Catastrophic wall rupture during conidial germination of a genetically tagged mutant of Glomerella graminicola

Video microscopy was used to examine a genetically tagged mutant (T30) of G lomerella graminicola whose conidia have a propensity to burst during the g ermination process. Before the germ tube is produced, the cell wail rupture s and the cytoplasm is extruded. The bursting takes place in less than 0.1 second. Bursting can be prevented by adding osmotica to the germination med ium. By phase-contrast microscopy of alkali-washed conidia. we found that e ach ruptured conidium had a single gaping hole that was oriented parallel t o the long axis of the spore. The holes were more frequent in the middle re gion of the conidia than at the apices. A mathematical model for stress bas ed on conidial shape indicated that conidia ruptured where the stress on th e wall was greatest. The microscopic observations on the orientation and di stribution of the rupture sites are consistent with the hypothesis that T30 conidia have weaker walls than the wild-type. Comparative tests of resista nce to mechanical breakage support this hypothesis.