PH governs most, if not all, processes of life. In fungi, ambient pH acts as a potent regulator of growth and development [1]. Studies conducted primarily in the 2 model organisms Saccharomyces cerevisiae and Aspergillus nidulans have cemented our understanding of how fungi sense and respond to pH. More recently, pH has emerged as a key player in the control of fungal pathogenicity. Infections caused by fungi are often associated with a pH shift in the surrounding host tissue [2±4]. Extracellular alkalinization contributes to fungal virulence, but the underlying mechanisms are not fully understood. Recent studies have revealed new and unexpected ways by which fungi induce host alkalinization to increase their infectious potential. Here, we provide a brief overview of the mechanisms that govern pH signaling in fungi and highlight how recent findings have advanced our understanding of pathogen-induced alkalinization and its role during infection. We also discuss the emerging view that intracellular pH (pHi) acts as a master switch to govern fungal development and pathogenicity

Summary: Soil‐inhabiting fungal pathogens use chemical signals to locate and colonise the host plant. In the vascular wilt fungus Fusarium oxysporum, hyphal chemotropism towards tomato roots is triggered by secreted plant peroxidases (Prx), which catalyse the reductive cleavage of reactive oxygen species (ROS). Here we show that this chemotropic response requires the regulated synthesis of ROS by the conserved fungal NADPH oxidase B (NoxB) complex, and their transformation into hydrogen peroxide (H₂O₂) by superoxide dismutase (SOD). Deletion of NoxB or the regulatory subunit NoxR, or pharmacological inhibition of SOD, specifically abolished chemotropism of F. oxysporum towards Prx gradients. Addition of isotropic concentrations of H₂O₂ rescued chemotropic growth in the noxBΔ and noxRΔ mutants, but not in a mutant lacking the G protein‐coupled receptor Ste2. Prx‐triggered rapid Nox‐ and Ste2‐dependent phosphorylation of the cell wall integrity mitogen‐activated protein kinase (CWI MAPK) Mpk1, an essential component of the chemotropic response. These results suggest that Ste2 and the CWI MAPK cascade function downstream of NoxB in Prx chemosensing. Our findings reveal a new role for Nox enzymes in directed hyphal growth of a filamentous pathogen towards its host and might be of broad interest for chemotropic interactions between plants and root‐colonising fungi.

Acetic acid is mostly known as a toxic by-product of alcoholic fermentation carried out by Saccharomyces cerevisiae, which it frequently impairs. The more recent finding that acetic acid triggers apoptotic programmed cell death (PCD) in yeast sparked an interest to develop strategies to modulate this process, to improve several biotechnological applications, but also for biomedical research. Indeed, acetate can trigger apoptosis in cancer cells, suggesting its exploitation as an anticancer compound. Therefore, we aimed to identify genes involved in the positive and negative regulation of acetic acid-induced PCD by optimizing a functional analysis of a yeast Euroscarf knock-out mutant collection. The screen consisted of exposing the mutant strains to acetic acid in YPD medium, pH 3.0, in 96-well plates, and subsequently evaluating the presence of culturable cells at different time points. Several functional categories emerged as greatly relevant for modulation of acetic acid-induced PCD (e.g.: mitochondrial function, transcription of glucose-repressed genes, protein synthesis and modifications, and vesicular traffic for protection, or amino acid transport and biosynthesis, oxidative stress response, cell growth and differentiation, protein phosphorylation and histone deacetylation for its execution). Known pro-apoptotic and anti-apoptotic genes were found, validating the approach developed. Metabolism stood out as a main regulator of this process, since impairment of major carbohydrate metabolic pathways conferred resistance to acetic acid-induced PCD. Among these, lipid catabolism arose as one of the most significant new functions identified. The results also showed that many of the cellular and metabolic features that constitute hallmarks of tumour cells (such as higher glycolytic energetic dependence, lower mitochondrial functionality, increased cell division and metabolite synthesis) confer sensitivity to acetic acid-induced PCD, potentially explaining why tumour cells are more susceptible to acetate than untransformed cells and reinforcing the interest in exploiting this acid in cancer therapy. Furthermore, our results clearly establish a connection between cell proliferation and cell death regulation, evidencing a conserved developmental role of programmed cell death in unicellular eukaryotes. This work advanced the characterization of acetic acid-induced PCD, providing a wealth of new information on putative molecular targets for its control with impact both in biotechnology and biomedicine.

Fungal phytopathogens cause devastating losses in global agriculture. All plant-infecting fungi use conserved MAPK signaling pathways to successfully locate, enter, and colonize their hosts.

Like many hemibiotrophic plant pathogens, the root-infecting vascular wilt fungus Fusarium oxysporum induces an increase in the pH of the surrounding host tissue. How alkalinization promotes fungal infection is not fully understood, but recent studies point towards the role of cytosolic pH (pH_c) and mitogen-activated protein kinase (MAPK) signaling. In fungi, pH_c is mainly controlled by the essential plasma membrane H⁺-ATPase Pma1. Here we created mutants of F. oxysporum lacking casein kinase 1 (Ck1), a known negative regulator of Pma1. We found that the ck1Δ mutants have constitutively high Pma1 activity and exhibit reduced alkalinization of the surrounding medium as well as decreased hyphal growth and conidiation. Importantly, the ck1Δ mutants exhibit defects in hyphal chemotropism towards plant roots and in pathogenicity on tomato plants. Thus, Ck1 is a key regulator of the development and virulence of F. oxysporum.