pH Response Transcription Factor PacC Controls Salt Stress Tolerance and Expression of the P-Type Na ⁺ -ATPase Ena1 in Fusarium oxysporum

Abstract

Fungi possess efficient mechanisms of pH and ion homeostasis, allowing them to grow over a wide range of environmental conditions. In this study, we addressed the role of the pH response transcription factor PacC in salt tolerance of the vascular wilt pathogen Fusarium oxysporum. Loss-of-function pacC ؉/؊ mutants showed increased sensitivity to Li ؉ and Na ؉ and accumulated higher levels of these cations than the wild type. In contrast, strains expressing a dominant activating pacC c allele were more salt tolerant and had lower intracellular Li ؉ and Na ؉ concentrations. Although the kinetics of Li ؉ influx were not altered by mutations in pacC, we found that Li ؉ efflux at an alkaline, but not at an acidic, ambient pH was significantly reduced in pacCloss-of-function mutants. To explore the presence of a PacC-dependent efflux mechanism in F. oxysporum, we cloned ena1 encoding an orthologue of the yeast P-type Na ؉ -ATPase ENA1. Northern analysis revealed that efficient transcriptional activation of ena1 in F. oxysporum required the presence of high Na ؉ concentrations and alkaline ambient pH and was dependent on PacC function. We propose a model in which PacC controls ion homeostasis in F. oxysporum at a high pH by activating expression of ena1 coordinately with a second Na ؉ -responsive signaling pathway.Fungi are a versatile class of organisms that have successfully occupied numerous ecological niches, including those of plant and animal pathogenesis. A striking property of fungi and a major determinant of their evolutionary success is their capacity to adapt to an extremely wide range of environmental conditions. This ability depends crucially on the presence of cellular sensory networks that monitor the environment and mediate changes in gene expression in response to shifts in the external conditions. We use the vascular wilt pathogen Fusarium oxysporum as a model to understand how environmental signals regulate gene expression in fungi and how these regulatory mechanisms determine fungal virulence.A key factor in fungal growth and development is ambient pH. Fungi grow over a wide range of pH conditions and must thus be able to tailor gene expression to the particular pH of their growth environment. A conserved signaling cascade integrated by the products of the pal genes, whose terminal component is the zinc-finger transcription factor PacC/ Rim101p, regulates gene expression in response to ambient pH (18). Upon shift to alkaline pH, an inactive PacC precursor is posttranscriptionally activated by proteolytic processing into a shorter functional form that activates genes preferentially expressed at alkaline pH and represses genes expressed under acidic growth conditions (18). The pacC orthologue of F. oxysporum was recently cloned, and the encoded protein was shown to regulate pH-dependent gene expression and to function as a negative regulator of virulence on plants (11). Thus, pacC ϩ/Ϫ loss-of-function mutants of F. oxysporum mimic growth at acidic ambient pH and exhibit increased virulence, whereas pac...

The RIM101/pacC Homologue from the Basidiomycete Ustilago maydis Is Functional in Multiple pH-Sensitive Phenomena

“…This pathway, named Pal/Rim, involves the action of different proteins that convey the pH stimulus to a key zinc finger transcription factor, PacC (Rim101), which activates transcription of genes induced at alkaline pH and represses genes induced at acid pH (8,15,26,44). Further studies proved the wide distribution of similar (although not identical) pathways in other ascomycetes and deuteromycetes (9,13,25,34,43). These studies have demonstrated that the function of PacC/Rim101 is more complex than simply regulating the transcription of pH-dependent specific enzymes.…”

“…These studies have demonstrated that the function of PacC/Rim101 is more complex than simply regulating the transcription of pH-dependent specific enzymes. Additional processes in which PacC/Rim101 plays a regulatory role include meiosis, morphogenesis, and salt stress tolerance (9,23,36,43; reviewed in references 32 and 33).…”

Ambient pH signalling in the yeast Yarrowia lipolytica involves YlRim23p/PalC, which interacts with Snf7p/Vps32p, but does not require the long C terminus of YlRim9p/PalI

“…It is required for the hyphal formation at alkaline pH in C. albicans, in particular by repressing NRG1 expression in a pHdependent way (El Barkani et al, 2000;Lotz et al, 2004). Moreover, the resistance of cells to high levels of Ca 2+ and Li + requires activity of the Rim pathway, and the subset of ESCRT components needed for Rim101p processing in S. cerevisiae (Lamb & Mitchell, 2003;Platara et al, 2006), in C. albicans (Kullas et al, 2007) and in Fusarium oxysporum (Caracuel et al, 2003). In addition, the Rim pathway participates directly in cell-wall assembly, and acts in parallel with the protein kinase C (PKC) pathway in S. cerevisiae (Castrejon et al, 2006).…”

Expression and promoter characterization of BbPacC, a pH response transcription factor gene of the entomopathogenic fungus Beauveria bassiana

“…Thus, it is also upregulated in response to alkaline conditions (Tilburn et al, 1995). In addition, PacC in some filamentous fungi was found to be involved in salt resistance (Caracuel et al, 2003). BbPacC was induced by both alkaline conditions and salt stress (Fig.…”

“…1b). In Fusarium oxysporum, PacC was found to be necessary for salt resistance (Caracuel et al, 2003). This prompted us to investigate the response of BbPacC to salt stress, and the results show that in addition to an alkaline pH, BbPacC was also induced by salt stress (Fig.…”