Functional analysis of the pH responsive pathway Pal/Rim in the phytopathogenic basidiomycete Ustilago maydis

The most important mechanism for fungal response to the environmental pH is the Rim or Pal pathway. Details on its operation are known through the analysis of ascomycete fungi. In this study we analyzed whether this pathway is conserved in a basidiomycete, Ustilago maydis. We could identify only five homologues of the seven known components of the pathway in the U. maydis as well as in other basidiomycete genomes. We determined that only genes encoding Rim20/PalA, Rim13/PalB and Rim23/PalC, that constitute the endosomal membrane complex, and Rim9/PalI of the complex located at the plasma membrane are conserved, but this latter lacked a detectable role in signal transduction. Mutants in this pathway showed a pleiotropic phenotype, but dimorphism and virulence were not affected. Our data reveal that the Rim/Pal pathway is conserved in basidiomycetes, but with notable differences to the ascomycete systems.