Deletion of Mid1, a putative stretch-activated calcium channel in Claviceps purpurea, affects vegetative growth, cell wall synthesis and virulence

The putative Claviceps purpurea homologue of the Saccharomyces cerevisiae stretch-activated calcium ion channel Midi was investigated for its role in vegetative growth, differentiation and pathogenicity on rye (Secale cereale). Gene replacement mutants of CL purpurea mid1 were not affected in polar growth and branching in axenic culture but showed a significantly reduced growth rate. The growth defect could not be complemented by Ca2+ supplementation, in contrast to mid1 mutants in yeast, but the altered sensitivity of the mutants to changes in external and internal Ca2+ concentrations indicates some role of Midi in Ca2+ homeostasis. The major effect of mid1 deletion, however, was the complete loss of virulence: infected rye plants showed no disease symptoms at all. Detailed analyses of in vitro-infected rye ovaries demonstrated that the Delta mid1 mutants had multiple apical branches and were unable to infect the host tissue, suggesting that Midi is essential for generating the necessary mechanical force for penetration. This is believed to be the first report of an essential role for a Mid 1 homologue in the virulence of a plant-pathogenic fungus.