Fusarium oxysporum G-protein β subunit Fgb1 regulates hyphal growth, development, and virulence through multiple signalling pathways

The vascular wilt fungus Fusarium oxysporum causes disease in a wide variety of crops. A signalling cascade controlled by the extracellular-regulated mitogen-activated protein kinase (MAPK) Fmk1 was previously found to be required for plant infection. To investigate the role of the heterotrimeric G-protein β subunit Fgb1 as a putative upstream component of the Fmk1 signalling cascade, we generated F. oxysporum strains carrying either a Δfgb1 loss-of-function allele or an fgb1W115G allele that mimicks the yeast STE4W136G mutation resulting in insensitivity to the cognate G-protein α subunit. Both types of mutants showed reduced virulence on tomato plants, similar to Δfmk1 strains. However, in contrast to the latter, Δfgb1 mutants displayed an abnormal hyphal growth phenotype with highly elongated cells, increased tip growth, a completely straight hyphal growth axis, and reduced subapical branching. Exogenous cAMP reversed part but not all of the Δfgb1 growth phenotypes. Likewise, expression of the fgb1W115G allele only partly reversed growth phenotypes and failed to restore virulence on plants, whereas reintroduction of a functional fgb1 allele fully restored the wild type phenotype. Immunoblot analysis showed that levels of Fmk1 phosphorylation in fgb1 mutants were comparable to those in the wild type strain. Our results support a model in which Fgb1 controls hyphal growth, development and virulence in F. oxysporum both through cAMP-dependent and -independent pathways.