The secreted lipase FGL1 is sufficient to restore the initial infection step to the apathogenic Fusarium graminearum MAP kinase disruption mutant Δgpmk1

Mitogen activated protein (MAP) kinases are key components in signalling networks. In Fusarium graminearum, one of the most devastating fungal plant pathogens, two MAP kinases are known to be involved in pathogenicity, Mgv1 and the Gibberella pathogenicity MAP kinase Gpmk1. Delta gpmk1 mutants with a disrupted GPMK1 gene are unable to infect wheat spikelets. They exhibit altered secretion of several extracellular enzymes. Among those, the lipase FGL1 is known to be a major virulence factor of F. graminearum. FGL1 gene expression was decreased in Delta gpmk1 strains during wheat head infection. To uncouple FGL1 expression from Gpmk1 activity, we generated Delta gpmk1 strains that constitutively express FGL1 under control of the Cochliobolus heterostrophus glyceraldehyde 3-phosphate dehydrogenase (GPD) promoter. The level of extracellular lipolytic activity of these strains in culture was comparable to the wild-type. These mutants showed fully restored conidiation and partial complementation of defects in development that were reported from the Delta gpmk1 mutant. They also partially complemented the apathogenic disease phenotype of the Delta gpmk1 mutants causing lesions in directly inoculated wheat spikelets. But in contrast to wild-type, their growth was restricted to directly inoculated spikelets. A barrier-like formation was observed at the rachis node. Based on these results, we could show that the lipase FGL1 is necessary but not sufficient to restore complete pathogenicity to the apathogenic Delta gpmk1 mutant. Hence, we hypothesize that the MAP kinase Gpmk1 is involved in the regulation of additional factors required for complete virulence of F. graminearum.