Functional characterization of Rho family small GTPases in Fusarium graminearum.

Rho GTPases have multiple cellular and metabolic functions, including vesicular trafficking and pathogenesis, as signaling molecules in fungi. Wheat scab, caused by Fusarium graminearum, is one of the most important wheat diseases worldwide, yet the mechanisms associated with making this fungus such a devastating pathogen remain largely ambiguous. In an effort to better understand F. graminearum virulence, we functionally characterized all six Rho GTPases in F. graminearum. FgRHO1 was determined to be essential for fungal survival, while FgRho3 demonstrated functions only in vegetative growth and conidiation. Other four Rho GTPases, FgRho2, FgRho4, FgCdc42 and FgRac1, were multifunctional and were involved in sexual development and pathogenesis. While FgRho2 and FgRho4 were both involved in cell wall integrity, only FgRho4 showed a role in nuclear division and septum formation. FgRho4, FgCdc42 and FgRac1 were also important for hyphal growth and conidiation. All single deletion mutants showed impaired growth, particularly in conidial morphology, when compared to the wild-type progenitor. FgRac1 deletion mutants displayed a precocious, multi-site germ tube formation as well as hyperbranching of hyphae. Significantly we learned that FgRac1 negatively controls DON production whereas FgRho4 plays a positive role. FgCla4 was identified as a downstream target of FgRac1, but was dispensable for sexual development. We determined that FgRho GTPases contribute diversely to growth, conidiogenesis, sexual reproduction, DON production and pathogenesis in F. graminearum.