PHYSIOL. PLANT. 53: 421-428. Copenhagen 1981 Tentoxin-induced loss of plastidic polyphenol oxidase Kevin C. Vaughn and Stephen O. Duke Vaughn, K. C. and Duke, S. O. 1981, Tentoxin-induced loss of plastidic polyphenol oxidase. -Physioi. Plant. 53: 421^28. Tentoxin-treated mung bean plants are shown to lack chloroplast polyphenol oxidase (PPO) by enzymatic, electrophoretic and cytochemical analysis. Incorporation of PPO (a protein coded by nuclear DNA) into the plastid may occur via concentration of the protein into inner envelope-derived vesicles. PPO integration into the plastid is apparently blocked by a tentoxin treatment although fraction I protein (and hence the proteins for chJoroplast ribosome production) is not affected by this fungai toxin. Both apical and etiolated piastids from tentoxin-treated plants lack PPO, Thus, it is unlikely that the primary effect of tentoxin is due to the binding of the chloroplast coupling factor, as previously supposed. Additional key-words: Ultrastructure, Vigna radiata, fungal toxin, mung bean, chloroplast. K. C. Vaughn and S. O. Duke, Southern Weed Science Laboratory, USDA—SEA—AR, Box 225, Stoneville, MS 38 776, U.S.A. Introduction Tentoxin is a cyclic tetrapeptide produced by Alternaria alternata (Fr.) Keissler (= A. tenuis Nees) which induces chlorosis when either excised cotyledons or germinating seedlings are incubated in aqueous solutions of this to- xin (Templeton 1972, Duke etal 1980). Attention has been placed upon the binding of tentoxin to the chloroplast coupling factor (CFj) (Steele et al. 1976). It has been reasoned that tentoxin-induced chlorosis occurs because formation of grana stacks and chlorophyll accumulation depends upon plastid ATP synthesis involving CFj, and, that if tentoxin binds to this molecule, energy required for granal formation is not available (Arntzen 1972,, Steele et al. 1976). Ul- trastructural analysis (Duke et al. 1980) has revealed, however, that not only are grana stacks formed in ten- toxin-treated Ipomoea hederacea cotyledons but super-stacked configurations known as magnograna (Vaughn et al. 1980a) are formed. Moreover, it has re- cently been shown that mutants deficient in CFj make grana stacks and have relatively nortnal appearing chloroplast structure (H0yer-Hansen <?( a/. 1979). Thus, chlorosis induced by tentoxin cannot solely be related to its binding to CFj. The tentoxin-treated piastids oi Ipomoea (Duke et al. 1980) are ultrastructural phenocopies of the magno- grana plastid DNA mutants described in a number of plants (DuBuy et al 1950, Keresztes el al. 1976, Vaughn and Wilson 1980). Because these magnograna mutants lack plastidic polyphenol oxidase (PPO) and show a reduction of chlorophyll similar to ten- toxin-treated tissues (DuBuy et al. 1950, K. C. Vaughn and K. G. Wilson, unpublished results), we have inves- tigated the PPO activity and contents of ten- toxin-treated mung bean plants. Our preliminary studies indicated that mung bean was extremely sensi- tive to tentoxin and its ease of culture makes it an ideal experimental object. Abbreviations: PPO = polyphenol oxidase; DDTC = diethyl- dithiocarbamate; DOPA = dihydroxyphenyialanine; SDS — sodiom dodecylsulfate. Materials and methods Plant material Mung bean (Vigna radiata (L.) Wilczek cv. Berken) seeds were imbibed for 24 h in 9-cm petri dishes (50 Received 7 May, 1981; revised 20 July, 1981 PhvsW. Planl. 53 0031-9317/81/120421-08 $03.00/0 © 1981 Physiologia Plantarum 421