MAACKIAIN DETOXIFICATION CONTRIBUTES TO THE VIRULENCE OF NECTRIA-HAEMATOCOCCA MP-VI ON CHICKPEA

Nectria haematococca mating population (MP) VI isolates that contain t he MAK1 gene are able to degrade maackiain, a chickpea (Cicer arietinu m) phytoalexin, to a less toxic compound, To test the contribution of MAK1 to the virulence of N. haematococca MP VI on chickpea, the MAK1 g ene was disrupted in a highly virulent Mak(+) isolate or added to a we akly virulent Ma(-) isolate via transformation, The disruption of MAK1 decreased virulence to a moderate level, while addition of multiple c opies of MAK1 increased virulence to either a moderate or a high level , These data demonstrate that maackiain detoxification is a determinan t, but not the only determinant, of virulence in N. haematococca MP VI isolates capable of causing disease on chickpea, MAK1 is located on a 1.6-Mb conditionally dispensable chromosome, To ascertain if there ar e additional genes influencing virulence toward chickpea stems on the MAK1 chromosome, the loss of this chromosome was chemically induced in an isolate containing the disrupted MAK1 gene, Loss of the MAK1 chrom osome did not reduce virulence toward chickpea stems further, thus ind icating that no additional genes for virulence on this part of the hos t plant are located on the MAK1 chromosome.