A Novel QTL for Resistance to Phytophthora Crown Rot in Squash

Abstract

Phytophthora capsici Leonian causes significant yield losses in commercial squash (Cucurbita pepo) production worldwide. The deployment of resistant cultivars can complement integrated management practices for P. capsici, but resistant cultivars are currently unavailable for growers. Moderate resistance to Phytophthora crown rot in a selection of accession PI 181761 (C. pepo) (designated line #181761-36P) is controlled by three dominant genes (R4, R5 and R6). Introgression of these loci into elite germplasm through marker-assisted selection (MAS) can accelerate the release of new C. pepo cultivars resistant to crown rot, but these tools are currently unavailable. Here we describe the identification of a quantitative trait locus (QTL), molecular markers and candidate genes associated with crown rot resistance in #181761-36P. Five hundred and twenty-three SNP markers were genotyped in an F2 (n = 83) population derived from a cross between #181761-36P (R) and Table Queen (S) using targeted genotyping by sequencing. A linkage map (2068.96 cM) consisting of twenty-one linkage groups and an average density of 8.1 markers/cM was developed for the F2 population. The F2:3 families were phenotyped in the greenhouse with a virulent strain of P. capsica, using the spore-spray method. A single QTL (QtlPC-C13) was consistently detected on LG 13 (chromosome 13) across three experiments and explained 17.92–21.47% of phenotypic variation observed in the population. Nine candidate disease resistance gene homologs were found within the confidence interval of QtlPC-C13. Single nucleotide polymorphism (SNP) markers within these genes were converted into Kompetitive Allele Specific PCR (KASP) assays and tested for association with resistance in the F2 population. One SNP marker (C002686) was significantly associated with resistance to crown rot in the F2 population (p < 0.05). This marker is a potential target for MAS for crown rot resistance in C. pepo.

Transcriptional Changes during Phytophthora capsici Infection Reveal Potential Defense Mechanisms in Squash

“…While several sources of resistance to Phytophthora crown rot [7][8][9][10][11] and their associated inheritance mechanisms [8,12] and QTLs [13][14][15] have been described in Cucurbita spp., the molecular mechanisms and gene pathways underlying the resistance remain poorly understood. In the current study, RNA sequencing revealed gene expression patterns in resistant and susceptible genotypes of Cucurbita following P. capsici infection.…”

“…In both C. moschata (breeding line #394-1-27-12 carrying resistance from C. lundeliana) and C. pepo (breeding line #181761-36P), resistance to Phytophthora crown rot is conferred by three independent dominant genes [8,12]. Furthermore, the quantitative trait loci (QTLs) associated with resistance to Phytophthora crown rot have recently been reported in C. moschata [13] and C. pepo [14,15], paving the way for the application of marker-assisted selection in germplasm improvement. However, despite these advancements, the molecular mechanisms underlying resistance to Phytophthora crown rot are still poorly understood.…”