Transcriptional induction of two phosphate transporter 1 genes and enhanced root branching in grape plants inoculated with Funneliformis mosseae.

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.

Use of a RT-qPCR Method to Estimate Mycorrhization Intensity and Symbiosis Vitality in Grapevine Plants Inoculated with Rhizophagus irregularis.

Assessing the mycorrhization level in plant roots is essential to study the effect of arbuscular mycorrhizal fungi (AMF) on plant physiological responses. Common methods used to quantify the mycorrhization of roots are based on microscopic visualization of stained fungal structures within the cortical cells. While this method is readily accessible, it remains time-consuming and does not allow checking of the symbiosis vitality. The aim of this work is thus to develop an efficient method for assessing the intensity and vitality of mycorrhiza associated with grapevine through gene expression analyses by RT-qPCR. To this end, grapevine plants were inoculated with the AMF Rhizophagus irregularis (Ri). The relationship between mycorrhization level, assessed by microscopy, and expression of several fungus and grapevine genes involved in the symbiosis was investigated. In AMF-inoculated plants, transcript amounts of fungal constitutively-expressed genes Ri18S, RiTEF1α and RiαTub were significantly correlated to mycorrhization intensity, particularly Ri18S. Grapevine (VvPht1.1 and VvPht1.2) and AMF (GintPT, Ri14-3-3 and RiCRN1) genes, known to be specifically expressed during the mycorrhizal process, were significantly correlated to arbuscular level in the whole root system determined by microscopy. The best correlations were obtained with GintPT on the fungal side and VvPht1.2 on the plant side. Despite some minor discrepancies between microscopic and molecular techniques, the monitoring of Ri18S, GintPT and VvPht1.2 gene expression could be a rapid, robust and reliable method to evaluate the level of mycorrhization and to assess the vitality of AMF. It appears particularly useful to identify AMF-inoculated plants with very low colonization level, or with non-active fungal structures. Moreover, it can be implemented simultaneously with the expression analysis of other genes of interest, saving time compared to microscopic analyses.

Arbuscular Mycorrhizal Symbiosis Triggers Major Changes in Primary Metabolism Together With Modification of Defense Responses and Signaling in Both Roots and Leaves of Vitis vinifera.

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.

Arbuscular mycorrhizal symbiosis stimulates key genes of the phenylpropanoid biosynthesis and stilbenoid production in grapevine leaves in response to downy mildew and grey mould infection.

Grapevine (Vitis spp) is susceptible to serious fungal diseases usually controlled by chemical treatments. Arbuscular mycorrhizal fungi (AMF) are obligate plant symbionts which can stimulate plant defences. We investigated the effect of mycorrhization on grapevine stilbenoid defences. Vitis vinifera cvs Chasselas, Pinot noir and the interspecific hybrid Divico, on the rootstock 41B, were mycorrhized with Rhizophagus irregularis before leaf infection by Plasmopara viticola or Botrytis cinerea. Gene expression analysis showed an up-regulation of PAL, STS, and ROMT, involved in the stilbenoid biosynthesis pathway, in plant leaves, 48 h after pathogen inoculation. This defense response could be potentiated under AMF colonization, with an intensity level depending on the gene, the plant cultivar and/or the pathogen. We also showed that higher amounts of active forms of stilbenoids (i.e trans-form of resveratrol, ε- and δ-viniferins and pterostilbene) were produced in mycorrhized plants of the three genotypes in comparison with non-mycorrhized ones, 10 days post-inoculation with either pathogen. These results support the hypothesis that AMF root colonization enhances defence reactions against a biotrophic and a necrotrophic pathogen, in the aerial parts of grapevine.

[Studies of several analytical methods for antioxidant potential evaluation in food]. / Méthodes d'évaluation du potentiel antioxydant dans les aliments.

Oxidation control is necessary to manage the evolution of complex biological system, particularly in food whose degradation could have consequences on food security. After description of context and oxidation mechanisms, several analytical methods to evaluate the additive antioxidant potential are presented. This evaluation is performed either by quantification of products (in particular hydroperoxydes) using direct or indirect photometric techniques and chemical titration with suitable reactants or on the effectiveness to trap free radicals with modelized systems that can generate them. Methods based on the comparison of radical trapping ability between an additive and Trolox (particularly Trolox(R) equivalent antioxydant capacity, TEAC) can be applied to many products whatever the hydrophily or the hydrophobia of the medium.