PacC-dependent adaptation and modulation of host cellular pH controls hemibiotrophic invasive growth and disease development by the rice blast fungus

Abstract

Many of the worlds most serious crop diseases are caused by hemibiotrophic fungi. These pathogens have evolved the ability to colonize living plant cells, suppressing plant immunity responses, before switching to necrotrophic growth, in which host cells die, providing the energy to fuel sporulation and spread of the fungus. How hemibiotrophic pathogens switch between these two lifestyles remains poorly understood. Here, we report that the devastating rice blast fungus, Magnaporthe oryzae, manipulates host cellular pH to regulate hemibiotrophy. During infection by M. oryzae, host plant cells are alkalinized to pH 7.8 during biotrophic growth, but later acidified to pH 6.5 during necrotrophy. Using a forward genetic screen, we identified alkaline-sensitive mutants of M. oryzae that were blocked in biotrophic proliferation and impaired in induction of host cell acidification and necrotrophy. These mutants defined components of the PacC-dependent ambient pH signal transduction pathway in M. oryzae. We report that PacC exists as a full-length repressor, PacC559, and a truncated transcriptional activator, PacC222, which localize to the fungal nucleus during biotrophic growth and to the cytoplasm during necrotrophy. During biotrophy, PacC222 directly activates genes associated with nutrient acquisition and fungal virulence, while PacC559 represses genes associated with saprophytic mycelial growth and sporulation, which are subsequently de-repressed during necrotrophy. When considered together, our results indicate that temporal regulation of hemibiotrophy by M. oryzae requires PacC-dependent sensing and manipulation of host cellular pH. Author SummaryCrop diseases caused by fungi represent some of the most serious threats to global food security. Many fungal pathogens have evolved the ability to invade living plant tissue and suppress host immunity, before switching to a completely different mode of growth, in which they are able to kill host plant cells. This lifestyle- called hemibiotrophy -is exemplified by the blast fungus, Magnaporthe oryzae, which causes devastating diseases of rice, wheat and many other grasses. We found that during infection by M. oryzae, host cells initially have an alkaline pH, when the fungus is growing in living tissue, but pH rapidly becomes acidic, as host tissue is killed. We identified mutants of the blast fungus that were sensitive to alkaline pH and this enabled us to identify the signal transduction pathway by which the fungus responds to changes in ambient pH. We found that mutants in the pH response pathway were blocked in invasive fungal growth and could not cause acidification of host tissue. Consequently, they are unable to cause blast disease. We characterized the central regulator of this pathway, the PacC transcription factor, which unusually can act as both a repressor and an activator of fungal gene expression. During biotrophic invasive growth, PacC activates many genes previously reported to be required for virulence, including several associated with nutrient acquisition, and at the same time represses genes associated with vegetative growth and sporulation. The PacC signaling pathway is therefore necessary for regulating the switch in fungal lifestyle associated with causing blast disease.