Gα and Gβ Proteins Regulate the Cyclic AMP Pathway That Is Required for Development and Pathogenicity of the Phytopathogen Mycosphaerella graminicola

We identified and functionally characterized genes encoding three Gα proteins and one Gβ protein in the dimorphic fungal wheat pathogenMycosphaerella graminicola , which we designatedMgGpa1 ,MgGpa2 ,MgGpa3 , andMgGpb1 , respectively. Sequence comparisons and phylogenetic analyses showed that MgGPA1 and MgGPA3 are most related to the mammalian Gαi and Gαs families, respectively, whereas MgGPA2 is not related to either of these families. On potato dextrose agar (PDA) and in yeast glucose broth (YGB),MgGpa1 mutants produced significantly longer spores than those of the wild type (WT), and these developed into unique fluffy mycelia in the latter medium, indicating that this gene negatively controls filamentation.MgGpa3 mutants showed more pronounced yeast-like growth accompanied with hampered filamentation and secreted a dark-brown pigment into YGB. Germ tubes emerging from spores ofMgGpb1 mutants were wavy on water agar and showed a nested type of growth on PDA that was due to hampered filamentation, numerous cell fusions, and increased anastomosis. Intracellular cyclic AMP (cAMP) levels ofMgGpb1 andMgGpa3 mutants were decreased, indicating that both genes positively regulate the cAMP pathway, which was confirmed because the WT phenotype was restored by adding cAMP to these mutant cultures. The cAMP levels inMgGpa1 mutants and the WT were not significantly different, suggesting that this gene might be dispensable for cAMP regulation. In planta assays showed that mutants ofMgGpa1 ,MgGpa3 , andMgGpb1 are strongly reduced in pathogenicity. We concluded that the heterotrimeric G proteins encoded byMgGpa3 andMgGpb1 regulate the cAMP pathway that is required for development and pathogenicity inM. graminicola .