A comparative study of plant-pathogen interaction in different genotypes and organs of grapevine (Vitis spp.), based on optimized and new screening methods for resistance to downy mildew (Plasmopara viticola - Berk. & Curt.)

Initially, I have studied the organ-specific interaction between grapevine and downy mildew (DM), caused by the biotrofic pathogen Plasmopara viticola (Berk. & M.A. Curtis) Berl. & De Toni. The studied genotypes have been chosen based on their susceptibility, tolerance and resistance to DM according to previous field observations. These two organs have been collected from 9 Vitis hybrids and a Vitis vinifera L. variety grown in an untreated experimental field at Edmund Mach Foundation (FEM). In particular, inflorescences have been harvested at three phenological stages (17, 25 and 29) of the Eichhorn-Lorenz (E-L) scale. Firstly, I have improved the annotation procedure of foliar resistance/susceptibility under controlled conditions (optimized and updated OIV descriptor 452-1). The in vitro leaf disc bioassay resulted significantly associated to the in-field DM response. Secondly, I have developed a new in vitro phenotyping method - from infection to symptom evaluation - for DM resistance assessment on grapevine inflorescence (developed and proposed OIV descriptor 453-1). The 17 E-L phenological stage of inflorescence upon in vitro assay resulted significantly correlated to the in-field DM response. Based on the latter DM response assessment, genotypes have been assigned to classes following the four thresholds set for leaf discs. Linear correlations between organs within the same genotype have confirmed that the E-L 17 stage is the most responsive to the DM attack. Genotypes classified as mid-resistant or resistant at leaf level have also resistant inflorescences; genotypes with mid-susceptible leaves present susceptible inflorescences except for Cabernet Cortis (CC) (Cabernet Sauvignon x Solaris), carrying mid-resistant leaves and mid-susceptible inflorescences. To verify that the CC had a significantly different response between organs under controlled conditions, I have carried out some experiments detaching organs from fruiting cuttings grown in phytotron, using the susceptible Pinot Noir organs as a reference. Unlike the latter, CC presented a significant difference in DM response. In the second part of my research project, I have used CC as a model to study divergent dual epidemics, compared to its parent Cabernet Sauvignon (CS) displaying out susceptible organs. Leaves and inflorescences – at the 17 E-L phenological stage – obtained from fruiting cuttings have been used. To assess the response of different grapevine organs to P. viticola attack, the disease development has been evaluated with different methods, from the macroscopic/microscopic to the ultrastructural point of view. The macroscopic, microscopic and ultrastructural response to DM showed that in the mid-resistant CC leaf the sporulation was related to the H2O2 production and to the callose deposition in specific areas, while in the mid-susceptible CC inflorescence they were not related each other. Since the divergent dual epidemics are poorly studied on a molecular point of view, I have decided to perform a trascriptomics study to identify the differential expressed genes (DEGs) in CC organs both at baseline and upon P. viticola-inoculation, using CS as a reference. DEGs were grouped in 14 clusters based on their expression profiles. Interestingly, in the mid-resistant leaves of CC, there was an up-regulation of genes implicated in biotic and oxidative stress responses, signal transduction and defence-related genes. Genes implicated in the hormone signalling were up-regulated in mid-susceptible CC inflorescence, while genes related to carbohydrate metabolism and photosynthetic process were down-regulated. Finally, to validate the organ-specific response to P. viticola-inoculation, the disease development and the expression level of some DEGs have been evaluated on an independent experiment, using the two studied genotypes, the reference V. vinifera variety Pinot noir and the CC-related Vitis hybrid Muscaris.