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

摘要

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 - 200 mu M) under continuous light of 25 degrees C, electrolyte leakage, chlorophyll loss, autolysis, and photobleaching were observed within 24 h. Electrolyte leakage, measured by the conductivity increase in the culture medium, began after 12 h with 200 mu M AAL-toxin T-A, but was observed after 24 h with 0.01 to 50 mu M AAL-toxin T-A, when it ranged from 25% to 63% of total releasable electrolytes, respectively. After 48 h incubation, leakage ranged from 39% to 79% of total for 0.01 to 200 mu M 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 10 mu M AAL-toxin T-A under continuous light at 25 degrees C revealed cytological damage beginning at 30 h, consistent with the time electrolyte leakage and chlorophyll reduction were observed. After 30 h 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 36-48 incubation with 10 mu M 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 72 h showed little phytotoxic damage. Within 6 h of exposure to greater than or equal to 0.5 mu M toxin, phytosphingosine and sphinganine in black nightshade leaf discs increased markedly, and continued to increase up to 24 h 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. (C) 1998 Elsevier Science Ltd. All lights reserved. [References: 34]