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

Departamento de Genética,¹ Departamento de Microbiología, Universidad de Córdoba, Córdoba, Spain²

Received 15 July 2003/ Accepted 1 October 2003

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 pacC^+/- loss-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.

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