Membrane Mucin Msb2 Regulates Invasive Growth and Plant Infection in Fusarium oxysporum.

Fungal pathogenicity in plants requires a conserved mitogen-activated protein kinase (MAPK) cascade homologous to the yeast filamentous growth pathway. How this signaling cascade is activated during infection remains poorly understood. In the soil-borne vascular wilt fungus Fusarium   oxysporum , the orthologous MAPK Fmk1 (Fusarium MAPK1) is essential for root penetration and pathogenicity in tomato (Solanum lycopersicum) plants. Here, we show that Msb2, a highly glycosylated transmembrane protein, is required for surface-induced phosphorylation of Fmk1 and contributes to a subset of Fmk1-regulated functions related to invasive growth and virulence. Mutants lacking Msb2 share characteristic phenotypes with the Δ fmk1 mutant, including defects in cellophane invasion, penetration of the root surface, and induction of vascular wilt symptoms in tomato plants. In contrast with Δ fmk1 , Δ msb2 mutants were hypersensitive to cell wall targeting compounds, a phenotype that was exacerbated in a Δ msb2 Δ fmk1 double mutant. These results suggest that the membrane mucin Msb2 promotes invasive growth and plant infection upstream of Fmk1 while contributing to cell integrity through a distinct pathway. [ABSTRACT FROM AUTHOR]