Graduate School of Bioagricultural Sciences, Nagoya University, Chikusa, Nagoya 464-8601, Japan.
Fusarium oxysporum produces three kinds of asexual spores, microconidia, macroconidia, and chlamydospores. We previously found that the transcript level of the nitrite reductase gene of F. oxysporum, named FoNIIA, was markedly upregulated during conidiation compared with during vegetative growth. FoNIIA was also found to be positively regulated by Ren1 that is a transcription regulator controlling development of microconidia and macroconidia. In this study, we analyzed the function of FoNIIA in conidiation of F. oxysporum. Conidiation cultures showed markedly higher level of accumulation of FoNiiA protein as well as FoNIIA mRNA than vegetative growth cultures. FoNIIA protein was significantly decreased in cultures of the REN1 disruption mutant compared with that of the wild type. These results confirmed that FoNIIA expression is upregulated during conidiation and is positively regulated by REN1. The FoNIIA disruption mutants produced microconidia, macroconidia, and chlamydospores, which were morphologically indistinguishable from those of the wild type. The mutants, however, produced significantly fewer macroconidia than the wild type, although the wild type and mutant strains produced similar numbers of microconidia and chlamydospores. These results demonstrate that nitrite reductase is involved in quantitative control of macroconidium formation as well as nitrate utilization in F. oxysporum.