Multi-trait assessment of Meloidogyne incognita resistance in developed Cucumis spp. inbred lines: horticultural, physiological, biochemical, and histological insights.

Abstract

This study aimed to develop nematode-resistant inbred lines from Cucumis sativus, C. sativus var. hardwickii, and C. metuliferus, and to elucidate their resistance mechanisms against Meloidogyne incognita. Three parental genotypes were self-pollinated across five generations, with simultaneous selection for superior horticultural traits and nematode resistance. Resistance levels were assessed based on root gall index, gall number, egg mass, and egg count per root system. To investigate the underlying mechanisms, histological, biochemical, and molecular analyses were conducted, focusing on salicylic acid (SA) content and the expression of SA pathway genes (PR1-1A, PR3, and LOX1). Based on the coefficient of variation (CV%) for horticultural traits, three inbred lines were selected from C. sativus (Cs), three from C. sativus var. hardwickii (Ch), and five from C. metuliferus (Cm). Among these, Cs-L1, Ch-L2, Cm-L1, and Cm-L2 exhibited superior horticultural traits, whereas Cs-L1, Ch-L3, Cm-L4, and Cm-L5 showed the lowest nematode infection parameters. Histological examination revealed localized necrosis and cellular degeneration in the resistant lines, suggesting a hypersensitive response (HR)-like mechanism that likely contributes to nematode containment and developmental arrest. Biochemical assays further demonstrated greater induction of defense-related enzymes, including peroxidase (POD), polyphenol oxidase (PPO), and superoxide dismutase (SOD), in the roots of resistant lines. Notably, SA levels increased significantly upon infection in all resistant roots, particularly in C. metuliferus, compared with the susceptible control (Barracuda F1). This elevated SA accumulation correlated with the upregulated expression of PR1-1A, PR3, and LOX1. Overall, resistance in the selected inbred lines is multifaceted, encompassing reduced nematode development, HR-associated necrosis, enhanced antioxidant enzyme activity, and coordinated SA-mediated gene upregulation. These responses most pronounced in C. metuliferus play a crucial role in mitigating root-knot nematode damage and provide valuable targets for breeding nematode-resistant cucumber genotypes.