Genetic and molecular characterization of host-plant resistance to root-knot nematodes and Fusarium wilt in cotton

Root-knot nematode (Meloidogyne incognita) is an important pathogen of cotton (Gossypium hirsutum L.) worldwide, and it interacts with Fusarium wilt (Fusarium oxysporum f.sp. vasinfectum, FOV), a major disease of cotton. In the presence of M. incognita, the incidence and severity of Fusarium wilt increases significantly. Host-plant resistance is the most economic and effective method to control nematode and wilt disease on cotton. Three cultivars ( G. hirsutum Acala SJ-2 and Acala Nemx; G. barbadense Pima S-7) were characterized based on the phenotype of reactions to M. incognita and FOV in the greenhouse. The greatest wilt symptoms occurred on wilt and nematode-susceptible SJ-2 in the presence of both nematodes and FOV, followed by wilt-resistant and nematode-susceptible Pima S-7, and then nematode-resistant and wilt-susceptible NemX. Nematode resistance was more effective than wilt resistance in suppressing wilt symptoms when either resistance was present alone. Based on genetic and molecular analysis of F 1, F2, F3, BC1F1, and F 7 recombinant inbred lines from the cross of nematode resistant NemX x susceptible SJ-2, a major resistance gene (rkn1) in NemX was identified. Four AFLP markers, one AFLP-derived CAPS marker and two SSR markers were found linked to rkn1 within an interval of 2.6 to 14.2 cM, and the genomic region around rkn1 was spanned to about 28.2 cM in the F2:7 (NemX x SJ-2). The rkn1 gene was mapped to cotton linkage group A03 based on the interspecific cross Pima S-7 x NemX, representing the first nematode resistance gene to be mapped in cotton. PCR-based markers closely linked to rkn1 may be utilized for marker-assisted selection. The rkn1 gene in the interspecific cross Pima S-7 x Nemx was found to interact with an independently assorting gene (RKN2) in susceptible Pima S-7, resulting in enhanced nematode resistance based on genetic and marker analysis. One major gene (Fov1) with allele dosage effects in Pima S-7 was found to confer resistance to FOV race 1 and two AFLP markers linked to Fov1 were identified based on genetic and AFLP marker analysis. Transgressive segregation for both wilt and nematode resistance was observed in this study.