New findings on ascus structure reveal forcible spore discharge mechanism in the fungal wheat pathogen Fusarium graminearum.

Fungi in the phylum Ascomycota disperse their sexual spores primarily by forcible discharge from the spore sac, the ascus, in which they develop. These fungi cause some of the most devastating diseases of agricultural crops worldwide. The mechanism of spore discharge is not well understood, despite its primary role in disease spread. Using confocal microscopy, we have elucidated the ascus structure and development as related to function for spore dispersal in Fusarium graminearum, a pathogen of small grains and corn. To determine the mechanism of spore discharge in this species, we examined ascus structure in both wild-type and spore discharge mutant strains and revealed the presence of two chambers within the ascus: a stiff-walled lower chamber that supports the developing spores and an upper chamber, extendable due to the presence of a chitin network across the surface that maintains the integrity of the sac as it is pressurized. The upper chamber includes an internal chute to confine and align the spores, while pressure builds up, with a minimum amount of fluid. Elucidation of the basis of spore dispersal for a member of this major group of fungi provides a solid beginning for exploring forcible discharge throughout this phylum. Previous studies have shown that the requirement for turgor is conserved, although the osmoticum may differ. This information will likely facilitate the development of new approaches to managing the spread of pathogenic species by targeting the very processes that allow them to efficiently colonize new environments.