A MAP kinase of the vascular wilt fungus Fusarium oxysporum is essential for root penetration and pathogenesis

The soil-borne vascular wilt fungus Fusarium oxysporum infects a wide varie ty of plant species by directly penetrating roots, invading the cortex and colonizing the vascular tissue. We have identified fmk1, encoding a mitogen -activated protein kinase (MAPK) of F. oxysporum that belongs to the yeast and fungal extracellular signal-regulated kinase (YERK1) subfamily. Targete d mutants of F. oxysporum f. sp. lycopersici carrying an inactivated copy o f fmk1 have lost pathogenicity on tomato plants but show normal vegetative growth and conidiation in culture. Colonies of the fmk1 mutants are easily wettable, and hyphae are impaired in breaching the liquid-air interface, su ggesting defects in surface hydrophobicity. Fmk1 mutants also show reduced invasive growth on tomato fruit tissue and drastically reduced transcript l evels of pl1 encoding the cell wall-degrading enzyme pectate lyase. Conidia of the mutants germinating in the tomato rhizosphere fail to differentiate penetration hyphae, resulting in greatly impaired root attachment. The ort hologous MAPK gene Pmk1 from the rice leaf pathogen Magnaporthe grisea comp lements invasive growth and partially restores surface hydrophobicity, root attachment and pathogenicity in an fmk1 mutant. These results demonstrate that FMK1 controls several key steps in the pathogenesis of F. oxysporum an d suggest a fundamentally conserved role for the corresponding MAPK pathway in soil-borne and foliar plant pathogens.