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

Abstract

The putativeClaviceps purpureahomologue of theSaccharomyces cerevisiaestretch-activated calcium ion channel Mid1 was investigated for its role in vegetative growth, differentiation and pathogenicity on rye (Secale cereale). Gene replacement mutants ofCl. purpurea mid1were 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 tomid1mutants in yeast, but the altered sensitivity of the mutants to changes in external and internal Ca2+concentrations indicates some role of Mid1 in Ca2+homeostasis. The major effect ofmid1deletion, however, was the complete loss of virulence: infected rye plants showed no disease symptoms at all. Detailed analyses ofin vitro-infected rye ovaries demonstrated that the Δmid1mutants had multiple apical branches and were unable to infect the host tissue, suggesting that Mid1 is essential for generating the necessary mechanical force for penetration. This is believed to be the first report of an essential role for a Mid1 homologue in the virulence of a plant-pathogenic fungus.