Candidate defense genes as predictors of partial resistance in 'President Roulin' against apple scab caused by Venturia inaequalis

Scab, caused by the fungus Venturia inaequalis, is one of the most important diseases of apple. Although major scab resistance genes (R gene) have been widely studied, little is known about the molecular mechanisms underlying partial resistance, thought to be more durable. We used a candidate gene approach to decipher the genetic determinism of the durable partial resistance in `President Roulin', an old Belgian apple cultivar. Pathological tests using monoconidial isolates of V. inaequalis on F1 'Gala' x 'President Roulin' progeny suggested that partial resistance was broad spectrum but resulted from the combination of several race-specific interactions and was governed by at least five R genes. From an earlier transcript profiling study, we selected 13 pathogenregulated genes in 'President Roulin' with a known role in plant defense and characterized their expression over a time-course experiment. These candidate defense genes (CDGs) were regulated between 6 and 120 h after inoculation. Most were significantly up-or downregulated in incompatible interactions only or were induced earlier compared with compatible interactions. Among them, eight were mapped in silico within chromosomal regions containing disease resistance factors (R gene analogues, major scab R genes or quantitative trait loci). We also investigated the extent of the correlation between CDG expression data and phenotypic variation in the progeny. We estimated that the induction of nine out of 10 CDGs accounted for up to 46 % of the phenotypic variance. Our study has improved the understanding of partial apple scab resistance and could be used in developing functional molecular markers for breeding new 'spray-free' cultivars with durable scab resistance.