Rax2 is important for directional establishment of growth sites, but not for reorientation of growth axes, during Candida albicans hyphal morphogenesis

Abstract

Hyphae of filamentous fungi maintain generally linear growth over long distances. In C. albicans, hyphae are able to reorient their growth in the direction of certain environmental cues. In previous work, the C. albicans bud-site selection proteins Rsr1 and Bud2 were identified as important for hyphae to maintain linear growth and were necessary for hyphal responses to directional cues in the environment (tropisms). To ask if hyphal directional responses are general functions of all yeast bud-site selection proteins, we studied the role of Rax2, ortholog of the S. cerevisiae bud-site selection protein Rax2, in C. albicans hyphal morphogenesis. Rax2-YFP localized to the hyphal cell surface in puncta and at the hyphal tip in a crescent. Strains lacking Rax2 had hyphal morphologies that did not differ from control strains. In non-cued growth conditions, rax2 mutant strains had defects in both yeast (bud) and hyphal (branch) site selection and mutant hyphae exhibited non-linear growth trajectories as compared to control hyphae. In contrast, when encountering a directional environmental cue, hyphae lacking Rax2 retained the ability to reorient growth in response to both topographical (thigmotropism) and electric-field (galvanotropism) stimuli but exhibited a reduced ability to establish hyphal growth in the direction of a cathodal stimulus. In conclusion, these results indicate that C. albicans Rax2 is important for establishing sites of emergence of yeast and hyphal daughters and for maintaining the linearity of hyphal growth. In contrast to Rsr1 and Bud2, Rax2 is not involved in responses that require a reorientation of the direction of already established hyphal growth (tropisms). Thus, it appears that some hyphal directionality responses are separable in that they are mediated by a different set of polarity proteins.

Comparative genomics reveals the origin of fungal hyphae and multicellularity

“…Several of the identified families contain genes with reported growth defects in A. fumigatus or S. cerevisiae , indicating that our searches recovered genes relevant for hyphal MC. For example, Rax proteins are major regulators of cellular morphogenesis and are involved in bud site selection in budding yeasts 74 , polarized growth in S. pombe 75 and polarity maintenance in filamentous fungi 76 . We further detected a fungal-specific cluster of tropomyosins (TPM1 in S. cerevisiae ), which originated in the MRCA of Blastocladiomycota and other fungi and is involved in polarized growth and the stabilization of actin microfilaments.…”

Comparative genomics reveals the origin of fungal hyphae and multicellularity

“…Several of the identified families contain genes with reported growth defects in A. fumigatus or S. cerevisiae , indicating that our searches recovered genes relevant for hyphal MC. For example, Rax proteins are major regulators of cellular morphogenesis and are involved in bud site selection in budding yeasts 73,74 , polarized growth in S. pombe 75 and polarity maintenance in filamentous fungi 76 . The finding that these families originated in BCZ nodes makes them candidates for being key contributors to the evolution of hyphal MC.…”

Comparative genomics reveals the origin of fungal hyphae and multicellularity

“…See the complete list of the 414 gene families in Appendix Several of the identified families contain genes with reported growth defects in Aspergillus fumigatus or Saccharomyces cerevisiae, indicating that our genome-wide search recovered genes relevant for hyphal multicellularity. For example, RAX proteins are major regulators of cellular morphogenesis and are involved in bud site selection in budding yeasts Gonia et al, 2013), polarized growth in Schizosaccharomyces pombe (Choi et al, 2006) and polarity establishment and maintenance in filamentous fungi (Borkovich et al, 2004). We further detected a fungalspecific cluster of tropomyosins (TPM1 and TPM2 in Saccharomyces cerevisiae), which originated in the MRCA of Blastocladiomycota and other fungi and is involved in polarized cell growth and the stabilization of actin microfilaments (Pruyne & Bretscher, 2000).…”