Abstract
Accurate regulation of chlorophyll synthesis is crucial for chloroplast formation during the greening process in angiosperms. In this study, we examined the role of phytochrome B (phyB) in the regulation of chlorophyll synthesis in rice seedlings (Oryza sativa L.) through the characterization of a pale-green phenotype observed in the phyB mutant grown under continuous red light (Rc) irradiation. Our results show that the Rc-induced chlorophyll accumulation can be divided into two components—a phyB-dependent and a phyB-independent component, and that the pale-green phenotype is caused by the absence of the phyB-dependent component. To elucidate the role of the missing component we established an Rc-induced greening experiment, the results of which revealed that several genes encoding proteins on the chlorophyll branch were repressed in the phyB mutant. Notable among them were ChlH and GUN4 genes, which encode subunit H and an activating factor of magnesium chelatase (Mg-chelatase), respectively, that were largely repressed in the mutant. Moreover, the kinetic profiles of chlorophyll precursors suggested that Mg-chelatase activity simultaneously decreased with the reduction in the transcript levels of ChlH and GUN4. These results suggest that phyB mediates the regulation of chlorophyll synthesis through transcriptional regulation of these two genes, whose products exert their action at the branching point of the chlorophyll biosynthesis pathway. Reduction of 5-aminolevulinic acid (5-ALA) synthesis could be detected in the mutant, but the kinetic profiles of chlorophyll precursors indicated that it was an event posterior to the reduction of the Mg-chelatase activity. It means that the repression of 5-ALA synthesis should not be a triggering event for the appearance of the pale-green phenotype. Instead, the repression of 5-ALA synthesis might be important for the subsequent stabilization of the pale-green phenotype for preventing excessive accumulation of hazardous chlorophyll precursors, which is an inevitable consequence of the reduction of Mg-chelatase activity.
Highlights
Among the various environmental signals that plants utilize to optimize their growth, development and reproduction, light is the most important one [1]
Chlorophyll content in the aerial parts of the phytochrome B (phyB) seedlings grown under Rc (15μmol m-2 s-1) for 9 days was in the 20–40% range compared to wild type (WT) (Fig 1B), indicating that the pale-green phenotype was due to reduced chlorophyll content
In the initial phase of chloroplast development, chlorophyll synthesis is massively activated to respond to the demand for building up the photosynthetic machinery
Summary
Among the various environmental signals that plants utilize to optimize their growth, development and reproduction, light is the most important one [1]. Physiological studies revealed that the phytochrome-mediated responses could be classified into at least three modes, namely very low fluence response (VLFR), low fluence response (LFR) and high irradiance response We have isolated a series of rice mutants deficient in all the phytochrome genes [14, 15] and generated comprehensive combinations of double and triple mutants [15, 16]. These mutants have contributed to the elucidation of the function of phytochromes in rice plants [17,18,19]
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