Cytosolic pH controls fungal MAPK signaling and pathogenicity
In fungi, ambient pH acts as a key regulator of development and virulence. The vascular wilt pathogen Fusarium oxysporum uses host alkalinization to promote infection of plant hosts through activation of the invasive growth mitogen-activated protein kinase (MAPK) Fmk1. The molecular events underlying pH-driven MAPK regulation are unknown. Using the ratiometric GFP-based pH sensor pHluorin, we find that both F. oxysporum and Saccharomyces cerevisiae respond to extracellular alkalinization or acidification with a transitory shift in cytosolic pH (pHc) and rapid changes in phosphorylation levels of the three fungal MAPKs Fmk1, Mpk1/Slt2 (cell wall integrity) and Hog1 (hyperosmotic stress). Pharmacological inhibition of the essential plasma membrane H+-ATPase Pma1, which leads to pHc acidification, is sufficient to trigger reprogramming of MAPK phosphorylation even in the absence of an extracellular pH shift. Screening of a subset of S. cerevisiae mutants identified the sphingolipid-regulated AGC kinase Ypk1/2 as a key upstream component of pHc-modulated MAPK responses. We further show that acidification of pHc in F. oxysporum leads to an increase of the long chain base (LCB) sphingolipid dihydrosphingosine (dhSph) and that exogenous addition of dhSph activates Mpk1 phosphorylation. Our results reveal a pivotal role of pHc in the regulation of MAPK signaling and suggest new ways to control fungal growth and pathogenicity.