviate lime-induced chlorosis in vineyard is apromising technique that requires further studies.
4.3 AM symbiosis increase grapevine tolerance to soilsalinity
Soil salinity refers to the amount of dissolved salts in soils andis one of the major abiotic factors limiting viticulture produc-tivity, mainly in arid and semi-arid zones of the Mediterraneanbasin (Belew et al. 2010). The main effects of salinity are: (i)the development of an osmotic stress that greatly impairs planttranspiration and photosynthesis (Shannon and Grieve 1998)and (ii) the induction of changes in the mineral balance lead-ing to metabolic dysfunctions and altered physiological mech-anisms (Hasegawa et al. 2000), leading to limiting plantgrowth. Saline soil conditions occur naturally in somevineyards. It could be due to primary salinization that occursnaturally when the soil parent material is rich in soluble salts.Moreover, salt-affected soils often occur on irrigatedvineyards, especially in arid and semi-arid regions, due tothe insufficient leaching, improper irrigation, poor drainage,or irrigation water high in salts. For example, in Australia, themajor ions in irrigation water are sodium (Na+) and chloride(Cl−) (Walker et al. 2010). Therefore, viticultural practicesshould be adapted to minimize the uptake and accumulationof those problematic ions in grapevines. Grapevine, by com-parison with other crop types, is classified as moderately sen-sitive to salinity (Maas and Hoffman 1977). However, highsalinity considerably inhibits shoot growth and affects berryquality (Walker et al. 2002).
It has been well established that AMF occur naturally insaline environments (García and Mendoza 2007) and alleviatesalinity stress in many plant cultures such as banana, Viciafaba, tomato, olive, Sorghum, Lotus, and palm oil tree(Yano-Melo et al. 2003; Rabie 2005; Al-Karaki 2006;Bouamri et al. 2006; Cho et al. 2006; and Sannazzaro et al.2006; Porras-Soriano et al. 2009) by improving uptake ofnutrients such as P, N, Zn, Cu, and Fe. The improvement inthe plant P status is probably the most important source ofsalinity stress tolerance in arbuscular mycorrhiza-colonizedplants (Giri et al. 2003; Giri and Mukerji 2004), even if otherphysiological processes occur concomitantly (Feng et al.2002; Ruiz-Lozano 2003; Giri and Mukerji 2004; Giri et al.2007). Indeed, when AM-colonized grapevines were irrigatedwith saline solutions, AM was observed at all salt levels used,albeit decreasing with increasing NaCl concentrations (Khalil2013). Moreover, although salinity reduced grapevine growth,vegetative growth increased as a result of AMF inoculationwhatever the rootstocks studied. These results clearly indicatethe beneficial role of AM symbiosis in improving grapevinesalt tolerance (Belew et al. 2010; Khalil 2013).
4.4 AM symbiosis increase grapevine tolerance to heavymetals
Copper-based fungicides have been used intensively in Eu-rope since the end of the nineteenth century to control vinefungal diseases, especially downymildew, and their long-termapplication and subsequent wash off from treated plants haveled to considerable Cu accumulation up to toxic concentra-tions in vineyard surface soils (Komárek et al. 2010). Whereasarable land usually presents quantities of Cu varying between5 and 30 mg kg−1 of Cu, the soils of numerous wine-growingareas could contain as much as 200 to 500 mg kg−1 of Cu(Brun et al. 1998). For example, Cu concentrations rangingfrom 130 to 1280 mg kg−1 have been detected in the surfacesoil of European vineyards with an established history ofcopper-based fungicide use (Magalhães et al. 1985; Deluisiaet al. 1996; Flores-Vélez et al. 1996; Besnard et al. 2001), Cuconcentrations in Australian vineyard soils were generallymuch lower (6–150 mg kg−1) (Wightwick et al. 2008), where-as Cu concentrations as high as 3216 mg kg−1 have beenfound in Brazil (Mirlean et al. 2007). Although this increasedCu availability does not appear to change grapevine nutrition-al status (Miotto et al. 2013), it has a negative influence on thesoil flora and fauna and may lead to phytotoxicity in acidicsoils, leaf oxidative stress, yield losses, and poorer wine qual-ity (Ninkov et al. 2012). In practice, soil copper is not expect-ed to induce phytotoxicity in mature grapevines as their rootsystems extend below the surface soils where most of theapplied copper tends to remain. In contrast, young vinesmay be affected as their root systems are much shallowerwhile establishing. In this context, Cu may have phytotoxicity
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implications for vine nurseries or when vineyards arereplanted (Conradie 2004).
Metal-tolerant AMF have been isolated from polluted soils.These indigenous populations cope better with metal toxicitythan those isolated from unpolluted soils (del Val et al. 1999;Meier et al. 2011). To persist in environments with high metalcontent, AMF have evolved a series of strategies to avoiddamage caused by metals and survive in Cu-polluted environ-ments (Ferrol et al. 2009; Cornejo et al. 2013). AMF inocula-tion has become a prospective tool for enhancing plant toler-ance to environmental stress conditions in metal-contaminatedsoils. Many examples from the literature illustrate this role ofarbuscular mycorrhiza symbiosis, but progress towards under-standing the cellular mechanisms utilized by AMF to metabo-lize heavy metals and alleviate their cytotoxicity has beenmade only recently (Lanfranco 2002), and the underlyingmechanisms are not yet fully understood (Khade and Alok2009). For example, the occurrence of AMF in Ni-hyperaccumulating plant species naturally found on metal-rich soils offers possibilities of using heavy metal-hyper-accumulating plants together with AMF for phytoremediationstrategies (Göhre and Paszkowski 2006). In another way,AMF not only improve Pi or N uptake by roots through theirmycelium network but also have a buffering effect on Cduptake and thus reduce the toxic effect of Cd on plant growth(Rivera-Becerril et al. 2002; Lopez-Millan et al. 2009). Immo-bilization of metals in the fungal biomass is proposed as amajor mechanism whereby AMFmay increase plant toleranceto heavy metals. Mycorrhizal roots may act as a barrier againstmetal transport, reducing transfer and enhancing root/shoot Cdratios (deAndrade et al. 2008). This effect is attributed tometaladsorption onto hyphal walls because chitin has an importantmetal-binding capacity (Joner et al. 2000; Christie et al. 2004).In soil, glomalin, a glycoprotein produced by AMF, may havea metal-chelating function and thereby diminish metal avail-ability for plants (González-Chávez et al. 2004; Khan 2006).
AMF play a signif icant ecological role in thephytostabilization of potentially toxic trace element-pollutedsoils through sequestration mechanisms and, in turn, help my-corrhizal plants to survive in polluted soils. Their potentialrole in phytoremediation of heavy metal-contaminated soilsis also emerging, and we can expect them to be useful inCu-contaminated vineyard soils. Moreover, since AMF arepresent on the roots of plants growing on such types of soils,isolation of indigenous Cu stress-adapted AMF could be apotential biotechnological tool for inoculating plants in dis-turbed ecosystems.
5 Protection against biotic stresses
Most grown grapevine varieties (V. vinifera cvs) are suscepti-ble to cryptogamic diseases such as downy mildew, causing
great yield losses and require numerous fungicide treatments.However, pesticides are often only partially effective againstsoil-borne diseases and they are detrimental to human healthand to the environment. Some of them are now forbidden insome countries, as fumigants and winegrowers have no moresolution to fight soil pathogens.
Plants can develop an enhanced defensive capacity afterinfection by AMF. This “mycorrhiza-induced resistance”(MIR) could provide systemic protection against a wide rangeof biotrophic and necrotrophic pathogens, nematodes, andherbivorous arthropods (Cameron et al. 2013 for review).Therefore, AMF can suppress or reduce plant pests and dis-eases through the induction of systemic resistance (Pozo andAzcon-Aguilar 2007; Pineda et al. 2010; Jung et al. 2012).Depending on the plant/pathogen/AMF interaction, MIRcould be associated with systemic acquired resistance(SAR)-like priming of salicylic acid (SA)-dependent genes(Khaosaad et al. 2007; Gallou et al. 2011), but more oftencoincides with priming of jasmonic acid (JA)-dependent de-fenses, especially in grapevine (Li et al. 2010; Hao et al.2012). In this case, MIR is thought to result from the activesuppression of components in the SA-dependent defense path-way, causing systemic priming of JA-dependent defenses(Pozo and Azcon-Aguilar 2007). Cameron et al. (2013) pre-sented a spatiotemporal model of MIR that differentiated fourmajor steps. During the first step, plant root exudation ofstrigolactones (a class of terpenoid lactones) induces hyphalbranching in AMF, thereby helping the fungus to localize hostroots and facilitating infection. Then (step 2), AMF initiateinfection of the root cortex, and microbe-associated molecularpatterns (MAMPs) from the mycorrhizal fungus are recog-nized by the plant innate immune system. A recent study(Wang et al. 2014) moreover reported that, in rice, a bifunc-tional plant receptor (OsCERK1) regulates both chitin-triggered immunity and arbuscular mycorrhizal symbiosis,suggesting that a single receptor could balance defense andsymbiosis. This perception step leads to transient expressionof MAMP-triggered immunity and generation of long-distance signals in the vascular tissues, which induce long-lasting priming of SA-dependent defenses and SAR. At thisstage, AMF use specific effector molecules and stimulateabscisic acid (ABA) production in the roots to suppressMAMP-triggered immunity locally. ABA can be transportedthrough the xylem to the aerial parts of the plant (shoot),where it can prime cell wall defenses to protect the plantagainst further aerial pathogen attacks. Moreover, formationof intracellular arbuscules increases transport of photosyn-thates (sugars) to the roots and improves phosphate uptake,hence quantitative and qualitative changes in the chemicalcomposition of root exudates. The combination of all theseeffects leads to changes in root exudation chemistry andrecruitment/selection of specific mycorrhizosphere bacteria.In the fourth and last step, the establishment of the
Arbuscular mycorrhiza symbiosis in viticulture 1459
mycorrhizosphere goes along with dense colonization by se-lected bacteria (including Pseudomonas and Burkholderiastrains) that metabolize mycorrhizal root exudates and deliverinduced systemic resistance (ISR)-eliciting signals to the rootsurface and/or fungal hyphae. The perception of these signalsby the host plant generates long-distance signals that primejasmonate- and ethylene-dependent plant defenses and induceISR. Bioprotection of mycorrhizal plants can be active againsta wide spectrum of pathogens; therefore, management ofAMF can be considered as an ecosystemic service (Fig. 3)and could complement innovative protective strategies.
In the case of grapevine, MIR has been described mainlyagainst root pathogens and especially against the fungiArmillaria and Cylindrocarpon spp. For example, local andsystemic MIR was evidenced against the ectoparasitic nema-tode Xiphinema index, the vector of grapevine fanleaf virus(GFLV) which provokes gall formation and severe damage tothe root system (Hao et al. 2012). This study suggests a prim-ing of grapevine rootstock (SO4) defense responses by thearbuscular mycorrhizal fungus R. irregularis BEG141 andtransmission of a plant-mediated signal to non-mycorrhizaltissues. This bioprotective effect was not linked to an im-proved P status of the plant, but directly involved a so farunknown induced systemic factor. White root rot disease(due to Armillaria mellea) symptoms developed more slowlyin mycorrhizal (G. intraradices BEG72) grapevine (110R)and exhibited greater root and shoot growth compared to thenon-mycorrhizal plants (Nogales et al. 2009). Mycorrhizalplants were therefore more tolerant to A. mellea than non-mycorrhizal ones. The authors revealed that polyamines wereprobably implied in the early signaling processes of increasedtolerance of mycorrhizal plants to the pathogen. Finally,V. rupestris inoculated with G. intraradices (INVAMCA501) was less susceptible to black foot disease (due toCylindrocarpon macrodidymum) that non-mycorrhizal ones(Petit and Gubler 2006). By enhancing plant resistance tothese biotic stresses, AMF may therefore decrease grape rootsusceptibility to black foot disease. However, in this study,grapevine root colonization with AMF preceded inoculationwith C. macrodidymum, so maybe very few root infectionsites were available for the pathogen.
6 Increase of soil stability
A large number of vineyards are planted on hills and submit-ted to risks of erosion. In these situations, soil structuration isessential. AMF symbionts grow out from the mycorrhizal rootto develop a complex, ramifying network into the surroundingsoil which can reach up to 30 m of fungal hyphae per gram ofsoil (Cavagnaro et al. 2005; Wilson et al. 2009). This mycelialnetwork can have a binding action on soil particles and im-prove soil structure. Moreover, the secretion of hydrophobic,
“sticky” proteinaceous substances by AMF, consisting ofglomalin (Rillig et al. 2002), also contributes to soil stabilityand water retention (Bedini et al. 2009). Therefore, the com-bination of all these factors is considered as an importantfactor that helps stabilize soil aggregates (Andrade et al.1998; Rillig andMummey 2006), thereby leading to increasedsoil structural stability and quality (Caravaca et al. 2006;Bedini et al. 2009; Rillig et al. 2014). In this way, a reductionin fungal biomass will result in a negative effect on soil sta-bility and consequently will increase the risk of soil erosion.
Erosion of cultivated land soil by water is detrimental on-site as well as off-site and poses problems throughout theworld. These include soil and nutrient loss (Ramos andMartinez-Casanovas 2004), long-term productivity loss of de-graded soils (Lal 1987; Gunatilake 2000), and a wide range ofenvironmental problems arising from sediment delivery to thedrainage network and reservoirs. Soil erosion in vineyards is aparticularly favorable context for soil loss as compared toother agricultural lands (Le Bissonnais et al. 2001; Brenotet al. 2008). This irreversible soil degradation may cause ma-jor environmental and economic damage (Pimentel 1995). Forexample, Martínez-Casanovas and Ramos (2006) estimatedthe cost of erosion in vineyard fields in the Penedès-Anoiaregion (NE Spain), with special focus on on-site effects, i.e.,the replacement cost of lost nutrients (loss of fertilizers, main-ly N and P) over the year and the cost incurred by drainagechannel maintenance and ephemeral gullies that appear in thevineyards as a result of high intensity rainfalls. In economicterms, they concluded that these costs represent respectively6.6 % (nutrient loss) and 7.8 % (damage to infrastructures) ofthe grape production income. The replacement value of lost Nand P represented 2.4 % for N and 1.2 % for P. In this context,endomycorrhizal fungi may have a double interest, firstly byenhancing soil structural stability (due to the external hyphanetwork and glomalin secretions) and secondly by enhancingN and P uptake from the soil and thereby limiting damagerelated to nutrient losses.
7 Impact of vineyard agricultural practiceson grapevine mycorrhization
During its life cycle, the grapevine is subject to several culturalpractices to allow it to grow in optimum conditions for a givenproduction objective. These practices include, among others,fertilization, soil management, weed vegetation cover (chem-ical or mechanical weed control, and weed), and pests. Withan objective of sustainable viticulture, efforts were made toreduce the use of pesticides and agroecological initiatives suchas green, organic, biologic, or biodynamic management havebeen developed (Altieri 2002). Grapevine mycorrhizationtherefore looks essential regarding the possible ecosystemicservices it can provide to the vineyard. However, viticultural
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practices have to be considered as some of them have trueimpact on AMF.
7.1 Impact of soil management practices
Tillage is known to be a prominent factor in shaping AMFcommunities in agricultural settings (Verbruggen and Kiers2010) because it directly affects the integrity of the mycelialnetwork. These effects seem to depend on tillage intensity ordepth (Stockinger et al. 2014) and to be modulated by fertili-zation (Peyret-Guzzon et al. 2015). Certain glomeromycotanspecies are able to tolerate the recurrent perturbation and thusaccumulate in arable soils. Studies on the effects of tillage onAMF in vineyards still remain scarce. Lumini et al. (2010), forinstance, used pyrosequencing of soil-derived DNA to ana-lyze AMF communities in a tilled vineyard in comparison toanother one covered by vegetation. Interestingly, these authorsfound that the tilled vineyard soil harbored a higher diversitythan the covered vineyard soil. The question arises whethertillage at the soil surface would affect AMF communities ac-tive in the roots which grow relatively deeply in the soil. Thisfield of research therefore definitely requires further study.
7.2 Impact of high fertilizer inputs
High fertilizer inputs (especially P) are known to reduce AMFroot colonization and propagules in many agrosystems, in-cluding vineyards (Karagiannidis and Nikolaou 1999). Forexample, in Oregon vineyards, foliar application of P waslinked to low AMF colonization. Depending on the rootstock,P sources in soil (calcium bis-dihydrogen phosphate, tri-calcium phosphate, aluminum phosphate, and iron phosphate)affected growth and mineral uptake of mycorrhizal grapevines(Nikolaou et al. 2002). Furthermore, grapevine N fertilizationaffected AMF colonization and, in turn, affected berry com-position. Karagiannidis et al. (2007) indeed compared the im-pact of different nitrogen fertilizers (CO(NH2)2, (NH4)2SO4,Ca(NO3)2, and NH4NO3). Although the nitrogen form had noeffect on non-inoculated plants mycorrhizal grapevine plantshad a higher growth response with calcium nitrate as the Nsource (see Section 3.3). It is worthwhile to note that ureasuppressed AMF root colonization and sporulation.
7.3 Impact of weeds/cover crops
Weed control practices are often used in vineyards as alterna-tives to herbicides. Such ecological practices promote weedsas hosts for soil beneficial microbes (Jordan et al. 2000). Asweed control and cover cropping impact vineyard soil com-munities (Baumgartner et al. 2005; 2010), we could expectAMF diversity and colonization of grapevine to increasewith the increasing frequency and diversity of mycorrhizalhosts on the vineyard floor. Bare reality seems to be more
complex as at least two studies give opposite results. On theone hand, Baumgartner et al. (2010) revealed that there was nocorrelation between mycorrhizal colonization of grapevineand frequency of mycorrhizal weeds. The authors suggestthat mycorrhizal hosts on the vineyard floor do notsignificantly influence mycorrhizal colonization ormycorrhizal fungal communities of grapevines. On the otherhand, Radic et al. (2012) showed that selected neighboringweeds (Plan tago lanceo la ta L. and Tanace tumcinerariifolium) could influence the formation of arbuscularmycorrhiza in grapevine. They stressed that plant identity anddensity significantly impact on the development of mycorrhi-zal intra- and extraradical mycelium and/or sporulation.Therefore, herbaceous weed species could promote a differentset of dominant mycorrhizal fungi, potentially providing awider spectrum of these fungi for colonizing grapevine roots.This study highlights that encouraging host plant diversity invineyards can be valuable, provided that it does not developnutrient and/or water competition with grapevine.
7.4 Impact of pest management practices
Soil pest management is complex, and the equilibrium be-tween fighting again pests and protecting/promoting benefi-cial microorganism as AMF is difficult to reach.
Mesocriconema xenoplax is a plant–parasitic nematodethat is widely distributed throughout vineyards in the USA,South Africa, and Europe. In Oregon, many of the vineyardshave high population densities of Mesocriconema xenoplax(Pinkerton et al. 1999), which may impact the establishmentand growth of young vines. Moreover, those nematodes andAMF coexist and interact with grapevine roots and with therhizosphere. The frequency of grapevine fine roots containingarbuscules was depressed by 5 to 65 % in plants initiallyinfested by this nematode as compared to controls (Pinkertonet al. 2004). Competition for photosynthates within the rootsystem may be a possible mechanism whereby plant–parasiticnematodes lower arbuscule frequency.
In the grape-growing areas of California, soil fumigationwith nematicides or biocides (such as methyl bromide) areuseful. Fumigation of soils (Menge et al. 1983) could be nec-essary to alleviate disease problems caused by soil-borne pests(againstM. xenoplax orC. macrodidymum especially), even ifthis practice is not authorized in all the vineyards. This methodis known to kill the endemic AM fungi (Cheng andBaumgartner 2004), and in the fumigated P deficient soils,vines are weaker due to the absence or the clear decrease ofAM fungi (Menge et al. 1978). Even if soil fumigation doesnot suppressed 100 % of the AMF inoculum in the soil(Menge 1982), it may decrease the diversity of AMF speciesto the extent that efficient colonizers of grapevine roots areeliminated from the AMF community (Cheng andBaumgartner 2004). It may also decrease the chances of
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grapevine roots encountering the most beneficial combinationof AMF species. Therefore, if soils are fumigated before plant-ing, then reintroducing AMF will likely greatly benefit plantestablishment and growth.
Present-day vineyard practices place several constraints on theuse of functions (Fig. 3) provided by mycorrhiza. The risk oflarge, costly, or irreversible changes is to be reduced oraverted. Future (modern) agriculture should be based on theimplementation of ecological management practices that de-liberately maintain resilience of ecosystem services. Thismeans integrating the development of vineyard managementstrategies that optimize the impact of beneficial microbes likemycorrhizal fungi on production. Furthermore, AMF vary intheir ability to provide ecological services so that suitabletools have to be defined to fully assess their contribution.Molecular tools have considerably improved the possibilityto identify and monitor mycorrhizal fungi in ecosystems, buta quick and reliable test for assessing their functionality is stilllacking (Gianinazzi et al. 2010). For producers’ expectationsto be met, a novel industry encompassing soil/mycorrhizaanalyses and advice to producers/managers is needed. Addi-tional barriers to rationally exploiting beneficial soil microbeslike mycorrhizal fungi as ecosystem services range from eco-nomical, technical, and cultural aspects to legislative ques-tions. In spite of this, considerable progress has been madein the last decade for crop plants in general, but also for grape-vine, towards the use of AMF.
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