A Novel Gene, ROA, Is Required for Normal Morphogenesis and Discharge of Ascospores in Gibberella zeae

Head blight, caused by Gibberella zeae, is a significant disease among cereal crops, including wheat, barley, and rice, due to contamination of grain with mycotoxins. G. zeae is spread by ascospores forcibly discharged from sexual fruiting bodies forming on crop residues. In this study, we characterized a novel gene, ROA, which is required for normal sexual development. Deletion of ROA (Delta roa) resulted in an abnormal size and shape of asci and ascospores but did not affect vegetative growth. The Delta roa mutation triggered round ascospores and insufficient cell division after spore delimitation. The asci of the Delta roa strain discharged fewer ascospores from the perithecia but achieved a greater dispersal distance than those of the wild-type strain. Turgor pressure within the asci was calculated through the analysis of osmolytes in the epiplasmic fluid. Deletion of the ROA gene appeared to increase turgor pressure in the mutant asci. The higher turgor pressure of the Delta roa mutant asci and the mutant spore shape contributed to the longer distance dispersal. When the Delta roa mutant was outcrossed with a Delta mat1-2 mutant, a strain that contains a green fluorescence protein (GFP) marker in place of the MAT1-2 gene, unusual phenotypic segregation occurred. The ratio of GFP to non-GFP segregation was 1: 1; however, all eight spores had the same shape. Taken together, the results of this study suggest that ROA plays multiple roles in maintaining the proper morphology and discharge of ascospores in G. zeae.