Acropetal: a genetic locus required for conidiophore architecture and pathogenicity in the rice blast fungus

Fungal spores are a primary means of dissemination and are the major sources of inoculum in pathogenic species. Sporulation in the rice blast fungus Magnaporthe grisea involves the production of three-celled conidia, borne sympodially on an aerial conidiophore. A disease cycle initiates when spores are dispersed and attach to the rice plant surface. Using insertional mutagenesis we have identified a major regulator of conidiophore morphogenesis in M. grisea. A null mutation in the acropetal (ACR1) locus causes a hypermorphic conidiation phenotype where indeterminate growth of the conidial tip cell results in the production of head-to-tail (acropetal) arrays of spores. acropetal mutants are nonpathogenic and fail to undergo infection-related morphogenesis. The ACR1 locus encodes a spore-specific transcript and acr1(-) mutants fail to turn off the expression of the hydrophobin encoding gene MPG1 in dormant spores. We propose that ACR1 is a stage-specific negative regulator of conidiation that is required to establish a sympodial pattern of spore formation. Interestingly a failure to establish the correct pattern of sporulation in M. grisea results in the production of spores that cannot progress through the disease cycle. Studies of Acropetal suggest that the diverse patterns of spore ontogeny in conidial fungi arose through alterations in major genes controlling spore-specific gene expression.