Phytochrome phosphorylation in plant light signaling.

Reversible protein phosphorylation is a switching mechanism used in eukaryotes to regulate various cellular signalings. In plant light signaling, sophisticated photosensory receptor systems operate to modulate growth and development. The photoreceptors include phytochromes, cryptochromes and phototropins. Despite considerable progresses in defining the photosensory roles of these photoreceptors, the primary biochemical mechanisms by which the photoreceptor molecules transduce the perceived light signals into cellular responses remain to be elucidated. The signal-transducing photoreceptors in plants are all phosphoproteins and/or protein kinases, suggesting that light-dependent protein phosphorylation and dephosphorylation play important roles in the function of the photoreceptors. This review focuses on the role of phytochromes' reversible phosphorylation involved in the light signal transduction in plants.     

Shining light on location-biased cAMP signaling

cAMP is the indispensable second messenger regulating cell metabolism and function in response to extracellular hormones and neurotransmitters. cAMP is produced via the activation of G protein–coupled receptors located at both the cell surface and inside the cell. Recently, Tsvetanova et al. explored cAMP generation in distinct locations and the impact on respective cell functions. Using a phospho-proteomic analysis, they provide insight into the unique role of localized cAMP production in cellular phospho-responses.     

Green light signaling and adaptive response

To a plant, the sun’s light is not exclusively energy for photosynthesis, it also provides a package of data about time and prevailing conditions. The plant’s surroundings may dampen or filter solar energies, altering spectral profiles of their light environment. Plants use this information to adjust form and physiology, tailoring gene expression to best match ambient conditions. Extensive literature exists on how blue, red and far-red light contribute to plant adaptive responses. A growing body of work identifies discrete effects of green light (500–565 nm) that also shape plant biology. Green light responses are known to be either mediated through, or independent of, the cryptochrome blue light receptors. Responses to green light share a general tendency to oppose blue- or red-light-induced responses, including stem growth rate inhibition, anthocyanin accumulation or chloroplast gene expression. Recent evidence demonstrates a role for green light in sensing a shaded environment, independent from far-red shade responses.     

Elucidating the role of ascorbate in light signaling

In a recent study, Bournonville et al. identified that the tomato PAS/LOV (PLP) photoreceptor downregulates ascorbate synthesis via inhibiting the GDP-L-galactose phosphorylase (VTC2; GGP) activity. This finding describes the role of PLP as a novel regulator for dark-light regulation of ascorbate and provides insight for future research in this field.     

Role of Arabidopsis BBX proteins in light signaling

Journal of Plant Biochemistry and Biotechnology - Light regulates numerous aspects of plant growth and development like seed germination, seedling de-etiolation, pigment accumulation, cotyledon...     

Integration of light and temperature signaling pathways in plants

As two of the most important environmental factors, light and temperature regulate almost all aspects of plant growth and development. Under natural conditions, light is accompanied by warm temperatures and darkness by cooler temperatures, suggesting that light and temperature are tightly associated signals for plants. Indeed, accumulating evidence shows that plants have evolved a wide range of mechanisms to simultaneously perceive and respond to dynamic changes in light and temperature. Notably, the photoreceptor phytochrome B (phyB) was recently shown to function as a thermosensor, thus reinforcing the notion that light and temperature signaling pathways are tightly associated in plants. In this review, we summarize and discuss the current understanding of the molecular mechanisms integrating light and temperature signaling pathways in plants, with the emphasis on recent progress in temperature sensing, light control of plant freezing tolerance, and thermomorphogenesis. We also discuss the questions that are crucial for a further understanding of the interactions between light and temperature signaling pathways in plants.     

Adaptive signaling behavior in stomatopods under varying light conditions

Stomatopod crustaceans (mantis shrimp) are aggressive benthic marine predators with extraordinary color vision. When communicating with conspecifics, many stomatopods display conspicuously colored body areas, often in combination with other types of signals such as motion and chemical cues. Some species occupy wide depth ranges (>30 m), where changing light conditions can influence color perception. To test the potential effects of differing ambient lights on signaling behavior, stomatopods (Gonodactylus smithii) interacted with conspecifics in aquaria, under full-spectrum, high intensity light or light restricted in either spectrum or intensity. During intrasexual and intersexual trials in full-spectrum, high intensity light, animals performed more aggressive acts using colored body parts (meral spread, lunge, strike). Stomatopods used significantly more antennular flicking, and performed aggressive acts at reduced distances under restricted light conditions. To compare the use of antennules in visual and chemical communication, additional experiments showed more antennular flicking in response to chemical stimuli from food or conspecifics compared to seawater controls. This response ceased immediately after ablation of antennular chemoreceptors but returned to pre-treatment levels after 5 days of recovery. These findings suggest that stomatopods can vary their use of signals during conspecific interactions under different photic conditions. These inducible, plastic behavioral responses can potentially improve signal transfer in varying light environments.     

Melanopsin: another way of signaling light

A subset of melanopsin-expressing retinal ganglion cells has been identified to be directly photosensitive (pRGCs), modulating a range of behavioral and physiological responses to light. Recent expression studies of melanopsin have provided compelling evidence that melanopsin is the photopigment of the pRGCs. However, the mechanism by which melanopsin transduces light information remains an open question. This review discusses the signaling pathways that may underlie melanopsin-dependent phototransduction in native pRGCs, as well as the many exciting challenges ahead.     

Phototropins and chloroplast activity in plant blue light signaling

In plants, phototropins 1 (phot1) and 2 (phot2) mediate chloroplast movement to blue light (BL). A recent report showed that phototropins (phot) are required for the expression of chloroplast genes in rice. The light-induced responses of phot1a rice mutants result in H₂O₂-mediated damage to chloroplast photosystems, indicating that phot-regulated responses might be associated with the other photoreceptor, such as cryptochrome (cry) BL receptor. This suggests diversification and specialization of photoreceptor signaling in plants.Key words: blue light, blue light receptor, chloroplast, cryptochrome, H₂O₂, phototropin, signalingIn order to counteract the adverse effects of environmental ight, plants have evolved sensory mechanisms that monitor their surroundings and adapt their growth and development through the use of a complex signaling network.¹ Plants sense their environmental light conditions by using three principal families of signal-transducing photoreceptors; the red/far-red (R/FR) light-absorbing phytochromes (phy) and the UV-A/blue light (BL)-absorbing cryptochromes (cry) and phototropins (phot).² The phys are reversibly photochromic biliproteins that absorb maximally in the R and FR light regions of the spectrum. Cry and phot possess a pair of flavin derivates. Two cry and two phot family members have been identified and well characterized in Arabidopsis. Photoreceptors regulate development throughout the plant lifecycle, from seed germination through to plant maturation and the onset of reproduction. BL regulates a wide variety of photoresponses in higher plants, including chloroplast movement, inhibition of hypocotyls elongation, circadian timing, regulation of gene expression and stomatal opening.^3–5 The roles of individual photoreceptors in mediating plant development have, however, often been confounded by redundant, synergistic and in some cases mutually antagonistic mechanisms of action. The mechanisms of photoreceptor signal transduction are far from being completely elucidated, but are believed to involve both cytosolic and nuclear components. The presence of putative kinase domains within photoreceptor proteins has suggested a role for phosphorylation in light signaling. The action of cry1 and cry2 has been demonstrated to involve BL-mediated autophosphorylation.^6,7 Phot1 was originally identified as a 120 kDa-membrane associated protein displaying BL-mediated autophosphorylation.⁸ It is now well accepted that phot mediates chloroplast movement, phototropism and stomatal opening in plants in response to BL.     

Genome-wide investigation of light and carbon signaling interactions in Arabidopsis

Background  

Light and carbon are two essential signals influencing plant growth and development. Little is known about how carbon and light signaling pathways intersect or influence one another to affect gene expression.     

Degradation of phytochrome interacting factor 3 in phytochrome-mediated light signaling

Plant photoreceptors regulate various developmental processes. Among the photoreceptors, phytochromes, red and far-red light receptors, regulate light responses through many signaling components, including phytochrome-interacting proteins. The functional relationships among phytochromes and their interacting proteins, however, have not been clearly established. Here, we sought to identify a functional relationship between phytochromes and phytochrome interacting factor 3 (PIF3). We demonstrate that PIF3 is polyubiquitinated rapidly and subsequently degraded in PHYA and PHYB-mediated light signaling. We also show that the degradation of PIF3 is mediated by the 26S proteasome. Our data indicate that light-stimulated phytochromes cause the degradation of their interacting protein, PIF3, by the 26S proteasome.     

Shedding light on Toll signaling through live imaging

The Toll receptor propagates the ventralizing signal designating dorsal/ventral cell fate in the Drosophila embryo. The application of live-imaging approaches to this classical developmental signaling pathway is yielding some surprising new insights into Toll receptor signaling. In addition to its previously known plasma membrane localization, Toll is present in Rab5+ early endosomes. Dominant, constitutively active forms of Toll preferentially partition into endosomes. Blocking endocytosis locally prevents Toll from signaling suggesting that endocytosis is required for Toll to signal. Augmenting endocytosis increases Toll signaling. Both interventions alter the shape of the Dorsal gradient globally indicating an important role of endocytosis in fixing spatial information for the Dorsal gradient.     

How to control cyclic nucleotide signaling by light

An optogenetic tool is composed of a photosensor domain, which absorbs light, and an effector domain, which executes the enzymatic function. Optionally, a targeting sequence for restricted expression in subcellular compartments can be added. Local light stimulation allows to control the enzymatic activity with high temporal and spatial resolution.

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