Phytochromes: Photosensory Perception and Signal Transduction

Phytochromes are photoreceptors that control many plant light responses. Phytochromes have two carboxyl-terminal structural domains called the PAS repeat domain and the histidine kinaserelated domain. These domains are each related to bacterial histidine kinase domains, and biochemical studies suggest that phytochromes are light-regulated kinases. The PAS repeat domain is important for proper phytochrome function and can interact with putative signaling partners. We have characterized several new phytochrome B mutants in Arabidopsis that express phyB protein, three of which affect the histidine kinase-related domain. Point mutations in the histidine kinase-related domain cause phenotypes similar to those of null mutants, indicating that this domain is important for phyB signaling. However, a truncation that removes most of the histidine kinase-related domain results in a phyB molecule with partial activity, suggesting that this domain is dispensable. These results suggest that phytochromes evolved in modular fashion. We discuss possible functions of the histidine kinase-related domain in phytochrome signaling. P hytochromes are a family of red͞far-red light photoreceptors in plants that regulate many developmental and cellular responses to light. They are present in all plant species examined and in many algae. The model plant Arabidopsis thaliana has five phytochromes called phyA-phyE that diverge from each other by as much as 50%, and genetic and physiological studies have revealed that they regulate distinct light responses (1-3). Phytochrome proteins interconvert between two stable spectral forms. They are synthesized in the dark in the Pr (red-absorbing) form. Red light converts Pr to the Pfr (far-red-absorbing) form, and far-red light reconverts Pfr to Pr. Pfr (and, for phyA, possibly ''cycled'' Pr that has passed through the Pfr form) is thought to be the biologically active form of phytochrome (ref. 4 and discussed in ref. 5).Phytochromes have two major structural domains (6, 7). The amino-terminal domain (Ϸ74 kDa) has a covalently attached linear tetrapyrrole chromophore (phytochromobilin) and is sufficient for light absorption and photoreversibility. The carboxylterminal domain (Ϸ55 kDa) is important for dimerization and downstream signaling and consists of two subdomains termed the PAS repeat domain (PRD) and the histidine kinase-related domain (HKRD). These domains are 9%-11% identical to each other within representative phytochromes, and they are each 13%-17% identical to the histidine kinase domain of sensory transducers of bacterial two-component regulatory systems (8).The cyanobacterium Synechocystis has a phytochrome-like molecule called cph1, which has an amino-terminal chromophore domain very similar to that of higher plant phytochromes and a single carboxyl-terminal histidine kinase domain. Like other two-component sensory transducers, cph1 autophosphorylates on a histidine residue and then transfers this phosphate to an aspartate on a second protein. Both of these activities are higher when ...

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confidence: 99%

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confidence: 99%

The histidine kinase-related domain participates in phytochrome B function but is dispensable

Krall

Reed

2000

“…Arabidopsis relies on at least nine photosensory receptors, including five phytochromes (phyA-phyE), two cryptochromes (cry1 and cry2), and two phototropins (phot1 and phot2), to regulate most of its light responses (13)(14)(15)(16). Among these photoreceptors, phytochromes and cryptochromes are known to regulate flowering time (5).…”

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confidence: 99%

Regulation of photoperiodic flowering byArabidopsisphotoreceptors

Mockler

Yang

et al. 2003

273

213

Photoperiodism is a day-length-dependent seasonal change of physiological or developmental activities that is widely found in plants and animals. Photoperiodic flowering in plants is regulated by photosensory receptors including the red͞far-red light-receptor phytochromes and the blue͞UV-A light-receptor cryptochromes. However, the molecular mechanisms underlying the specific roles of individual photoreceptors have remained poorly understood. Here, we report a study of the day-length-dependent response of cryptochrome 2 (cry2) and phytochrome A (phyA) and their role as day-length sensors in Arabidopsis. The protein abundance of cry2 and phyA showed a diurnal rhythm in plants grown in short-day but not in plants grown in long-day. The short-day-specific diurnal rhythm of cry2 is determined primarily by blue light-dependent cry2 turnover. Consistent with a proposition that cry2 and phyA are the major day-length sensors in Arabidopsis, we show that phyA mediates far-red light promotion of flowering with modes of action similar to that of cry2. Based on these results and a finding that the photoperiodic responsiveness of plants depends on light quality, a model is proposed to explain how individual phytochromes and cryptochromes work together to confer photoperiodic responsiveness in Arabidopsis.P hotoperiodic flowering in plants was the first photoperiodism phenomenon documented (1). The flowering of longday (LD) or short-day (SD) plants occurs or is accelerated in the LD or SD condition, respectively. Arabidopsis is a facultative LD plant for which flowering-time regulation has been extensively studied (2-5). Although the detailed mechanism underlying photoperiodism is not well understood, extensive plant physiological studies support a hypothesis referred to as the external coincidence model (6-8). According to this hypothesis, the light signal must interact at the appropriate time of the day (or ''coincide'') with the photoperiodic response rhythm (PRR) of a cellular activity to confer photoperiodic responsiveness. It has been found that mRNA expression of flowering-time genes in Arabidopsis, including CO, GI, and FT, exhibited circadian rhythms, which have different phase shapes in plants grown in LD compared with plants grown in SD (9-12). Therefore, the day-length-dependent circadian expression of one or more flowering-time genes may represent the PRR.Arabidopsis relies on at least nine photosensory receptors, including five phytochromes (phyA-phyE), two cryptochromes (cry1 and cry2), and two phototropins (phot1 and phot2), to regulate most of its light responses (13-16). Among these photoreceptors, phytochromes and cryptochromes are known to regulate flowering time (5). It has also been found that phyA and cry2 protein abundance is regulated by light (17, 18) and that cry2 expression changes in response to photoperiod (19). These studies indicate that cry2 and phyA may act as major day-length sensors. Indeed, it has been found that the coincidence of light perception by cry2 and phyA with the peak circadian ...

“…Joanne Chory, Associate Professor of the Salk Institute for Biological Studies, gave the first talk of the session, an overview of molecular genetic approaches that delineate the signal transduction pathway from light absorption by photoreceptors to the light-dependent developmental changes that prepare a plant for photosynthetic activity (photomorphogenesis) (2). Plants utilize at least three systems to perceive light: UV photoreceptors; blue light photoreceptors; and the red light photoreceptor known as phytochrome.…”

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confidence: 99%

Signaling in plants

Mulligan

Chory

Ecker

1997

This paper serves as a summary of a symposium session as part of the Frontiers of Science series, held November 7-9, 1996, at the Arnold and Mabel Beckman Center of the National Academies of Sciences and Engineering in Irvine, CA. Signaling in plants ABSTRACTHigher plants are sessile organisms that perceive environmental cues such as light and chemical signals and respond by changing their morphologies. Signaling pathways utilize a complex network of interactions to orchestrate biochemical and physiological responses such as f lowering, fruit ripening, germination, photosynthetic regulation, and shoot or root development. In this session, the mechanisms of signaling systems that trigger plant responses to light and to the gaseous hormone, ethylene, were discussed. These signals are first sensed by a receptor and transmitted to the nucleus by a complex network. A signal may be transmitted to the nucleus by any of several systems including GTP binding proteins (G proteins), which change activity upon GTP binding; protein kinase cascades, which sequentially phosphorylate and activate a series of proteins; and membrane ion channels, which change ionic characteristics of the cells. The signal is manifested in the nucleus as a change in the activity of DNA-binding proteins, which are transcription factors that specifically interact and modulate the regulatory regions of genes. Thus, detection of an environmental signal is transmitted through a transduction pathway, and changes in transcription factor activity may coordinate changes in the expression of a portfolio of genes to direct new developmental programs.