INVOLVEMENT OF THYLAKOID OVERENERGIZATION IN TENTOXIN-INDUCED CHLOROSIS IN NICOTIANA SPP

The purpose of this work was to clarify the mechanism of tentoxin-indu ced chlorosis in Nicotiana spp, seedlings. We found that chlorosis doe s not correlate with the inhibition of chloroplast ATP synthesis in vi vo, since it occurs at tentoxin concentrations far higher than that re quired for the inhibition of photophosphorylation measured in the same seedlings. However, tentoxin-induced chlorosis does correlate with in vivo overenergization of thylakoids. We show that tentoxin induces ov erenergization in intact plants and isolated thylakoids, probably via multiple interactions with ATP synthase. Furthermore, gramicidin D, a protonophore that relieves overenergization, also relieves chlorosis. Two lines of evidence suggest that reactive oxygen species may be invo lved in the process of chlorosis: ascorbate, a quencher of oxygen radi cals, significantly protects against chlorosis, whereas transgenic Nic otiana spp. mutants overexpressing chloroplast superoxide dismutase ar e partially resistant to tentoxin-induced chlorosis. It is proposed th at chlorosis in developing seedlings results from overenergization of thylakoids, which leads to the generation of oxygen radicals.