The G Protein Alpha Subunit Tga1 of Trichoderma Atroviride is Involved in Chitinase Formation and Differential Production of Antifungal Metabolites

Trichoderma mycoparasitism includes recognition, attack, overgrowth and lysis of the host fungus accompanied by morphological changes and secretion of hydrolytic enzymes and antibiotics. Studying the underlying signal transduction pathways, the tga1 gene encoding a Galpha subunit of Trichoderma atroviride P1 was analysed. A Deltatga1 mutant showed continuous sporulation and elevated internal steady-state cAMP levels. tga1 gene deletion resulted in a complete loss of mycoparasitic overgrowth and lysis of Rhizoctonia solani, Botrytis cinerea, and Sclerotinia sclerotiorum during direct confrontation, although infection structure formation was unaffected. The reduced mycoparasitic abilities were reflected by strongly decreased chitinase activities and reduced nag1 and ech42 gene transcription. Furthermore, production of 6-pentyl-alpha-pyrone and of metabolites with sesquiterpene structure was reduced in the Deltatga1 mutant. Regardless of these deficiencies, the mutant displayed an enhanced growth inhibition of the host fungi by over-producing other low molecular weight antifungal metabolites, suggesting opposite roles of Tga1 in regulating the biosynthesis of different antifungal substances in T. atroviride.