The Effects of Tentoxin on Chlorophyll Synthesis and Plastid Structure in Cucumber and Cabbage

Abstract

To determine if chlorosis caused by tentoxin, a toxin produced by Alternaria tenuis Nees., is due to interference with chlorophyll synthesis directly or to disruption of normal chloroplast development, the effects of the toxin on these processes in cucumber (Cucumis sativus L.) and cabbage (Brassica oleracea L., var. capitata) were studied. Cucumber cotyledons are highly sensitive to the toxin but exhibited no interference with the conversion of protochlorophyll(ide) to chlorophyll(ide) or with the general time course pattern of chlorophyll synthesis, although there was a 90% reduction in chlorophyll concentration. In cabbage, which shows no chlorosis in the presence of the toxin, there was a slight stimulation of chlorophyll synthesis in the presence of the toxin. Electron microscopy revealed that in cucumber, toxin treatment interferes with development of prolamellar bodies and lamellae, and results in deformed plastids. No such effects were noted in toxin-treated cabbage tissues. Plastids in toxin-treated cotyledons of both cucumber and cabbage contained more starch than plastids in nontreated tissues. It was concluded that tentoxin acts through disruption of normal plastid development, rather than through direct interference with chlorophyll synthesis.A chlorosis-inducing toxin (trivial name: tentoxin) produced by the fungus Alternaria tenuis Nees. has been characterized (8) as cycloleucyl-N-methyl alanylglycyl-N-methyl dehydrophenylalanyl. The toxin causes a sharply delimited, variegated, yellow chlorosis in germinating seedlings of many dicotyledonous species, but it has no apparent effect on tomato and members of the Cruciferae and Gramineae (1, 7). Templeton et al. (7) observed that cucumber seedlings were more sensitive to tentoxin in darkness than under continuous light, and that those exposed 48 to 50 hr after the initiation of germination were insensitive to the toxin. They suggested that activity of the toxin is associated with some step late in the development of chloroplasts. Saad et al. (4) (b) interfere with plastid development, thereby affecting chlorophyll synthesis indirectly. The present investigation was undertaken to determine if either of these hypotheses is correct and to determine if cellular organelles other than plastids are affected by the toxin. A preliminary report of this study has been published (2). MATERIALS AND METHODSProcedures for the production, purification, and quantitation of tentoxin have been described (4). Seeds of cucumber (Cucumis sativus L., c.v. SMR-18) and cabbage (Brassica oleracea L., var. capitata, c.v. Jersey Queen) were germinated in distilled water or tentoxin (30 ,ug/ml), in darkness, at 24 C, for 96 hr, unless otherwise noted. Seedlings were either harvested under a green safelight or exposed to 200 ft-c from General Electric Cool-white fluorescent lamps at 24 C.Protochlorophyll and chlorophyll were extracted by homogenizing 1 g (fresh wt) of cotyledons in 50 volumes (w/v) of acetone in a Sorvall Omni-mixer. The acetone solution was centrifuged a...

Effect of fusaric acid on biomass and photosynthesis of watermelon seedlings leaves

“…Cucumber cotyledons are highly sensitive to the toxin (produced by Alternaria tenuis Nees. ); there was a 90% reduction in chlorophyll concentration (HALLOIN et al 1970). Tentoxin from Alternaria tenuis Nees acts through disruption of normal plastid development, rather than direct interference with chlorophyll synthesis (HALLOIN et al 1970).…”

Chloroplast coupling factor 1: A species-specific receptor for tentoxin

“…The chlorosis results from a selective disruption of chloroplast function in that the amounts of lipids (6) and proteins (7) specific to chloroplasts are reduced and the ultrastructural alterations found are confined to the chloroplast (8). The sharp demarcation between sensitive and insensitive species suggested that chloroplasts from sensitive species might possess a specific receptor site for tentoxin.…”

“…Produced by the phytopathogenic fungus Alternaria tenuis, it induces chlorosis in many plants, including lettuce, potato, cucumber, and spinach, but has little or no effect on others such as radish, tobacco, and corn (4,5). The chlorosis results from a selective disruption of chloroplast function in that the amounts of lipids (6) and proteins (7) specific to chloroplasts are reduced and the ultrastructural alterations found are confined to the chloroplast (8). The sharp demarcation between sensitive and insensitive species suggested that chloroplasts from sensitive species might possess a specific receptor site for tentoxin.…”

Alternaria leaf blight of maize

“…Tentoxin induces stunting, wilting, and chlorosis of many dicotyledonous species, particularly members of the Compositae and Umbelliferae (22,38,87). The Solanaceae, Leguminosae, and Gramineae contain both sensitive and insensitive species (22,38,78). Maize is reported to be insensitive to the toxin (22,106).…”

“…The mechanisms by which tentoxin affects germinating seedlings are disruption of chloroplast development and inhibition of photophosphorylation (3,38,90) and reduction in transpiration rate (21,23). Tentoxin acts as an energy transfer inhibitor, interfering with the phosphoryla tion coupling mechanism but having no direct effect on electron trans port (3).…”