Mechanisms of resistance to phytophthora blight in pumpkin: A pathosystem perspective

Abstract

Phytophthora melonis severely affects Cucurbita pepo (pumpkin) production worldwide; however, the molecular mechanisms underlying resistance remain poorly understood, and only a limited number of resistant cultivars are available. This study investigates the biological responses of pumpkin to P. melonis, focusing on two resistant cultivars, "Ghelyani" and "Tanbal," and two susceptible cultivars, "Marmari" and "Khoreshti". Ten key marker genes (bHLH87, ERF014, HSF, MYB, PR-1, WRKY21, CPI, POD, PSK, and SGT) were analyzed alongside major antioxidant and hydrolytic enzymes at 24, 48, and 72 hours' post-inoculation (hpi). Resistant genotypes displayed robust transcriptomic responses at 48-72 hpi, with a 1.1-2.7-fold increase in gene expression in roots and leaves, strongly correlated with enzyme activities (r = 0.857-0.974). Key genes, including PR-1, CPI, PSK, SGT1, and POD, basically resistance to other pathogens such as Alternaria cucumerina, Verticillium dahliae, and Podosphaera xanthii, acted as dual-function regulators, and also contributes resistance to P. melonis. Resistant cultivars exhibited significantly higher ROS-scavenging enzyme activities than controls: SOD and POX increased six-fold, PPO and PAL 5.3-fold, and CAT 1.6-fold. beta-1,3-glucanase, associated with salicylic acid-mediated defense, increased 2.4-3-fold in resistant genotypes at 24 hpi, with strong gene-enzyme correlations (e.g., PPO-PR1, POD-WRKY21), indicating coordinated defense mechanisms. Significant differences in leaf and root traits were also observed between genotypes. Overall, these findings provide new insights into the integrated molecular and enzymatic defense strategies of C. pepo and identify key targets for breeding cultivars with improved resistance to P. melonis.