ROLE OF CALCIUM IN ADHESION AND GERMINATION OF ZOOSPORE CYSTS OF PYTHIUM - A MODEL TO EXPLAIN INFECTION OF HOST PLANTS

Newly-formed zoospore cysts of Pythium aphanidermatum adhered to glass and germinated without a further trigger, but both adhesion and germination were suppressed by EGTA. Older (10 min) cysts adhered and germinated poorly unless supplied with Ca2+. Adhesion and germination were linked but separable, because young cysts germinated when dislodged from glass after 10 min, and older cysts kept in suspension germinated in response to high (7-10 mM) concentrations of Ca2+, Mg2+ or Sr2+. Other cations (Li+, Na+, K+, Ba2+, Mn2+, Fe3+, Cu3+) had no effect up to 25 mM concentration or were toxic. The naturally low germination of suspended cysts was suppressed by EGTA (chelator), La3+ or verapamil (Ca2+ entry blockers), amiloride (Ca2+ flux inhibitor), trifluoperazine or dibucaine (calmodulin antagonists), TMB-8 (intracellular Ca2+ antagonist) or A23187 (Ca2+ ionophore). Suppression by TMB-8 or A23187 was not relieved by any post-treatment. Suppression by EGTA was relieved only by divalent cations. Suppression by all other compounds was relieved only by specific L-amino acids that stimulated germination of control cysts (asparagine, aspartic acid and glutamic acid but not lysine or glutamine, where tested). Alanine relieved suppression by verapamil or dibucaine in Pythium dissotocum but not P. aphanidermatum. The findings indicated a central role of Ca2+ (presumably released during encystment) in adhesion and germination, such that zoospores would germinate autonomously by a Ca2+-mediated cascade when they dock and encyst on roots with the fixed point of germination adjacent to the host. Amino acids are not essential but apparently synergize this process by species-specific receptor-binding which facilitates Ca2+ uptake or Ca2+ signalling.