Regulation of Fungal Growth by Calcium Signalling

A central dogma of fungal biology is that hyphae grow by tip growth, a situation analogous to growth of plant pollen tubes and root hairs. Calcium is widely recognised as a crucial molecule in signalling pathways regulating polarised growth. In plants, calcium ions play a crucial role in root hair growth and elongation of pollen tubes. The tip of the growing pollen tube has a cytosolic Ca2+ gradient, which is essential for tube growth and is formed by an influx of extracellular Ca2+ through stretch-activated ion channels. In the pollen protoplast, these channels are found at the site of tube germination and only after incubation in germination medium (Feijo et al. 2001; Dutta and Robinson 2004).

While tip-localised Ca2+ gradients have been observed in polarised fungal hyphal growth there is some debate about the in vivo significance of these gradients because of the lack of reliable techniques for the measurement of cytosolic Ca2+ concentrations. Some studies suggest that a Ca2+ gradient at the hyphal tip, formed by Ca2+ release from the endoplasmic reticulum (ER) in response to activation of stretch-activated phospholipase C and generation of IP3, is essential for hyphal growth (Silverman-Gavrila and Lew 2002, 2003). However, IP3-regulated Ca2+ channels are yet to be identified in fungi. Additional studies are needed in order to fully understand the role of Ca2+ gradients at the hyphal tip in regulating growth. However, the role of calcium signalling pathways in regulating hyphal growth and branching is well documented (Jackson and Heath 1993; Torralba and Heath 2001). In N. crassa, disturbance of the intracellular calcium concentration causes hyperbranching of the growing tip (Schmid and Harold 1988) and application of high concentrations of exogenous calcium restores a wild-type growth pattern to hyperbranched mutants (Kawano and Said 2005). Furthermore, disruption of calcineurin, a Ca2+-regulated phosphatase, induced hyperbranching in N. crassa, dissipation of the tip-high Ca2+ gradient and eventually growth arrest, suggesting calcineurin is essential for normal tip growth (Prokisch et al. 1997). While nothing is known so far about the importance of Ca2+ gradients in the regulation of hyphal growth of E. festucae, genes encoding homologues of calcium signalling genes identified in other fungi are found in the E. festucae genome (Nguyen et al. 2008) (Fig. 4).

Fig. 4 Proposed components of Ca + signalling pathways in E. festucae. Calcium ions enter the cytoplasm from the extracellular spaces and intracellular depots through a system of Ca2+ channels and are actively transported out of the cytoplasm through various Ca2+ pumps and exchangers. In the cytoplasm, Ca2+ regulates activity of various enzymes like phospholipase C and phosphatases. E. festucae contains three Ca2+ channels, various Ca2+ pumps and transporters, four PLCs, three Ca2+/calmodulin regulated kinases, one or two copies of calcineurin A and various other calcium regulated proteins

Fig. 4 Proposed components of Ca + signalling pathways in E. festucae. Calcium ions enter the cytoplasm from the extracellular spaces and intracellular depots through a system of Ca2+ channels and are actively transported out of the cytoplasm through various Ca2+ pumps and exchangers. In the cytoplasm, Ca2+ regulates activity of various enzymes like phospholipase C and phosphatases. E. festucae contains three Ca2+ channels, various Ca2+ pumps and transporters, four PLCs, three Ca2+/calmodulin regulated kinases, one or two copies of calcineurin A and various other calcium regulated proteins

4 Signalling Pathways

Undoubtedly, some of the most important signal transduction pathways in filamentous fungi are MAP kinase, heterotrimeric G protein and two-component regulatory pathways. However, with the exception of the stress-activated MAP kinase, these pathways are yet to be studied in epichloe endophytes or any other mutualistic plant symbiotic fungi. This section, therefore, provides an overview of the roles these pathways play in pathogenesis by phytopathogenic fungi. Results of a bioinformatic survey of the E. festucae Fl1 (E894) and E2368 genomes to identify which components of these pathways are encoded in these genomes will also be presented, allowing for predictions as to what roles these pathways may play in endophyte-grass associations.

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