Candidate resistance genes to foliar phylloxera identified at Rdv3 of hybrid grape

The foliage of the native grape species Vitis riparia and certain cold-hardy hybrid grapes are particularly susceptible to the insect pest phylloxera, Daktulosphaira vitifoliae Fitch. A previous study using a cold-hardy hybrid grape biparental F1 population (N ~ 125) detected the first quantitative trait locus (QTL) for foliar resistance on chromosome 14, designated as resistance to Daktulosphaira vitifoliae 3 (Rdv3). This locus spans a ~ 7-Mbp (10-20 cM) region and is too wide for effective marker-assisted selection or identification of candidate genes. Therefore, we fine mapped the QTL using a larger F1 population, GE1783 (N ~ 1023), and genome-wide rhAmpSeq haplotype markers. Through three selective phenotyping experiments replicated in the greenhouse, we screened 184 potential recombinants of GE1783 using a 0 to 7 severity rating scale among other phylloxera severity traits. A 500-kb fine mapped region at 4.8 Mbp on chromosome 14 was identified. The tightly linked rhAmpSeq marker 14_4 805 213 and flanking markers can be used for future marker-assisted breeding. This region contains 36 candidate genes with predicted functions in disease resistance (R genes and Bonzai genes) and gall formation (bifunctional 3-dehydroquinate dehydratase/shikimate dehydrogenase). Disease resistance genes suggest a traditional R-gene-mediated resistance mechanism often accompanied by a hypersensitive response, which has been widely studied in the plant pathology field. A novel resistance mechanism, non-responsiveness to phylloxera gall formation is proposed as a function of the bifunctional dehydratase gene, which plays a role in gallic acid biosynthesis and is important in gall formation. This study has implications for improvement of foliar phylloxera resistance in cold-hardy hybrid germplasm and is a starting place to understand the mechanism of resistance in crops to gall-forming insects.