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Microbial pathogens grow in a wide range of different morphologies that provide distinct advantages for virulence. In the fungal pathogen Candida albicans, adenylyl cyclase (Cyr1) is thought to be a master regulator of the switch to invasive hyphal morphogenesis and biofilm formation. However, faster growing cyr1Δ/Δ pseudorevertant (PR) mutants were identified that form hyphae in the absence of cAMP. Isolation of additional PR mutants revealed that their improved growth was due to loss of one copy of BCY1, the negative regulatory subunit of protein kinase A (PKA) from the left arm of chromosome 2. Furthermore, hyphal morphogenesis was improved in some of PR mutants by multigenic haploinsufficiency resulting from loss of large regions of the left arm of chromosome 2, including global transcriptional regulators. Interestingly, hyphal-associated genes were also induced in a manner that was independent of cAMP. This indicates that basal protein kinase A activity is an important prerequisite to induce hyphae, but activation of adenylyl cyclase is not needed. Instead, phosphoproteomic analysis indicated that the Cdc28 cyclin-dependent kinase and the casein kinase 1 family member Yck2 play key roles in promoting polarized growth. In addition, integrating transcriptomic and proteomic data reveals hyphal stimuli induce increased production of key transcription factors that contribute to polarized morphogenesis., Author summary The human fungal pathogen Candida albicans switches between budding and filamentous hyphal morphologies to gain advantages for virulence and survival in the host. Although adenylyl cyclase has been thought to be a master regulator that controls this switch, we identified C. albicans pseudorevertant mutants that grow better and form hyphae in the absence of adenylyl cyclase and cAMP. The mutant cells were also able to induce hyphal-associated genes in the absence of cAMP that are needed for virulence. Integrating information from different omics approaches identified cAMP-independent mechanisms that promote hyphal growth. This includes phosphoproteomic studies that revealed key roles for the Cdc28 cyclin-dependent kinase and casein kinase 1 in promoting hyphal growth. In addition, integrating transcriptomic and proteomic data revealed that post-transcriptional mechanisms regulate the levels of a set of key transcription factors that are important for hyphal induction, suggesting a special type of translational regulation. These studies better define the pathways that stimulate C. albicans to switch from budding to hyphal growth, which is important for invasion into tissues, escape from the immune system, and biofilm formation.

Summary The fungal pathogen Candida albicans can transition from budding to hyphal growth, which promotes biofilm formation and invasive growth into tissues. Stimulation of adenylyl cyclase to form cAMP induces hyphal morphogenesis. The failure of cells lacking adenylyl cyclase (cyr1Δ) to form hyphae has suggested that cAMP signaling is essential for hyphal growth. However, cyr1Δ mutants also grow slowly and have defects in morphogenesis, making it unclear whether hyphal inducers must stimulate cAMP, or if normal basal levels of cAMP are required to maintain cellular health needed for hyphal growth. Interestingly, supplementation of cyr1Δ cells with low levels of cAMP enabled them to form hyphae in response to the inducer N-acetylglucosamine (GlcNAc), suggesting that a basal level of cAMP is sufficient for stimulation. Furthermore, we isolated faster-growing cyr1Δ pseudorevertant strains that can be induced to form hyphae even though they lack cAMP. The pseudorevertant strains were not induced by CO2, consistent with reports that CO2 directly stimulates adenylyl cyclase. Mutational analysis showed that induction of hyphae in a pseudorevertant strain was independent of RAS1, but was dependent on the EFG1 transcription factor that acts downstream of protein kinase A. Thus, cAMP-independent signals contribute to the induction of hyphal responses.

The yeast bud site selection system represents a paradigm for understanding how fungal cells regulate the formation of a polarity axis. In Saccharomyces cerevisiae, Bud4 and Axl2 are components of the axial bud site marker. To address the possibility that these proteins regulate cellular morphogenesis in filamentous fungi, we have characterized homologues of Bud4 and Axl2 in Aspergillus nidulans. Our results show that Bud4 is involved in septum formation in both hyphae and developing conidiophores. Whereas Axl2 appears to have no obvious role in hyphal growth, it is required for the regulation of phialide morphogenesis during conidiation. In particular, Axl2 localizes to the phialide-spore junction, where it appears to promote the recruitment of septins. Furthermore, the developmental regulators BrlA and AbaA control the expression of Axl2. Additional studies indicate that Axl2 is also involved in the regulation of sexual development, not only in A. nidulans, but also in the phylogenetically unrelated fungus Fusarium graminearum. Our results suggest that Axl2 plays a key role in phialide morphogenesis and/or function during conidiation in the aspergilli.

The formation of hyphae that grow solely by apical extension is a defining feature of filamentous fungi. Hyphal morphogenesis involves several key steps, including the establishment and maintenance of a stable polarity axis, as well as cell division via the deposition of septa. Several filamentous fungi have been employed in attempts to decipher the mechanisms underlying these steps. Amongst these fungi, Aspergillus nidulans has proven to be a particularly valuable model. The genetic tractability of this fungus coupled with the availability of sophisticated post-genomics resources has enabled the identification and characterization of numerous genes involved in hyphal morphogenesis. Here, we summarize current progress towards understanding the function of these genes and the mechanisms involved in polarized hyphal growth and septation in A. nidulans . We also highlight important areas for future investigation.

Summary The ability of filamentous fungi to form hyphae requires the establishment and maintenance of a stable polarity axis. Based on studies in yeasts and animals, the GTPases Cdc42 and Rac1 are presumed to play a central role in organizing the morphogenetic machinery to enable axis formation and stabilization. Here, we report that Cdc42 (ModA) and Rac1 (RacA) share an overlapping function required for polarity establishment in Aspergillus nidulans. Nevertheless, Cdc42 appears to have a more important role in hyphal morphogenesis in that it alone is required for the timely formation of lateral branches. In addition, we provide genetic evidence suggesting that the polarisome components SepA and SpaA function downstream of Cdc42 in a pathway that may regulate microfilament formation. Finally, we show that microtubules become essential for the establishment of hyphal polarity when the function of either Cdc42 or SepA is compromised. Our results are consistent with the action of parallel Cdc42 and microtubule-based pathways in regulating the formation of a stable axis of hyphal polarity in A. nidulans.

The ability to switch between yeast and filamentous forms is central to Candida albicans biology. The yeast-hyphal transition is implicated in adherence, tissue invasion, biofilm formation, phagocyte escape, and pathogenesis. A second form of morphological plasticity in C. albicans involves epigenetic switching between white and opaque forms, and these two states exhibit marked differences in their ability to undergo filamentation. In particular, filamentous growth in white cells occurs in response to a number of environmental conditions, including serum, high temperature, neutral pH, and nutrient starvation, whereas none of these stimuli induce opaque filamentation. Significantly, however, we demonstrate that opaque cells can undergo efficient filamentation but do so in response to distinct environmental cues from those that elicit filamentous growth in white cells. Growth of opaque cells in several environments, including low phosphate medium and sorbitol medium, induced extensive filamentous growth, while white cells did not form filaments under these conditions. Furthermore, while white cell filamentation is often enhanced at elevated temperatures such as 37°C, opaque cell filamentation was optimal at 25°C and was inhibited by higher temperatures. Genetic dissection of the opaque filamentation pathway revealed overlapping regulation with the filamentous program in white cells, including key roles for the transcription factors EFG1, UME6, NRG1 and RFG1. Gene expression profiles of filamentous white and opaque cells were also compared and revealed only limited overlap between these programs, although UME6 was induced in both white and opaque cells consistent with its role as master regulator of filamentation. Taken together, these studies establish that a program of filamentation exists in opaque cells. Furthermore, this program regulates a distinct set of genes and is under different environmental controls from those operating in white cells., Author Summary Candida albicans is the most common human fungal pathogen, capable of growing as a commensal organism or as an opportunistic pathogen. Perhaps the best-studied aspect of C. albicans biology is the transition between the single-celled yeast form and the multicellular filamentous form. This transition is necessary for virulence, as cells locked in either state are avirulent. Here, we demonstrate that the yeast-filament transition is tightly regulated by another morphological switch, the white-opaque phenotypic switch. White cells undergo filamentation in response to a wide range of established physiological cues, while opaque cells do not. We further show that opaque cells can indeed undergo filamentation, but that they do so in response to different environmental cues than those of white cells. We define the genetic regulation of filamentous growth in opaque cells, as well as the transcriptional profile of these cell types, and contrast them with the established program of filamentation in white cells. Our results reveal a close relationship between the white-opaque switch and the yeast-hyphal transition, and provide further evidence of the morphological plasticity of this pathogen. They also establish that epigenetic switching allows two fungal cell types with identical genomes to respond differently to environmental cues.