MVE1, Encoding the Velvet Gene Product Homolog in Mycosphaerella graminicola, Is Associated with Aerial Mycelium Formation, Melanin Biosynthesis, Hyphal Swelling, and Light Signaling

The ascomycete fungus Mycosphaerella graminicola is an important pathogen of wheat that causes Septoria tritici blotch. Despite the serious impact of M. graminicola on wheat production worldwide, knowledge about its molecular biology is limited. The velvet gene, veA, is one of the key regulators of diverse cellular processes, including development and secondary metabolism in many fungi. However, the species analyzed to date are not related to the Dothideomycetes, the largest class of plant-pathogenic fungi, and the function of veA in this group is not known. To test the hypothesis that the velvet gene has similar functions in the Dothideomycetes, a veA-homologous gene, MVE1, was identified and gene deletion mutations (Delta mve1) were generated in M. graminicola. All of the MVE1 mutants exhibited consistent pleiotropic phenotypes, indicating the involvement of MVE1 in multiple signaling pathways. Delta mve1 strains displayed albino phenotypes with significant reductions in melanin biosynthesis and less production of aerial mycelia on agar plates. In liquid culture, Delta mve1 strains showed abnormal hyphal swelling, which was suppressed completely by osmotic stress or lower temperature. In addition, MVE1 gene deletion led to hypersensitivity to shaking, reduced hydrophobicity, and blindness to light-dependent stimulation of aerial mycelium production. However, pathogenicity was not altered in Delta mve1 strains. Therefore, the light-signaling pathway associated with MVE1 does not appear to be important for Septoria tritici blotch disease. Our data suggest that the MVE1 gene plays crucial roles in multiple key signaling pathways and is associated with light signaling in M. graminicola.