Roles of three Fusarium oxysporum calcium ion (Ca²⁺) channels in generating Ca²⁺ signatures and controlling growth

Spatial and temporal changes of cytoplasmic calcium ions ([Ca²⁺]_c), caused by external stimuli, are known as the Ca²⁺ signature and presumably control cellular and developmental responses. Multiple types of ion channels, pumps, and transporters on plasma and organellar membranes modulate influx and efflux of Ca²⁺ to and from the extracellular environment and internal Ca²⁺ stores to form Ca²⁺ signatures. Expression of a fluorescent protein-based Ca²⁺ probe, Cameleon YC3.60, in Fusarium oxysporum enabled us to study how disruption of three Ca²⁺ channel genes, including FoCCH1, FoMID1 and FoYVC1, affects Ca²⁺ signature formation at polarized hyphal tips and whether specific changes in the Ca²⁺ signature caused by these mutations are related to growth-related phenotypes. Resulting mutants displayed altered amplitude, interval, and duration of Ca²⁺ pulses under various external Ca²⁺ concentrations as well as changes in sporulation and growth. Loss of FoMID1 and FoCCH1, genes encoding putative plasma membrane channel proteins, had a major impact on Ca²⁺ signatures and growth, while disruption of FoYVC1, which encodes a vacuolar channel, only subtly affected both traits. Results from our study provide new insights into the underpinning of Ca²⁺ signaling in fungi and its role in controlling growth and also raise several new questions.