Differential responses of Plasmopara halstedii developmental forms to various steroid alkaloids

Significant differences were found in the sensitivity of various biotrophic and abiotrophic stages of Plasmopara halstedii to representative Solanum and Veratrum alkaloids and batrachotoxin with zoospores and cystospores being the most sensitive developmental forms. While Solanum alkaloids with pregnane skeleton inhibited the asexual spores without influencing biotrophic stages, Veratrum alkaloids that contain a highly hydroxylated C -nor- D -homosteroid skeleton inhibited both asexual spores and the systemic invasion of parasiting thallus. Cevagenine and 3- O -oleoylveracevine, the most active alkaloids, interrupted the life cycle of the fungus at several points: they influenced the parasiting thallus, delayed the zoosporogenesis and inhibited both zoospores and cystospores. The amphibian steroid alkaloid batrachotoxin affected asexual spores similarly. The zoospores responded rapidly to the alkaloid treatments; their motion was inhibited before the semipermeability of plasmalemma was completely lost. The antiperonospora efficacy of cevagenine and 3- O -oleoylveracevine was comparable to that of metalaxyl which inhibits only parasitic hyphae without affecting asexual spores. All alkaloids tested exhibited characteristic effect on the growth of the host plant, sunflower. These amphiphilic compounds are thought to act by direct incorporation into the fungal membrane thus disrupting its structural and functional integrity. Keywords : oomycota; sunflower; batrachotoxin; veratridine; solanine; tomatine; digitonin; tridemorph DOI: 10.3126/ijls.v4i0.2791 International Journal of Life Sciences Vol.4 2010 pp.1-15