Ultrastructural effects of AAL-toxin T(A) from the fungus Alternaria alternata on black nightshade (Solanum nigrum L.) leaf discs and correlation with biochemical measures of toxicity

Ultrastructural effects of AAL-toxin T(A) from Alternaria alternata on black nightshade (Solanum nigrum L.) leaf discs and correlation with biochemical measures of toxicity. In black nightshade (Solanum nigrum L.) leaf discs floating in solutions of AAL-toxin T(A) (0.01-200μM) under continuous light at 25°C, electrolyte leakage, chlorophyll loss, autolysis, and photobleaching were observed within 24h. Electrolyte leakage, measured by the conductivity increase in the culture medium, began after 12h with 200μM AAL-toxin T(A), but was observed after 24h with 0.01 to 50μM AAL-toxin T(A), when it ranged from 25% to 63% of total releasable electrolytes, respectively. After 48h incubation, leakage ranged from 39% to 79% of total for 0.01 to 200μM AAL-toxin T(A), respectively, while chlorophyll loss ranged from 5% to 32% of total, respectively. Ultrastructural examination of black nightshade leaf discs floating in 10μM AAL-toxin T(A) under continuous light at 25°C revealed cytological damage beginning at 30h, consistent with the time electrolyte leakage and chlorophyll reduction were observed. After 30h incubation chloroplast starch grains were enlarged in control leaf discs, but not in AAL-toxin T(A)-treated discs, and the thylakoids of treated tissue contained structural abnormalities. After 36-48h incubation with 10μM AAL-toxin T(A), all tissues were destroyed with only cell walls, starch grains, and thylakoid fragments remaining. Toxicity was light-dependent, because leaf discs incubated with AAL-toxin T(A) in darkness for up to 72h showed little phytotoxic damage. Within 6h of exposure to ≥0.5μM toxin, phytosphingosine and sphinganine in black nightshade leaf discs increased markedly, and continued to increase up to 24h exposure. Thus, physiological and ultrastructural changes occurred in parallel with disruption of sphingolipid synthesis, consistent with the hypothesis that AAL-toxin T(A) causes phytotoxicity by interrupting sphingolipid biosynthesis, thereby damaging cellular membranes. Copyright (C) 1998 Elsevier Science Ltd.