Chickpea (Cicer arietium L.) produces the antimicrobial compounds (
phytoalexins)
medicarpin and
maackiain in response to
infection by microorganisms. Nectria haematococca mating population (MP) VI, a fungus pathogenic on chickpea, can metabolize
maackiain and
medicarpin to less toxic products. These reactions are thought to be detoxification mechanisms in N. haematococca MP VI and required for pathogenesis by this fungus on chickpea. In the present study, these hypotheses were tested by examining the phenotypes of progeny from crosses of the fungus that segregated for genes (Mak genes) controlling phytoalexin metabolism. Mak1 and Mak2, two genes that individually confer the ability to convert
maackiain to its 1a-hydroxydienone derivative, were linked to higher tolerance of the
phytoalexins and high virulence on chickpea. These results indicate that this metabolic reaction is a mechanism for increased phytoalexin tolerance in the fungus, which thereby allows a higher virulence on chickpea. Mak3, a gene conferring the ability to convert
maackiain to its
6a-hydroxypterocarpan derivative, also increased tolerance to
maackiain in strains which carried it; however, the contribution of Mak3 to the overall level of pathogenesis could not be evaluated because most progeny from the cross segregating for this gene were low in virulence. Thus, metabolic detoxification of
phytoalexins appeared to be necessary, as demonstrated in the Mak1 and Mak2 crosses, but not sufficient by itself, as in the Mak3 cross, for high virulence of N. haematococca MP VI on chickpea.