Involvement of Thylakoid Overenergization in Tentoxin-Induced Chlorosis in Nicotiana spp.

The purpose of this work was to clarify the mechanism of tentoxin-induced chlorosis in Nicotiana spp. seedlings. We found that chlorosis does not correlate with the inhibition of chloroplast ATP synthesis in vivo, since it occurs at tentoxin concentrations far higher than that required 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 overenergization 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 involved in the process of chlorosis: ascorbate, a quencher of oxygen radicals, significantly protects against chlorosis, whereas transgenic Nicotiana spp. mutants overexpressing chloroplast superoxide dismutase are partially resistant to tentoxin-induced chlorosis. It is proposed that chlorosis in developing seedlings results from overenergization of thylakoids, which leads to the generation of oxygen radicals.