Nitrate Nonutilizing Mutants ofFusarium oxysporumand Their Use in Vegetative Compatibility Tests

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Abstract
Over 1,300 nitrate nonutilizing (nit) mutants were recovered from seven strains of Fusarium oxysporum cultured on two media, potato-dextrose agar or minimal agar, amended with 1.5% potassium chlorate. The mutants could be divided into three phenotypic classes by their growth on supplemented minimal agar medium. These classes presumably reflect mutations at a nitrate reductase structural locus (nit1), a nitrate-assimilation pathway-specific regulatory locus (nit3), and loci (at least five) that affect the assembly of a molybdenum-containing cofactor necessary for nitrate reductase activity (NitM). Nit mutants in each phenotypic class were recovered from each of the seven strains of F. oxysporum tested. The majority (59-96%) of nit mutants recovered were nit1 mutants. With all seven strains, nit3 and NitM mutants were recovered at a much higher frequency from minimal agar medium amended with chlorate than from potato-dextrose agar amended with chlorate. For six of the seven strains, physiological complementation occurred (as a result of heterokaryon formation) between the different nit mutant phenotypes derived from the same parental strain. Complementation also was observed between some nit mutants with the same phenotype. No complementation was observed between any of the nit mutants recovered from a strain of F. o. f. sp. melonis; this strain has been designated "heterokaryon self-incompatible". The phenotypes of complementary nit mutants (arbitrarily deisgnated nitA and nitB) recovered by other workers in three previous studies also were determined. Neither the nitA nor the nitB mutants corresponded to a particular mutant phenotype; all three phenotypes were found among the nitA and nitB mutants. With the exception of the heterokaryon self-incompatible strain, all nit1 and nit3 mutants from an individual strain readily complemented the NitM mutants derived from the same strain. The NitM mutants form heterokaryons rapidly and reliably and should be used as one of the nit mutant testers to identify the vegetative compatibility group to which isolates of F. oxysporum belong. Several procedures should expedite the screening of natural populations of F. oxysporum for vegetative compatibility.