Significance of circadian gene expression in higher plants.

The significance of the circadian clock for living organisms is not fully understood. Recent findings demonstrate circadian control of transcription of quite a number of genes with individual maxima throughout the entire day. Evidence in favor of circadian-clock-controlled translation has also been documented. In this article, we want to promote the idea that in plants the clock functions as a regulator which coordinates critical cellular processes, such as cell division, nitrate reduction, or synthesis of chlorophyll-protein complexes, in such a way that the generation of dangerous, oxidative radicals or exposure to harmful light is minimized. This has been achieved by plant organisms either by confining gene expression to the dark phase or by a tight coordination of different tiers of gene expression during the light phase. This leads to the consequence for the researcher that the time of experimentation needs to be carefully considered and documented. It also follows that one might lose important findings if only a particular portion of the day is investigated.

The expression of mRNAs for light-stress proteins in barley: inverse relationship of mRNA levels of individual genes within the leaf gradient.

Two cDNAs coding for putative light-stress proteins of barley (Hordeum vulgare L.) were cloned and the expression of the corresponding mRNAs analyzed in the barley leaf and compared to that of the well-studied ELIP (early-inducible protein) mRNA. During greening the mRNA for clone HL No. 2, which shows homology to two rice proteins of as yet unknown function, was transiently induced; its level rose more slowly and remained elevated for a longer time than was described for ELIP mRNAs. The mRNA corresponding to clone HL No. 13 was recognized as homologous to subunit P of pea glycine decarboxylase, a nuclear-encoded mitochondrial protein involved in photorespiration. Its mRNA level rose more slowly with cellular development than that of the mRNA for LHC II, the apoprotein of the chlorophyll-a/b-binding protein of PSII. The mRNAs of both novel proteins were induced by high light up to an irradiance of 2000 W.m-2. Their levels remained elevated under high light for up to 9 h, the longest time span examined, while after return to culture light conditions the mRNAs rapidly decayed, each with an individual time course. In green barley leaves the mRNA for clone HL No. 2 was expressed to the highest level in the most basal segment, similar to that of ELIPs, while in contrast the mRNA for subunit P of glycine decarboxylase accumulated to the highest level in the leaf apex where the fully developed cells and mitochondria reside. The latter finding strongly indicates that photorespiration is regulated by high light also at the level of mRNA transcription or mRNA accumulation. In addition, we show that perception of light stress is under the control of cellular development and differentiation.

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

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 (Hordeum 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 [delta]-aminolevulinic acid ([delta]-ALA) synthesis. [delta]-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.

The effect of heat shock on morphogenesis in barley : coordinated circadian regulation of mRNA levels for light-regulated genes and of the capacity for accumulation of chlorophyll protein complexes.

The effect of daily heat-shock treatments on gene expression and morphogenesis of etiolated barley (Hordeum vulgare) was investigated. Heat-shock treatments in the dark induced shortening of the primary leaves and the coleoptiles to the length of those in light-grown plantlets. In addition, the mRNA levels of the light-induced genes that were investigated were raised under these conditions and showed distinct oscillations over a period of at least 3 d. While the mRNA levels for chlorophyll a/b binding protein (LHC II), plastocyanin, and the small subunit of ribulose-1,5-bisphosphate carboxylase had maxima between 8 and 12 pm (12-16h after the last heat-shock treatment), the mRNA levels for thionin oscillated with a phase opposed to that of LHC II. Etiolated barley, the circadian oscillator of which was synchronized by cyclic heatshock treatments, was illuminated for a constant interval at different times of the day; this led to the finding that greening was fastest at the time when the maximal levels of mRNA for LHC II were also observed. Whereas accumulation of chlorophyll a during a 4-h period of illumination oscillated by a factor of 3, chlorophyll b accumulation changed 10- to 15-fold. Similarly, accumulation of LHC II was highest when pigments accumulated maximally. Hence, greening or, in other words, thylakoid membrane assembly is under control of the circadian oscillator.