ACROPETAL - A GENETIC-LOCUS REQUIRED FOR CONIDOPHORE ARCHITECTURE ANDPATHOGENICITY IN THE RICE BLAST FUNGUS

Fungal spores are a primary means of dissemination and are the major s ources of inoculum in pathogenic species. Sporulation in the rice blas t fungus Magnaporthe grisea involves the production of three-celled co nidia, borne sympodially on an aerial conidiophore. A disease cycle in itiates when spores are dispersed and attach to the rice plant surface . Using insertional mutagenesis we have identified a major regulator o f conidiophore morphogenesis in M. grisea. A null mutation in the acro petal (ACR1) locus causes a hypermorphic conidiation phenotype where i ndeterminate growth of the conidial tip cell results in the production of head-to-tail (acropetal) arrays of spores. acropetal mutants are n onpathogenic and fail to undergo infection-related morphogenesis. The ACR1 locus encodes a spore-specific transcript and acr1-mutants fail t o turn off the expression of the hydrophobin encoding gene MPG1 in dor mant spores. We propose that ACR1 is a stage-specific negative regulat or of conidiation that is required to establish a sympodial pattern of spore formation. Interestingly a failure to establish the correct pat tern of sporulation in M. grisea results in the production of spores t hat cannot progress through the disease cycle. Studies of Acropetal su ggest that the diverse patterns of spore ontogeny in conidial fungi ar ose through alterations in major genes controlling spore-specific gene expression. (C) 1998 Academic Press.