Light-induced changes in the amounts of the 36000-Mr polypeptide of NADPH-protochlorophyllide oxidoreductase and its mRNA in barley plants grown under a diurnal light/dark cycle

Seedlings of barley were grown either in continuous darkness or under a diurnal 12 h light/12 h dark cycle and the effects on NADPH-protochlorophyllide oxidoreductase were followed at two different levels. Firstly, the relative content of the mRNA encoding the NADPH-protochlorophyllide oxidoreductase was measured by dot-blot hybridization. Secondly, changes in the enzyme polypeptide were monitored either by the method of immunoblotting or by immunogold labelling of ultrathin sections of Lowicryl-embedded leaf tissue. Our results demonstrate that drastic diurnal changes in the level of mRNA sequences and the enzyme protein are unlikely to occur in plants which have been grown under natural light/dark conditions. In the dark, protein and mRNA accumulation occurs at an early developmental stage. These results are difficult to reconcile with the suggestion that the massive accumulation of mRNA and enzyme protein in dark-grown seedlings is primarily the consequence of an artificially extended darkperiod. In addition to the plastid-specific NADPH-protochlorophyllide oxidoreductase a closely related polypeptide has been detected outside the plastid in the surrounding cytoplasm (Dehseh et al. 1986b, Planta 169, 172–183). During the diurnal light/dark treatment of seedlings the concentrations of the two protein populations did not show any variation indicative of an exchange between the two protein populations across the plastid envelope.Abbreviation poly(A)⁺RNA polyadenylated RNA