IDENTIFICATION OF RESISTANCE SOURCES AGAINST PLASMOPARA VITICOLA IN VITIS VINIFERA CULTIVARS FOR BREEDING PROGRAMS

The Eurasian grapevine (Vitis vinifera), an Old World species now cultivated worldwide for high-quality wine production, is extremely susceptible to the agent of downy mildew, Plasmopara viticola (Berk. et Curt.) Berl. and de Toni. This Oomycete is one of the most important pathogens of European grapevine. The discovery of resistant cultivars for breeding programs could be a solution to decreasing fungicides application for downy mildew disease worldwide. Extensive evaluation of Georgian cultivated grapevine germplasm has highlighted unique resistance behavior through the reduction of disease severity and pathogen sporulation. Unraveling the genetic architecture of grapevine response to P. viticola infection is crucial to develop resistant varieties. The aim of this project was to identify loci related to P. viticola resistance traits and to obtain new insights in the mechanism of resistance of Georgian germplasm. To address the first aim a genome-wide association (GWA) approach has been applied to a panel of Georgian-derived accessions phenotyped for P. viticola susceptibility and genotyped with Vitis18kSNP chip array. GWA identified three new loci (Rpv29, Rpv30 and Rpv31) associated with a low level of disease incidence. Rpv29, Rpv30, and Rpv31 loci appeared to be associated with genes related to plant defense mechanism against biotic stresses (pathogen recognition and signal transduction). Regarding the second objective, the role of leaf VOCs in the resistance mechanism of two resistant cultivars (Mgaloblishvili, a pure Georgian V. vinifera cultivar, and Bianca, an interspecific hybrid) has been investigated. The leaf VOC profiles analyzed through solid-phase microextraction gas chromatography-mass spectrometry analysis, and the expression of six terpene synthases (TPSs), through, real-time RT-PCR, were determined upon pathogen inoculation. In both cultivars, an increment of VOCs (such as farnesene, nerolidol, ocimene and valencene) has been detected after pathogen inoculation, contextually to an increment of the expression pattern of six TPSs. Finally, the transcripts of P. viticola in the early interaction with grapevine cultivars have been characterized. In this study, the early P. viticola development in susceptible host cells (2008-059-020, Rpv–) was compared two resistance 2008-059-121 (carrier of Rpv3 and Rpv10) and 2011-003-013 (homozygous for the locus Rpv10) using RNA sequencing data and microscopic observation. In total six novel genes of TAR 1 protein, cellulose synthase, a regulator of G- protein in signaling and Ras-related proteins were identified in P. viticola which are differentially expressed during the initial infection. This primary signaling induction by the pathogen in host cell could be used in the future coupled with the first report on resistance loci in V. vitinifera, VOC induction and genome regions involved in resistance response, for further genetic study of V. vinifera and breeding programs.