G-Protein ß Subunit of Cochliobolus heterostrophusInvolved in Virulence, Asexual and Sexual Reproductive Ability, and Morphogenesis

ABSTRACTPrevious work established that mutations in mitogen-activated protein (MAP) kinase (CHK1) and heterotrimeric G-protein a (Ga) subunit (CGA1) genes affect the development of several stages of the life cycle of the maize pathogen Cochliobolus heterostrophus. The effects of mutating a third signal transduction pathway gene, CGB1, encoding the Gß subunit, are reported here. CGB1is the sole Gß subunit-encoding gene in the genome of this organism. cgb1mutants are nearly wild type in vegetative growth rate; however, Cgb1 is required for appressorium formation, female fertility, conidiation, regulation of hyphal pigmentation, and wild-type virulence on maize. Young hyphae of cgb1mutants grow in a straight path, in contrast to those of the wild type, which grow in a wavy pattern. Some of the phenotypes conferred by mutations in CGA1are found in cgb1mutants, suggesting that Cgb1 functions in a heterotrimeric G protein; however, there are also differences. In contrast to the deletion of CGA1, the loss of CGB1is not lethal for ascospores, evidence that there is a Gß subunit-independent signaling role for Cga1 in mating. Furthermore, not all of the phenotypes conferred by mutations in the MAP kinase CHK1gene are found in cgb1mutants, implying that the Gß heterodimer is not the only conduit for signals to the MAP kinase CHK1module. The additional phenotypes of cgb1mutants, including severe loss of virulence on maize and of the ability to produce conidia, are consistent with CGB1being unique in the genome. Fluorescent DNA staining showed that there is often nuclear degradation in mature hyphae of cgb1mutants, while comparable wild-type cells have intact nuclei. These data may be genetic evidence for a novel cell death-related function of the Gß subunit in filamentous fungi.