Regulation by Ions of Zoospore Release in Pythium

Abstract

In a Pythium species causing equine phycomycosis, release of zoospores from vesicles is regulated by K⁺ in the presence of either Ca²⁺ or Mg²⁺ . In the presence of K⁺ (16 mM ) , numerous vesicles were formed but, in most, mature zoospores failed to develop to maturity and both immature zoospores and vesicles disintegrated unless Ca,²⁺ (0.3µM -3mM ) or Mg²⁺ (0.3µM - 0.3mM) were present. The effect of K⁺ can be replaced partially by Rb⁺. Addition of Li⁺, Cs⁺ or NH₄⁺ ions to colonies 4 h after zoospore induction had commenced led to vesicle lysis and to the formation of abnormal spores and bizarre-shaped bodies in vesicles; simultaneous addition of Ca²⁺ (3mM) exerted a particularly marked stabilising effect on vesicle structure. Zoospore motility both in the vesicle and after release was reduced or completely inhibited by K⁺ (16 mM) . Group I elements generally inhibited zoospore motility with the notable exception of Na+. Some group 11 elements inhibited zoospore motility but Ca²⁺ and Mg²⁺ were notable exceptions. Vesicle membranes induced in the presence of K⁺ and Ca²⁺ (16 and 3 mM respectively) were up to eight times thicker than membranes induced in distilled water. Zoospore motility, zoospore encystment and membrane stability appear to be critical factors in the release of zoospores from vesicles.