Susceptibility assessment and genetic analysis of cucumber genotypes to damping-off disease by Phytophthora melonis

Abstract

Cucumbers (Cucumis sativus L.) are an essential vegetable crop in the globe, they confront severe problems due to Phytophthora blight, which is mostly caused by Phytophthora melonis and causes large economic losses, particularly in Iran. The aim of this study was to investigate the resistance mechanisms against this disease in different cucumber genotypes. In the current study, the resistance to damping-off in 38 different local and exotic commercial cucumber genotypes was evaluated during the seedling and maturity stages. The genotypes were divided into four classes, with the average percent of damping-off ranging from 7.92 (resistant) to 88.01 (highly susceptible). Genetic diversity was found by molecular analysis utilizing ISSR markers, with ISSR29 being the most informative primer that highlights the potential of ISSR markers in providing a basis for identifying new resistant cucumber varieties. Important antifungal genes (CsWRKY20, CsLecRK6.1, PR3, PR1-1a, and LOX1) were found to be elevated in resistant and moderately resistant genotypes, especially in root collars, according to gene expression analysis, indicating their role in initiating defense mechanisms. At the same time, the related marker genes modulating immunity not only against P. melonis, but also are basically resistant against other pathogenic fungi, nematodes, and abiotic stress. For instant, CsWRKY20- Sphaerotheca fusca (powdery mildew), salt, drought, and cold stress; CsLecRK6.1 and PR1-1a- S. fusca; PR-3 and LOX1 Meloidogyne incognita (root-knot nematode) and Alternaria brassicae (leaf spot). Moreover, enzyme activity assays showed that PAL and POX enzymes were linked to resistance, whereas PPO, SOD, and CAT did not show a significant correlation. The importance of certain genes and metabolic pathways, in particular POX and PAL, in enhancing cucumber resistance to P. melonis has highlighted the potential of the related defense-resistant genes and enzymes in breeding efforts, providing a basis for developing new resistant cucumber varieties.