Dissecting the genetic determinants of powdery mildew resistance on grape

Grapevine (Vitis vinifera L.) in European viticulture is threatened by pathogens like powdery and downy mildew, requiring heavy fungicide applications. Concerns about environmental impacts request more sustainable production methods. New high quality cultivars with genetic resistance are therefore generated. Resistance traits from wild Vitis species are introgressed into the cultivated susceptible V. vinifera. However, this breeding is a long term process. Molecular markers linked to traits of interest help to accelerate it, rationalizing the choice of parental types and replacing part of the phenotypic evaluation by genetic testing at early stages of plant growth. Molecular markers linked to downy and powdery mildew resistance were recently identified by genetic mapping. In a complementary study, differential gene expression was investigated under powdery mildew challenge of resistant (`Regent´) and susceptible (‘Chardonnay’) grapes grown in vitro. A set of 27 candidate genes was studied in detail by quantitative Real Time PCR. Results confirm an early induction of some transcription factor genes whose products are instrumental for reprogramming the cellular transcriptome during early defense reactions. The sequence diversity of their genes and additional key regulators of signal cascades was investigated in a set of 45 genotypes with varied levels of powdery mildew resistance. A high number of single nucleotide polymorphisms was found and analyzed for their implications on encoded amino acids or potential protein domains. More than 100 SNPs identified in six genes were subjected to association analysis with powdery mildew resistance expression. Among those, 14 turned out to be strong candidates. One was preliminarily found to be associated with powdery mildew resistance of grapevine leaves in a comprehensive sample set. In conclusion, targeted diversity analysis in candidate genes combined with tests for association to a specific trait seems to be a promising approach to identify new gene-linked markers for breeding purposes.