Mutational analysis of genes determining antagonism of Alcaligenes sp. strain MFA1 against the phytopathogenic fungus Fusarium oxysporum.

Alcaligenes sp. strain MFA1 inhibits microconidial germination and germination-tube elongation of Fusarium oxysporum f.sp. dianthi and reduces the severity of fusarium wilt of carnation, presumably as a result of its production of a siderophore (G.Y. Yuen and M.N. Schroth. 1986. Phytopathology, 76:171-176). Derivative strains of MFA1, deficient in antagonism against F. oxysporum and in iron-limited growth, were obtained by Tn5 mutagenesis. The presence of a single Tn5 insertion in the genomic DNA of each derivative strain was detected by Southern analysis. Marker-exchange mutagenesis of strain MFA1 with DNA fragments, containing Tn5 and flanking sequences cloned from representative mutants, confirmed the association of single Tn5 insertions with the loss of antifungal activity and iron-independent growth of MFA1. These results are consistent with the involvement of siderophore biosynthesis by MFA1 in the inhibition of F. oxysporum.