Hydrogen peroxide mediates the induction of chloroplastic Ndh complex under photooxidative stress in barley

Chloroplast-encoded NDH polypeptides (components of the plastid Ndh complex ) and the NADH dehydrogenase activity of the Ndh complex (NADH-BH) increase d under photooxidative stress. The possible involvement of H2O2-mediated si gnaling in the photooxidative induction of chloroplastic ndh genes was thor oughly studied. We have analyzed the changes in the NABH-DH and steady-stat e levels of NDH-F polypeptide and ndhB and ndhF transcripts in barley ((Hor deum vulgare cv Hassan) leaves. Subapical leaf segments were incubated in g rowing light (GL), photooxidative light (PhL), GL and H2O2 (GL + H2O2) or P hL and 50 nM paraquat in the incubation medium. Treatments with H2O2 under GL mimicked the photooxidative stimulus, causing a dose-dependent increase of NADH-DH and NDH-F polypeptide. The kinetic of Ndh complex induction was further studied in leaves pre-incubated with or without the H2O2-scavenger dimethyltiourea. NADH-DH and NDH-F polypeptide rapidly increased up to 16 h in PhL, GL+ H2O2, and, at higher rate, in PhL, and paraquat. The observed increases of NADH-DH and NDH-F after 4 h in PhL and GL + H2O2 were not acco mpanied by significant changes in ndhB and ndhF transcripts. However, at 16 -h incubations NADH-DH and NDH-F changes closely correlated with higher ndh B and ndhF transcript levels. All these effects were prevented by dimethylt hiourea, It is proposed that the induction of chloroplastic ndh genes under photooxidative stress is mediated by H2O2 through mechanisms that involve a rapid translation of pre-existing transcripts and the increase of the ndh transcript levels.