Genetics and phenotypic characterisation of the hypersensitive resistance of Solanum sparsipilum to Meloidogyne incognita

The hypersensitive resistance of the genotype '385.484.13' of Solanum sparsipilum to Meloidogyne incognita, one of the southern root-knot nematodes and their Mi virulent populations was analysed. Genetic control of the hypersensitive reaction was assessed based on segregation of the necrotic reaction in infected roots of diploid F1 plants obtained from the cross of the genotype '385.484.13' with the dihaploid susceptible potato genotype 'Keltia H12'. Two distinct tests showed a distorted segregation compared to the hypothetical ratio expected for a monogenic control. We hypothesised that the resistance is based on one dominant gene designated Mh and linked to the self incompatibility locus-S. Hypersensitive plants prevented juveniles from feeding, developing and reproducing. Ninety percent of invading juveniles remained undeveloped 6 weeks after inoculation. Almost all the adults were males and only very rarely were females observed. Inhibition of the development of juveniles into females showed a bimodal distribution of the genotypes: hypersensitive ones allowed almost no development of females whilst all the juveniles became females in nonhypersensitive plants. That result supports the hypothesis of a monogenic control of the resistance. The resistance was broken when plants were grown at 30 degrees C. The histology of the hypersensitive reaction was very similar to that of the Mi gene of tomato and of some Me genes of pepper. Necrotic cells were localised not only around the head of the juveniles but also surrounding their bodies. In addition, healthy cells adjacent to necrotised ones underwent divisions parallel to the juveniles. Intercellular spaces between these cells and necrotised ones enlarged markedly, which is consistent with a process leading to isolation of the necrotic area from healthy tissue.