Grapevine (Vitis vinifera L.) is one of the most important crops worldwide but is subjected to multiple biotic and abiotic stresses, especially related to climate change. In this context, the grapevine culture could take advantage of symbiosis through association with arbuscular mycorrhizal fungi (AMF), which are able to establish symbiosis with most terrestrial plants. Indeed, it is well established that mycorrhization improves grapevine nutrition and resistance to stresses, especially water stress and resistance to root pathogens. Thus, it appears essential to understand the effect of mycorrhization on grapevine metabolism and defense responses. In this study, we combined a non-targeted metabolomic approach and a targeted transcriptomic study to analyze changes induced in both the roots and leaves of V. vinifera cv. Gewurztraminer by colonization with Rhizophagus irregularis (Ri). We showed that colonization of grapevine with AMF triggers major reprogramming of primary metabolism in the roots, especially sugar and fatty acid metabolism. On the other hand, mycorrhizal roots had decreased contents of most sugars and sugar acids. A significant increase in several fatty acids (C16:1, linoleic and linolenic acids and the C20 arachidonic and eicosapentaenoic acids) was also detected. However, a downregulation of the JA biosynthesis pathway was evidenced. We also found strong induction of the expression of PR proteins from the proteinase inhibitor (PR6) and subtilase (PR7) families in roots, suggesting that these proteins are involved in the mycorrhiza development but could also confer higher resistance to root pathogens. Metabolic changes induced by mycorrhization were less marked in leaves but involved higher levels of linoleic and linolenic acids and decreased sucrose, quinic, and shikimic acid contents. In addition, Ri colonization resulted in enhanced JA and SA levels in leaves. Overall, this study provides a detailed picture of metabolic changes induced by AMF colonization in a woody, economically important species. Moreover, stimulation of fatty acid biosynthesis and PR protein expression in roots and enhanced defense hormone contents in leaves establish first insight in favor of better resistance of grapevine to various pathogens provided by AMF colonization.

SummaryIn the past few years, the usefulness of transient expression assays has continuously increased for the characterization of unknown gene function and metabolic pathways. In grapevine (Vitis vinifera L.), one of the most economically important fruit crops in the world, recent systematic sequencing projects produced many gene data sets that require detailed analysis. Due to their rapid nature, transient expression assays are well suited for large-scale genetic studies. Although genes and metabolic pathways of any species can be analysed by transient expression in model plants, a need for homologous systems has emerged to avoid the misinterpretation of results due to a foreign genetic background. Over the last 10 years, various protocols have thus been developed to apply this powerful technology to grapevine. Using cell suspension cultures, somatic embryos, leaves or whole plantlets, transient expression assays enabled the study of the function, regulation and subcellular localization of genes involved in specific metabolic pathways such as the biosynthesis of phenylpropanoids. Disease resistance genes that could be used for marker-assisted selection in conventional breeding or for stable transformation of elite cultivars have also been characterized. Additionally, transient expression assays have proved useful for shaping new tools for grapevine genetic improvement: synthetic promoters, silencing constructs, minimal linear cassettes or viral vectors. This review provides an update on the different tools (DNA constructs, reporter genes, vectors) and methods (Agrobacteriummediated and direct gene transfer methods) available for transient gene expression in grapevine. The most representative results published thus far are then described.

We investigated the effects of the arbuscular mycorrhizal fungus (AMF) Funneliformis mosseae on the growth and root architecture of plantlets of the grape rootstock 41B MGt under hydroponic conditions, and analyzed the concomitant expression of putative mycorrhizal-specific phosphate transporter 1 (PHT1) genes. In vitro propagated plantlets were acclimatized to ex vitro culture before AMF inoculation and grown under low phosphate (Pi) nutrition conditions during 6 weeks. Grape roots could be efficiently colonized by F. mosseae in this culture system, as shown by high mycorrhization frequency and intensity. The presence of many arbuscules in the cortex was coupled with high-level expression of two PHT1 genes in grape roots. These two very similar genes, named VvPht1-1 and VvPht1-2, present P1BS and MYCS regulatory motifs in their promoter, consistent with a specific role in the mycorrhizal pathway of Pi uptake. Although AMF inoculation significantly increased shoot growth, no effect on root biomass was observed. However, inoculated grapes exhibited an enhanced branched root system compared with non-inoculated controls, with a twofold higher number of tips and a higher proportion of fine roots usually involved in nutrient uptake from the soil. Taken together, these results suggest that root colonization by F. mosseae improved grape growth by favoring the uptake of Pi from the substrate via VvPht1-1 and VvPht1-2 high-level expression.

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