Role of calcium in adhesion and germination of zoospore cysts of Pythium: a model to explain infection of host plants

Abstract

Newly-formed zoospore cysts of Pythiurn aphaniderrnaturn 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 Pythiurn dissotocum but not P.aphanidevmaturn. 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.

CanPhytophthora quercinahave a negative impact on mature pedunculate oaks under field conditions?

“…We also wanted to evaluate if the root damage was related to the crown defoliation and mineral nutrition of the trees, and thereby elucidate whether this pathogen may contribute to oak decline in southern Sweden. Since acidificationinduced nutrient imbalances of trees have been discussed as a cause for tree decline in Sweden, and the asexual as well as sexual reproduction of Phytophthora species are known to be influenced by soil chemistry [12,14,16,38,54], we also wanted to investigate if the root damage caused by P. quercina was related to the chemical conditions in the soil surrounding the tree. A field study, comparing the root systems, the tree nutrient status and the soil chemistry between healthy, moderately declining and severely declining oak trees in five stands with P. quercina was thus conducted.…”

“…It is assumed that these correlations are the result of an impeded water and nutrient uptake as a consequence of root damage caused by the pathogens. However, Phytophthora species are highly sensitive to environmental conditions, such as water availability [16], temperature [16] and soil chemistry [12,14,16,38,45,54]. In addition, various environmental factors may also affect the susceptibility of the trees to infection [19,34].…”

Zoospore Homing and Infection Events: Effects of the Biocontrol Bacterium Burkholderia cepacia AMMDR1 on Two Oomycete Pathogens of Pea ( Pisum sativum L.)

“…The most important reason for this is probably that zoospore encystment and cyst germination are influenced more by Ca 2ϩ fluxes and/or host surface compounds than by host exudates (11). Zoospores and cysts do not take up amino acids or glucose until germination (41), and the effects of these compounds on zoospores and cysts are hypothesized to be indirect by facilitating Ca 2ϩ channeling across the membranes (11,13). Because the role of sugars and amino acids in zoospore encystment and germination is probably secondary, bacterial competition for these compounds in exudates probably cannot result in large effects on these events.…”

Pseudosphaerita euglenae, a fungal parasite ofEuglenaspp. in the Mangere Oxidation Ponds, Auckland, New Zealand

“…Two factors appeared to control the initial stages of invasion by Rozella: a soluble exudate which attracts Rozella zoospores, and a receptor on the cell wall surface which causes Rozella zoospores to adhere. Research on other zoosporic plant pathogens (Pythium and Phytophthora) suggested that zoospore adhesion and germination is regulated at a biochemical level by Ca 2+ and amino acids (Donaldson & Deacon 1992). The hypothesis that optimal biochemical rather than strictly environmental conditions must exist to cause an epidemic was not investigated in the present study, but warrants further investigation.…”