Regulation of Plant Photoresponses by Protein Kinase Activity of Phytochrome A

Abstract

Extensive research has been conducted for decades to elucidate the molecular and regulatory mechanisms for phytochrome-mediated light signaling in plants. As a result, tens of downstream signaling components that physically interact with phytochromes are identified, among which negative transcription factors for photomorphogenesis, PHYTOCHROME-INTERACTING FACTORs (PIFs), are well known to be regulated by phytochromes. In addition, phytochromes are also shown to inactivate an important E3 ligase complex consisting of CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) and SUPPRESSORs OF phyA-105 (SPAs). This inactivation induces the accumulation of positive transcription factors for plant photomorphogenesis, such as ELONGATED HYPOCOTYL 5 (HY5). Although many downstream components of phytochrome signaling have been studied thus far, it is not fully elucidated which intrinsic activity of phytochromes is necessary for the regulation of these components. It should be noted that phytochromes are autophosphorylating protein kinases. Recently, the protein kinase activity of phytochrome A (phyA) has shown to be important for its function in plant light signaling using Avena sativa phyA mutants with reduced or increased kinase activity. In this review, we highlight the function of phyA as a protein kinase to explain the regulation of plant photoresponses by phyA.

Phytochrome phosphorylation in plant light signaling

“…PIFs are integral for phytochrome signaling as they repress various photomorphogenic responses through transcriptional regulation of over a thousand genes ( Pham et al., 2018 ; Jing and Lin, 2020 ). Thus, plant growth and development are likely regulated by phytochrome-PIF signaling modules comprising five phytochromes and eight PIFs in Arabidopsis ( Wang et al., 2022 ; Choi et al., 2023 ). For example, photoactivated phytochromes induce phosphorylation and degradation of PIF1 and PIF3 in plants, which positively regulates seed germination and de-etiolation responses, respectively.…”

“…This regulation generates de-etiolated seedlings through inhibition of hypocotyl elongation, chloroplast differentiation with chlorophyll accumulation, and cotyledon expansion ( Figure 1 , right). Phytochromes also participate in many other stages of plant growth and development ( Song et al., 2020 ; Choi et al., 2023 ).…”

“…Thus, it is apparent that phosphorylation and dephosphorylation are involved in phytochrome signaling. Although a recent review describes the regulation of plant photomorphogenesis by the PK activity of phyA ( Choi et al., 2023 ), it is worthwhile to review phytochrome phosphorylation in plant light signaling by focusing not only on phyA but also on other phytochromes including phyB, with recent studies on PKs that can phosphorylate phyB ( Liu et al., 2023 ; Zhao et al., 2023 ). Therefore, this review highlights regulation of phytochromes through phosphorylation and dephosphorylation, and their roles as PKs in plant light signaling.…”

Phytochrome B phosphorylation expanded: site‐specific kinases are identified

“…; Hunt & Pratt, 1980; Wong et al ., 1986; Lapko et al ., 1999; Han et al ., 2010). To cover our knowledge on phyA, phosphorylation in detail is outside the scope of this work; also, it was reviewed recently (Choi et al ., 2023). Instead, here we focus on the recent advances in the phosphorylation of plant phyB.…”

Two Distinct Molecular Types of Phytochrome A in Plants: Evidence of Existence and Implications for Functioning

“…This approach predetermined the choice of the material for the analysis and citation. For general comprehensive information on phyA properties and mechanisms of action, the reader is addressed to a number of excellent reviews [23,52,[59][60][61][62][63].…”