The small GTPase Rac and the p21‐activated kinase Cla4 in Claviceps purpurea: interaction and impact on polarity, development and pathogenicity

Abstract

SummaryClaviceps purpurea, the ergot fungus, is a highly specialized pathogen of grasses; its colonization of host ovarian tissue requires an extended period of strictly polarized, oriented growth towards the vascular tissue. To understand this process, we study the role of signalling factors affecting polarity and differentiation. We showed that the small GTPase Cdc42 is involved in polarity, sporulation and in planta growth in C. purpurea. Here we present evidence that the GTPase Rac has an even stronger and, in some aspects, inverse impact on growth and development: Drac mutants form coralline-like colonies, show hyper-branching, loss of polarity, sporulation and ability to penetrate. Functional analyses and yeast two-hybrid studies prove that the p21-activated kinase Cla4 is a major downstream partner of Rac. Phosphorylation assays of MAP kinases and expression studies of genes encoding reactive oxygen species (ROS)-scavenging and -generating enzymes indicate a function of Rac and Cla4 in fungal ROS homoeostasis which could contribute to their drastic impact on differentiation.

CDC-42 and RAC-1 regulate opposite chemotropisms inNeurospora crassa

“…When considered together, these data indicate that repositioning of activated RAC-1 is probably relayed to MAK-2 via CLA-4. This is consistent with previous work indicating that two separate but complementary GTPase and PAK signaling modules operate in fungi (Li et al, 2004;Mahlert et al, 2006;Rolke and Tudzynski, 2008;Frieser et al, 2011). Recently, BEM-1 has been identified as the likely scaffolding protein for the assembly of a plasma membrane-associated signaling complex capable of interacting with the NRC-1-MEK-2-MAK-2 kinase module during cell fusion (Schürg et al, 2012).…”

Branching of fungal hyphae: regulation, mechanisms and comparison with other branching systems

“…Nevertheless, this would seem to be a reasonable possibility. When the plant pathogen Claviceps purpurea infects rye ovarian tissue, it forms hyphae that exhibit extreme apical dominance until they reach the vascular tissue, whereupon they undergo extensive branching (Rolke and Tudzynski 2008). The related endophytic fungus Epichloe festucae forms long, unbranched hyphae in ryegrass leaves before switching to a branched pattern of morphogenesis during the formation of stroma (Scott 2001).…”

Small-GTPase-Associated Signaling by the Guanine Nucleotide Exchange Factors CpDock180 and CpCdc24, the GTPase Effector CpSte20, and the Scaffold Protein CpBem1 in Claviceps purpurea

“…The importance of Cla4 kinase signaling appears to be more important in higher fungi, while in yeasts the closely related PAK Ste20 fulfils a more crucial role (10,(42)(43)(44)(45)(46). In C. purpurea, Cla4 was previously shown to be the major downstream effector of Rac but not Cdc42 (15).…”

“…Due to the defects in polarity during infection, the ⌬cpcdc42 strain is probably recognized as a pathogen by the plant (6). In contrast, CpRac is a positive mediator of polarity, sporulation, and general growth and is essential for infection (15). In comparison, Ras-GTPases are known to be involved in upstream Rho-GEF activation in various organisms, such as Saccharomyces cerevisiae, Schizosaccharomyces pombe, and Cryptococcus neoformans (16)(17)(18).…”