The use of amino acid fungal auxotrophs to study the predisposition phenomena in the root-knot-wilt fungus disease complex of tomato

Abstract

Root-knot nematode ( Meloidogyne incognita) infections of tomato plants resistant to the wilt fungus ( Fusarium oxysporum f. sp. lycopersici) often predispose the tomato to this fungus. The rôle of free amino acids, which are abundant in nematode-galled tissue, in the predisposition of a resistant host to the fungus was investigated. Amino acid levels in the exudates of healthy and nematode-galled tomato cultivars were determined. Single step amino acid mutants (auxotrophs) were induced in the wild type (race 1) wilt fungus. Those auxotrophs that lost their relative pathogenicity as compared to the wild type were selected for the study of the influence of increased or decreased levels of the corresponding free amino acids in the exudates of host cultivars. Auxotrophs requiring threonine, proline, methionine, histidine and glycine were pathogenic only when the relevant amino acid was present above a certain threshold level in the exudates of genetically susceptible, nematode-galled cultivars. However, these caused negligible pathogenicity in nematode-inoculated cultivars that are genetically fungal resistant. A similar trend in pathogenicity was observed when threonine, proline, methionine and histidine were applied exogenously to the tomato cultivars before inoculation with the fungal auxotrophs. The rôle of these and other amino acid auxotrophs are discussed in the light of the nutritional hypotheses of Lewis and Garber [ 12, 18]. These experiments show that amino acids may play a significant role in predisposing fungal-resistant plants in a disease complex.