Abstract
The cultivated Eurasian grapevine (Vitis vinifera L.) is highly susceptible to downy mildew (DM) – caused by the biotrophic oomycete Plasmopara viticola (Berk. & M.A. Curtis) Berl. & De Toni – the major disease of temperate-humid climates among various pathogen threats. DM control relies mainly on the massive use of fungicides leading to environmental pollution, development of resistance and residual toxicity. The exploitation of DM-resistant wild genetic resources for the development of new resistant cultivars represents a promising alternative. Taking advantage of a segregating population derived from ‘Merzling’ (M, a mid-resistant hybrid) and ‘Teroldego’ (T, a susceptible landrace), recent studies have highlighted the importance of stilbenoids among phenolic compounds in conferring resistance to this oomycete. In order to elucidate the genetic bases of DM resistance and polyphenol biosynthesis upon P. viticola inoculation, 136 M×T F1 individuals were characterized by an integrative approach combining genetic, phenotypic and gene-expression data. An improved M×T linkage map was obtained by scoring 192 microsatellite markers. The progeny was further screened for degree of resistance and production of 42 phenolic compounds (including 18 different stilbenoids). Quantitative trait locus (QTL) mapping showed that DM resistance is associated with a specific haplotype at the Rpv3 locus – herein named Rpv3-3, derived from the French hybrid ‘Seyval’ – and identified 46 novel metabolic (m)QTLs linked to 30 polyphenol-related parameters. A list of the 76 most relevant candidate genes was generated by specifically exploring the genomic regions underlying the mQTLs associated with the stilbenoids induced by the infection. Finally, expression analysis of 13 genes in Rpv3-3+/- genotypes, displaying divergent DM resistance and stilbenoid accumulation, revealed significant candidates for the genetic control of stilbenoid biosynthesis and oligomerization. These findings emphasize that DM resistance can be mediated by the major Rpv3-3 locus and stilbenoid induction.
Highlights
Around 68,000 tons of fungicides per year are used in Europe to manage grape diseases, i.e., 65% of all fungicides used in agriculture viticulture encompasses only 4% of the EU arable land (Eurostat, 2007)
The Merzling” × “Teroldego” (M×T) genetic map built in this study is an improved version in terms of progeny individuals and linkage groups (LGs) number as well as marker number/order compared to the one by Salmaso et al (2008)
The high percentage of markers (68%) that was mapped in both parents is close to the 62% of markers positioned into the reference grapevine genetic map, while the 5% of distorted markers is about half of the value previously observed (Adam-Blondon et al, 2004)
Summary
Around 68,000 tons of fungicides per year are used in Europe to manage grape diseases, i.e., 65% of all fungicides used in agriculture viticulture encompasses only 4% of the EU arable land (Eurostat, 2007). A total of 27 Quantitative Trait Loci (QTLs) associated with DM resistance in different genetic backgrounds are known and described (VIVC, 2018). The major Rpv loci originated from Muscadinia rotundifolia (Merdinoglu et al, 2003), Vitis riparia (Marguerit et al, 2009; Moreira et al, 2011), V. amurensis (Blasi et al, 2011; Schwander et al, 2012; Venuti et al, 2013), V. cinerea (Ochssner et al, 2016), and V. rupestris (Divilov et al, 2018). The Rpv locus is a major determinant of grapevine DM resistance. Seven conserved Rpv haplotypes were identified in five descent groups of resistant varieties and traced back to their founders, which belong to V. rupestris, V. lincecumii, V. riparia, and V. labrusca (Di Gaspero et al, 2012). Until now only two haplotypes at this locus were validated in segregating populations derived from different DM resistance donors (Welter et al, 2007; Zyprian et al, 2016)
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