The Circadian Oscillator Coordinates the Synthesis of Apoproteins and Their Pigments during Chloroplast Development

Abstract

Greening has been studied at circadian times of maximal and minimal levels of mRNA for the light-harvesting chlorophyll a/b binding protein in photosystem II (Cab mRNA) after circadian synchronization of etiolated barley plantlets (Hordeurn vulgare cv Apex) by heat-shock treatments. It was found that greening occurs faster and without a lag period when illumination was started at the time of maximal Cab mRNA accumulation. This holds true for the rate of accumulation of Cab and early light-inducible protein mRNAs, the levels of their correspondent proteins, and the levels of chlorophyll a and b. When illumination was started at the time of Cab mRNA minimum, a lag in the appearance of all components mentioned above was observed. Under these conditions, the lag in chlorophyll b accumulation was by far more pronounced than that found for chlorophyll a. The circadian oscillation in the capacity of chlorophyll synthesis appears to be controlled via 6-aminolevul i n k acid (6-ALA) synthesis. 6-ALA accumulation after levulinic acid treatment is itself under circadian control; the maxima in stationary concentrations coincide with those of Cab mRNA levels. The amounts of protochlorophyllide and photoconvertible protochlorophyllide showed only minor differences between circadian minima and maxima, the levels being slightly lower during the time of minimum.

Expression studies in tetrapyrrole biosynthesis: inverse maxima of magnesium chelatase and ferrochelatase activity during cyclic photoperiods

“…In another experimental system using other plant species the chlorophyll-synthesizing capacity parallels the cycling rate of ALA synthesis (Beator et al 1992;Beator and Kloppstech 1993;Bei-Paraskevopoulou et al 1995). A simultaneous activation of Mg-chelatase would tentatively ensure the supply of Mg-porphyrins.…”

Gene Expression Profiling of the Tetrapyrrole Metabolic Pathway in Arabidopsis with a Mini-Array System

“…In mature leaves that possess developed chloroplasts, the synthesis of Chl and Chl-binding proteins is closely coordinated and regulated by light and the endogenous clock for the assembly of a functional photosynthetic apparatus (Beator and Kloppstech, 1993). To determine the gene expression profiles for tetrapyrrole biosynthesis under diurnal and circadian rhythms, samplings were carried out from 3-week-old seedlings of Arabidopsis harvested after growth under a cyclic 12-h light/12-h dark regime or subsequently after a second round of continuous illumination.…”

Identification of NADPH:Protochlorophyllide Oxidoreductases A and B: A Branched Pathway for Light-Dependent Chlorophyll Biosynthesis in Arabidopsis thaliana

“…On the other hand, experiments performed in barley suggest that the coordinated synthesis of LHC B polypeptides and Chl may not be controlled at the level of Pchlide reduction (Beator and Kloppstech, 1993). Whether coordinated diurnal oscillations of A. thaliana PorB and Lhcbl mRNAs help to fulfill a biological function in co-regulating t h e supply of Chl and Chl-binding sites remains to be investigated.…”