Abstract
Extensive research over several decades in plant light signaling mediated by photoreceptors has identified the molecular mechanisms for how phytochromes regulate photomorphogenic development, which includes degradation of phytochrome-interacting factors (PIFs) and inactivation of COP1-SPA complexes with the accumulation of master transcription factors for photomorphogenesis, such as HY5. However, the initial biochemical mechanism for the function of phytochromes has not been fully elucidated. Plant phytochromes have long been known as phosphoproteins, and a few protein phosphatases that directly interact with and dephosphorylate phytochromes have been identified. However, there is no report thus far of a protein kinase that acts on phytochromes. On the other hand, plant phytochromes have been suggested as autophosphorylating serine/threonine protein kinases, proposing that the kinase activity might be important for their functions. Indeed, the autophosphorylation of phytochromes has been reported to play an important role in the regulation of plant light signaling. More recently, evidence that phytochromes function as protein kinases in plant light signaling has been provided using phytochrome mutants displaying reduced kinase activities. In this review, we highlight recent advances in the reversible phosphorylation of phytochromes and their functions as protein kinases in plant light signaling.
Highlights
As sessile organisms, plants utilize light as the energy source for photosynthesis and as environmental cues for photomorphogenesis, i.e., growth and development in response to light signals such as wavelength, intensity, direction, and duration [1,2]
We focus on the molecular mechanisms of phytochromes for the photomorphogenic development
The results of interactions are the inactivation of phytochrome-interacting factors (PIFs) and COP1-SPA complexes by protein degradation and dissociation of the complex, respectively, which eventually induces the accumulation of HY5, i.e., one of the master transcriptional factors for photomorphogenesis
Summary
Plants utilize light as the energy source for photosynthesis and as environmental cues for photomorphogenesis, i.e., growth and development in response to light signals such as wavelength, intensity, direction, and duration [1,2]. Phytochromes are encoded by small gene families, for example, five members (phyA to phyE) in Arabidopsis thaliana [6] They are further classified into light-labile type I (phyA) and light-stable type II (phyB to phyE) species, among which phyA is responsible for sensing FR light and phyB to phyE play roles in R light-mediated photomorphogenic development [7,8]. When germinated in the dark, plant seedlings undergo etiolated growth, i.e., skotomorphogenesis that is characterized by elongated hypocotyls and closed cotyledons with apical hooks. This etiolated development allows the buried seeds to emerge through the soil in search of light. We highlight current knowledge about the reversible phosphorylation of phytochromes and their functions as protein kinases in plant light signaling
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