A G alpha subunit gene is essential for conidiation and potassium efflux but dispensable for pathogenicity of Alternaria alternata on citrus.

Heterotrimeric G proteins play a profound role in the recognition and transduction of extracellular signals in eukaryotic cells. We characterized the AaG alpha1 gene, encoding a fungal Class I G alpha subunit of the GTP-binding protein, in Alternaria alternata of citrus. Interruption of AaG alpha1 with a marker gene resulted in fungal transformants producing fewer conidia, becoming hypersensitive to KCl, and displaying elevated lipolytic and pectolytic activities. Expression of a functional copy of AaG alpha1 in a null mutant restored all altered phenotypes to the wild type. The AaG alpha1 mutants, whose conidia germinate normally, caused necrotic lesions on citrus indistinguishable from wild type. Application of cAMP, its inhibitors (atropine and theophylline), or 3-isobutyl-1-methylxanthine (IBMX) decreased conidiation in the fungal strains carrying a functional AaG alpha1. In contrast, conidial formation in the null mutants was restored by dibutyryl-cAMP or by a low concentration of cAMP or theophylline (1 mM). Unlike the oxidative stress-responsive AaAP1 transcription activator or the AaFUS3 mitogen-activated protein kinase (MAPK), AaG alpha1 is not required for cellular resistance to oxidative, osmotic, or chemical stress. AaFUS3 has also been demonstrated to be essential for conidial formation, suggesting a possible interaction between AaG alpha1 and AaFUS3 during conidiation even though expression of AaG alpha1 was not affected by AaFUS3 and vice versa. Inactivation of AaG alpha1 suppressed accumulation of transcripts of the AaAP1 gene and the AaHSK1 gene encoding a histidine kinase. These are novel features that have not been previously characterized to be associated with the GTP-binding protein.