Mid1, a Mechanosensitive Calcium Ion Channel, Affects Growth, Development, and Ascospore Discharge in the Filamentous Fungus Gibberella zeae

The role of Mid1, a stretch-activated ion channel capable of being permeated by calcium, in ascospore development and forcible discharge from asci was examined in the pathogenic fungus Gibberella zeae (anamorph Fusarium graminearum). The Delta mid1 mutants exhibited a > 12-fold reduction in ascospore discharge activity and produced predominately abnormal two-celled ascospores with constricted and fragile septae. The vegetative growth rate of the mutants was similar to 50% of the wild-type rate, and production of macroconidia was > 10-fold lower than in the wild type. To better understand the role of calcium flux, Delta mid1 Delta cch1 double mutants were also examined, as Cch1, an L-type calcium ion channel, is associated with Mid1 in Saccharomyces cerevisiae. The phenotype of the Delta mid1 Delta cch1 double mutants was similar to but more severe than the phenotype of the Delta mid1 mutants for all categories. Potential and current-voltage measurements were taken in the vegetative hyphae of the Delta mid1 and Delta cch1 mutants and the wild type, and the measurements for all three strains were remarkably similar, indicating that neither protein contributes significantly to the overall electrical properties of the plasma membrane. Pathogenicity of the Delta mid1 and Delta mid1 Delta cch1 mutants on the host (wheat) was not affected by the mutations. Exogenous calcium supplementation partially restored the ascospore discharge and vegetative growth defects for all mutants, but abnormal ascospores were still produced. These results extend the known roles of Mid1 to ascospore development and forcible discharge. However, Neurospora crassa Delta mid1 mutants were also examined and did not exhibit defects in ascospore development or in ascospore discharge. In comparison to ion channels in other ascomycetes, Mid1 shows remarkable adaptability of roles, particularly with regard to niche-specific adaptation.