Abstract/Summary

Apple (Malus x domestica; Md) and strawberry (Fragaria x ananassa; Fxa) are two of the most culturally and economically important cultivated fruit varieties in the world. Phytophthora cactorum (Pc) isolates cause substantial damages in both plants’ growing systems. Its ability to spread infection through waterlogged orchard soils as well as through the irrigation systems used in tabletop strawberry cultivation, coupled with the lack of available chemical management options, reinforce the need for resistant varieties. Despite the extended resistance breeding efforts in both hosts, and the recent mapping of resistance in Fxa, much remains to be elucidated of the mechanisms underlying plant resistance to this pathogen. Resistance in an apple was mapped using a bi-parental population generated from the cross of two popular rootstock varieties (‘M.27’ and ‘M.116’), revealing the presence of a largeeffect quantitative trait locus (QTL) on chromosome 6 (MdRPc1). Moreover, a preliminary genome-wide association study (GWAS) performed on a panel of 99 apple accessions from the wider germplasm confirmed the presence of the MdRPc1 locus, as well as two additional loci (MdRPc2 and MdRPc3) on chromosomes 5 and 15. The transcriptional response to infection was studied in both hosts through the whole-transcriptome sequencing of root tissue samples of susceptible and resistant varieties from time course inoculation experiments. This allowed to identify pathways regulated upon Pc infection, as well as candidate resistance/susceptibility genes. Finally, the transcriptome analysis of Pc during apple infection revealed the regulation of a large effector array and highlighted candidate pathogenicity genes. Further, comparisons with the previously published transcriptome analysis of a Pc isolate during infection of strawberry has provided insights into factors determining host specificity. Taken together these findings help elucidate the mechanisms underlying host-Pc interactions and provide valuable data to be used in future resistance breeding efforts.