Interplay among photoreceptors determines the strategy of coping with excess light in tomato.

This study investigates photoreceptor's role in the adaption of photosynthetic apparatus to high light (HL) intensity by examining the response of tomato wild type (WT) (Solanum lycopersicum L. cv. Moneymaker) and tomato mutants (phyA, phyB1, phyB2, cry1) plants to HL. Our results showed a photoreceptor-dependent effect of HL on the maximum quantum yield of photosystem II (Fv/Fm) with phyB1 exhibiting a decrease, while phyB2 exhibiting an increase in Fv/Fm. HL resulted in an increase in the efficient quantum yield of photosystem II (ΦPSII) and a decrease in the non-photochemical quantum yields (ΦNPQ and ΦN0) solely in phyA. Under HL, phyA showed a significant decrease in the energy-dependent quenching component of NPQ (qE), while phyB2 mutants showed an increase in the state transition (qT) component. Furthermore, ΔΔFv/Fm revealed that PHYB1 compensates for the deficit of PHYA in phyA mutants. PHYA signaling likely emerges as the dominant effector of PHYB1 and PHYB2 signaling within the HL-induced signaling network. In addition, PHYB1 compensates for the role of CRY1 in regulating Fv/Fm in cry1 mutants. Overall, the results of this research provide valuable insights into the unique role of each photoreceptor and their interplay in balancing photon energy and photoprotection under HL condition.

Blue light photoreceptor cryptochrome 1 promotes wood formation and anthocyanin biosynthesis in Populus.

Blue light photoreceptor cryptochrome 1 (CRY1) in herbaceous plants plays crucial roles in various developmental processes, including cotyledon expansion, hypocotyl elongation and anthocyanin biosynthesis. However, the function of CRY1 in perennial trees is unclear. In this study, we identified two ortholog genes of CRY1 (PagCRY1a and PagCRY1b) from Populus, which displayed high sequence similarity to Arabidopsis CRY1. Overexpression of PagCRY1 substantially inhibited plant growth and promoted secondary xylem development in Populus, while CRISPR/Cas9-mediated knockout of PagCRY1 enhanced plant growth and delayed secondary xylem development. Moreover, overexpression of PagCRY1 dramatically increased anthocyanin accumulation. The further analysis supported that PagCRY1 functions specifically in response to blue light. Taken together, our results demonstrated that modulating the expression of blue light photoreceptor CRY1 ortholog gene in Populus could significantly influence plant biomass production and the process of wood formation, laying a foundation for further investigating the light-regulated tree growth.

Plant photoreceptors and their signalling components in chloroplastic anterograde and retrograde communication.

The red phytochrome and blue cryptochrome plant photoreceptors play essential roles in promoting genome-wide changes in nuclear and chloroplastic gene expression for photomorphogenesis, plastid development, and greening. While their importance in anterograde signalling has been long recognized, the molecular mechanisms involved remain under active investigation. More recently, the intertwining of the light signalling cascades with the retrograde signals for the optimization of chloroplast functions has been acknowledged. Advances in the field support the participation of phytochromes, cryptochromes, and key light-modulated transcription factors, including HY5 and the PIFs, in the regulation of chloroplastic biochemical pathways that produce retrograde signals, including the tetrapyrroles and the chloroplastic MEP-isoprenoids. Interestingly, in a feedback loop, the photoreceptors and their signalling components are targets themselves of these retrograde signals, aimed at optimizing photomorphogenesis to the status of the chloroplasts, with GUN proteins functioning at the convergence points. High light and shade are also conditions where the photoreceptors tune growth responses to chloroplast functions. Interestingly, photoreceptors and retrograde signals also converge in the modulation of dual-localized proteins (chloroplastic/nuclear) including WHIRLY and HEMERA/pTAC12, whose functions are required for the optimization of photosynthetic activities in changing environments and are proposed to act themselves as retrograde signals.

Cryptochromes are the dominant photoreceptors mediating heliotropic responses of Arabidopsis inflorescences.

Inflorescence movements in response to natural gradients of sunlight are frequently observed in the plant kingdom and are suggested to contribute to reproductive success. Although the physiological and molecular bases of light-mediated tropisms in vegetative organs have been thoroughly investigated, the mechanisms that control inflorescence orientation in response to light gradients under natural conditions are not well understood. In this work, we have used a combination of laboratory and field experiments to investigate light-mediated re-orientation of Arabidopsis thaliana inflorescences. We show that inflorescence phototropism is promoted by photons in the UV and blue spectral range (≤500 nm) and depends on multiple photoreceptor families. Experiments under controlled conditions show that UVR8 is the main photoreceptor mediating the phototropic response to narrowband UV-B radiation, and phototropins and cryptochromes control the response to narrowband blue light. Interestingly, whereas phototropins mediate bending in response to low irradiances of blue, cryptochromes are the principal photoreceptors acting at high irradiances. Moreover, phototropins negatively regulate the action of cryptochromes at high irradiances of blue light. Experiments under natural field conditions demonstrate that cryptochromes are the principal photoreceptors acting in the promotion of the heliotropic response of inflorescences under full sunlight.

Identification of a dual orange/far-red and blue light photoreceptor from an oceanic green picoplankton.

Photoreceptors are conserved in green algae to land plants and regulate various developmental stages. In the ocean, blue light penetrates deeper than red light, and blue-light sensing is key to adapting to marine environments. Here, a search for blue-light photoreceptors in the marine metagenome uncover a chimeric gene composed of a phytochrome and a cryptochrome (Dualchrome1, DUC1) in a prasinophyte, Pycnococcus provasolii. DUC1 detects light within the orange/far-red and blue spectra, and acts as a dual photoreceptor. Analyses of its genome reveal the possible mechanisms of light adaptation. Genes for the light-harvesting complex (LHC) are duplicated and transcriptionally regulated under monochromatic orange/blue light, suggesting P. provasolii has acquired environmental adaptability to a wide range of light spectra and intensities.

A constitutively monomeric UVR8 photoreceptor confers enhanced UV-B photomorphogenesis.

The plant ultraviolet-B (UV-B) photoreceptor UVR8 plays an important role in UV-B acclimation and survival. UV-B absorption by homodimeric UVR8 induces its monomerization and interaction with the E3 ubiquitin ligase COP1, leading ultimately to gene expression changes. UVR8 is inactivated through redimerization, facilitated by RUP1 and RUP2. Here, we describe a semidominant, hyperactive allele, namely uvr8-17D, that harbors a glycine-101 to serine mutation. UVR8G101S overexpression led to weak constitutive photomorphogenesis and extreme UV-B responsiveness. UVR8G101S was observed to be predominantly monomeric in vivo and, once activated by UV-B, was not efficiently inactivated. Analysis of a UVR8 crystal structure containing the G101S mutation revealed the distortion of a loop region normally involved in stabilization of the UVR8 homodimer. Plants expressing a UVR8 variant combining G101S with the previously described W285A mutation exhibited robust constitutive photomorphogenesis. This work provides further insight into UVR8 activation and inactivation mechanisms and describes a genetic tool for the manipulation of photomorphogenic responses.

Expression of Tomato UVR8 in Arabidopsis reveals conserved photoreceptor function.

UV RESISTANCE LOCUS 8 (UVR8) is a photoreceptor that regulates UV-B photomorphogenesis in plants. UV-B photon perception promotes UVR8 homodimer dissociation into monomer, which is reverted to homodimer post UV-B, forming a complete photocycle. UVR8 monomer interacts with CONSTITUTIVELY PHOTOMORPHOGENEIC 1 (COP1) to initiate UV-B signaling. The function and mechanism of Arabidopsis UVR8 (AtUVR8) are extensively investigated, however, little is known about UVR8 and its signaling mechanisms in other plant species. Tomato is a widely used model plant for horticulture research. In this report we tested whether an ortholog of AtUVR8 in Tomato (SIUVR8) can complement Arabidopsis uvr8 mutant and whether the above-mentioned key signaling mechanisms of UVR8 are conserved. Heterologous expressed SIUVR8 in an Arabidopsis uvr8 null mutant rescued the uvr8 mutant in the tested UV-B responses including hypocotyl elongation, UV-B target gene expression and anthocyanin accumulation, demonstrating that the SIUVR8 is a putative UV-B photoreceptor. Moreover, in response to UV-B, SIUVR8 forms a protein complex with Arabidopsis COP1 in plants, suggesting conserved signaling mechanism. SIUVR8 exhibits similar photocycle as AtUVR8 in plants, which highlights conserved photoreceptor activation and inactivation mechanisms.

An Optogenetic Tool for Induced Protein Stabilization Based on the Phaeodactylum tricornutum Aureochrome 1a Light-Oxygen-Voltage Domain.

Control of cellular events by optogenetic tools is a powerful approach to manipulate cellular functions in a minimally invasive manner. A common problem posed by the application of optogenetic tools is to tune the activity range to be physiologically relevant. Here, we characterized a photoreceptor of the light-oxygen-voltage (LOV) domain family of Phaeodactylum tricornutum aureochrome 1a (AuLOV) as a tool for increasing protein stability under blue light conditions in budding yeast. Structural studies of AuLOVwt, the variants AuLOVM254, and AuLOVW349 revealed alternative dimer association modes for the dark state, which differ from previously reported AuLOV dark-state structures. Rational design of AuLOV-dimer interface mutations resulted in an optimized optogenetic tool that we fused to the photoactivatable adenylyl cyclase from Beggiatoa sp. This synergistic light-regulation approach using two photoreceptors resulted in an optimized, photoactivatable adenylyl cyclase with a cyclic adenosine monophosphate production activity that matches the physiological range of Saccharomyces cerevisiae. Overall, we enlarged the optogenetic toolbox for yeast and demonstrated the importance of fine-tuning the optogenetic tool activity for successful application in cells.

Central clock components modulate plant shade avoidance by directly repressing transcriptional activation activity of PIF proteins.

Light-environment signals, sensed by plant phytochrome photoreceptors, are transduced to target genes through direct regulation of PHYTOCHROME-INTERACTING FACTOR (PIF) transcription factor abundance and activity. Previous genome-wide DNA-binding and expression analysis has identified a set of genes that are direct targets of PIF transcriptional regulation. However, quantitative analysis of promoter occupancy versus expression level has suggested that unknown "trans factors" modulate the intrinsic transcriptional activation activity of DNA-bound PIF proteins. Here, using computational analysis of published data, we have identified PSEUDO-RESPONSE REGULATORS (PRR5 and PRR7) as displaying a high frequency of colocalization with the PIF proteins at their binding sites in the promoters of PIF Direct Target Genes (DTGs). We show that the PRRs function to suppress PIF-stimulated growth in the light and vegetative shade and that they repress the rapid PIF-induced expression of PIF-DTGs triggered by exposure to shade. The repressive action of the PRRs on both growth and DTG expression requires the PIFs, indicating direct action on PIF activity, rather than a parallel antagonistic pathway. Protein interaction assays indicate that the PRRs exert their repressive activity by binding directly to the PIF proteins in the nucleus. These findings support the conclusion that the PRRs function as direct outputs from the core circadian oscillator to regulate the expression of PIF-DTGs through modulation of PIF transcriptional activation activity, thus expanding the roles of the multifunctional PIF-signaling hub.

Babo1, formerly Vop1 and Cop1/2, is no eyespot photoreceptor but a basal body protein illuminating cell division in Volvox carteri.

In photosynthetic organisms many processes are light dependent and sensing of light requires light-sensitive proteins. The supposed eyespot photoreceptor protein Babo1 (formerly Vop1) has previously been classified as an opsin due to the capacity for binding retinal. Here, we analyze Babo1 and provide evidence that it is no opsin. Due to the localization at the basal bodies, the former Vop1 and Cop1/2 proteins were renamed V.c. Babo1 and C.r. Babo1. We reveal a large family of more than 60 Babo1-related proteins from a wide range of species. The detailed subcellular localization of fluorescence-tagged Babo1 shows that it accumulates at the basal apparatus. More precisely, it is located predominantly at the basal bodies and to a lesser extent at the four strands of rootlet microtubules. We trace Babo1 during basal body separation and cell division. Dynamic structural rearrangements of Babo1 particularly occur right before the first cell division. In four-celled embryos Babo1 was exclusively found at the oldest basal bodies of the embryo and on the corresponding d-roots. The unequal distribution of Babo1 in four-celled embryos could be an integral part of a geometrical system in early embryogenesis, which establishes the anterior-posterior polarity and influences the spatial arrangement of all embryonic structures and characteristics. Due to its retinal-binding capacity, Babo1 could also be responsible for the unequal distribution of retinoids, knowing that such concentration gradients of retinoids can be essential for the correct patterning during embryogenesis of more complex organisms. Thus, our findings push the Babo1 research in another direction.

Engineering the phototropin photocycle improves photoreceptor performance and plant biomass production.

The ability to enhance photosynthetic capacity remains a recognized bottleneck to improving plant productivity. Phototropin blue light receptors (phot1 and phot2) optimize photosynthetic efficiency in Arabidopsis thaliana by coordinating multiple light-capturing processes. In this study, we explore the potential of using protein engineering to improve photoreceptor performance and thereby plant growth. We demonstrate that targeted mutagenesis can decrease or increase the photocycle lifetime of Arabidopsis phototropins in vitro and show that these variants can be used to reduce or extend the duration of photoreceptor activation in planta Our findings show that slowing the phototropin photocycle enhanced several light-capturing responses, while accelerating it reduced phototropin's sensitivity for chloroplast accumulation movement. Moreover, plants engineered to have a slow-photocycling variant of phot1 or phot2 displayed increased biomass production under low-light conditions as a consequence of their improved sensitivity. Together, these findings demonstrate the feasibility of engineering photoreceptors to manipulate plant growth and offer additional opportunities to enhance photosynthetic competence, particularly under suboptimal light regimes.

Optical Control of Transcription: Genetically Encoded Photoswitchable Variants of T7 RNA Polymerase.

Light-sensing protein domains that link an exogenous light signal to the activity of an enzyme have attracted much attention for the engineering of new regulatory mechanisms into proteins and for studying the dynamic behavior of intracellular reactions and reaction cascades. Light-oxygen-voltage (LOV) photoreceptors are blue-light-sensing modules that have been intensely characterized for this purpose and linked to several proteins of interest. For the successful application of these tools, it is crucial to identify appropriate fusion strategies for combining sensor and enzyme domains that sustain activity and light-induced responsivity. Terminal fusion of LOV domains is the natural strategy; however, this is not transferrable to T7 RNA polymerase because both of its termini are involved in catalysis. It is shown herein that it is possible to covalently insert LOV domains into the polymerase protein, while preserving its activity and generating new light-responsive allosteric coupling.

Plant photoreceptors: Multi-functional sensory proteins and their signaling networks.

Light is a crucial environmental cue not only for photosynthetic energy production but also for plant growth and development. Plants employ sophisticated methods to detect and interpret information from incoming light. Five classes of photoreceptors have been discovered in the model plant Arabidopsis thaliana. These photoreceptors act either distinctly and/or redundantly in fine-tuning many aspects of plant life cycle. Unlike mobile animals, sessile plants have developed an enormous plasticity to adapt and survive in changing environment. By monitoring different information arising from ambient light, plants precisely regulate downstream signaling pathways to adapt accordingly. Given that changes in the light environment is typically synchronized with other environmental cues such as temperature, abiotic stresses, and seasonal changes, it is not surprising that light signaling pathways are interconnected with multiple pathways to regulate plant physiology and development. Indeed, recent advances in plant photobiology revealed a large network of co-regulation among different photoreceptor signaling pathways as well as other internal signaling pathways (e.g., hormone signaling). In addition, some photoreceptors are directly involved in perception of non-light stimuli (e.g., temperature). Therefore, understanding highly inter-connected signaling networks is essential to explore the photoreceptor functions in plants. Here, we summarize how plants co-ordinate multiple photoreceptors and their internal signaling pathways to regulate a myriad of downstream responses at molecular and physiological levels.

Molecular cloning and functional analysis of a UV-B photoreceptor gene, BpUVR8 (UV Resistance Locus 8), from birch and its role in ABA response.

As a photoreceptor specifically for UV-B light, UVR8 gene plays an important role in the photomorphogenesis and developmental growth of plants. In this research, we isolated the UVR8 gene from birch, named BpUVR8 (AHY02156). BpUVR8 overexpression rescued the uvr8 mutant phenotype using functional complementation assay of BpUVR8 in Arabidopsis uvr8 mutants, which showed that the function of UVR8 is conserved between Arabidopsis and birch. The expression analysis of BpUVR8 indicated that this gene is expressed in various tissues, but its expression levels in leaves are higher than in other organs. Moreover, abiotic stress factors, such as UV-B, salinity, and abscisic acid (ABA) can induce the expression of BpUVR8 gene. Interestingly, the analysis of promoter activity indicated that BpUVR8 promoter not only has the promoting activity but can also respond to the induction of abiotic stress and ABA signal. So, we analyzed its function in ABA response via transgenic UVR8 overexpression in Arabidopsis. The BpUVR8 enhances the susceptibility to ABA, which indicates that BpUVR8 is regulated by ABA and can inhibit seed germination. The root length of 20-day-old 35S::BpUVR8/WT transgenic plants was 18% reduced as compared to the wild-type under the ABA treatment. The membrane of the BpUVR8-overexpressing in Arabidopsis thaliana was the most damaged after ABA treatment and 35S::BpUVR8/WT transgenic plant was more sensitive to ABA than the wild type. These results showed that BpUVR8 is a positive regulator in the ABA signal transduction pathway. In the presence of low dose of UV-B, the sensitivity of wild-type and 35S::BpUVR8/WT plants to ABA was reduced. Moreover, BpUVR8 regulates the expression of a subset of ABA-responsive genes, both in Arabidopsis and Betula platyphylla, under the ABA treatment. Our data provide evidence that BpUVR8 is a positive regulator in the UV-B-induced photomorphogenesis in plants. Moreover, we propose from this research that BpUVR8 might have an important role in integrating plant growth and ABA signaling pathway.

Blue-Light Receptors for Optogenetics.

Sensory photoreceptors underpin light-dependent adaptations of organismal physiology, development, and behavior in nature. Adapted for optogenetics, sensory photoreceptors become genetically encoded actuators and reporters to enable the noninvasive, spatiotemporally accurate and reversible control by light of cellular processes. Rooted in a mechanistic understanding of natural photoreceptors, artificial photoreceptors with customized light-gated function have been engineered that greatly expand the scope of optogenetics beyond the original application of light-controlled ion flow. As we survey presently, UV/blue-light-sensitive photoreceptors have particularly allowed optogenetics to transcend its initial neuroscience applications by unlocking numerous additional cellular processes and parameters for optogenetic intervention, including gene expression, DNA recombination, subcellular localization, cytoskeleton dynamics, intracellular protein stability, signal transduction cascades, apoptosis, and enzyme activity. The engineering of novel photoreceptors benefits from powerful and reusable design strategies, most importantly light-dependent protein association and (un)folding reactions. Additionally, modified versions of these same sensory photoreceptors serve as fluorescent proteins and generators of singlet oxygen, thereby further enriching the optogenetic toolkit. The available and upcoming UV/blue-light-sensitive actuators and reporters enable the detailed and quantitative interrogation of cellular signal networks and processes in increasingly more precise and illuminating manners.

Geomagnetic field impacts on cryptochrome and phytochrome signaling.

The geomagnetic field (GMF) is an environmental element whose instability affects plant growth and development. Despite known plant responses to GMF direction and intensity, the mechanism of magnetoreception in plants is still not known. Magnetic field variations affect many light-dependent plant processes, suggesting that the magnetoreception could require light. The objective of this work was to comprehensively investigate the influence of GMF on Arabidopsis thaliana (Col-0) photoreceptor signaling. Wild-type Arabidopsis seedlings and photoreceptor-deficient mutants (cry1cry2, phot1, phyA and phyAphyB) were exposed to near null magnetic field (NNMF, ≤40 nT) and GMF (~43 µT) under darkness and different light wavelengths. The GMF did not alter skotomorphogenic or photomorphogenic seedling development but had a significant impact on gene expression pathways downstream of cryptochrome and phytochrome photoactivation. GMF-induced changes in gene expression observed under blue light were partially associated with an alteration of cryptochrome activation. GMF impacts on phytochrome-regulated gene expression could be attributed to alterations in phytochrome protein abundance that were also dependent on the presence of cry1, cry2 and phot1. Moreover, the GMF was found to impact photomorphogenic-promoting gene expression in etiolated seedlings, indicating the existence of a light-independent magnetoreception mechanism. In conclusion, our data shows that magnetoreception alters photoreceptor signaling in Arabidopsis, but it does not necessarily depend on light.

Tomato UV-B receptor SlUVR8 mediates plant acclimation to UV-B radiation and enhances fruit chloroplast development via regulating SlGLK2.

Plants utilize energy from sunlight to perform photosynthesis in chloroplast, an organelle that could be damaged by solar UV radiation. The ultraviolet-B (UV-B) photoreceptor UVR8 is required for UV-B perception and signal transduction. However, little is known about how UVR8 influence chloroplast development under UV-B radiation. Here, we characterized tomato UVR8 gene (SlUVR8) and our results indicated that SlUVR8 facilitate plant acclimation to UV-B stress by orchestrating expression of the UVB-responsive genes (HY5 and CHS) and accumulating UV-absorptive compounds. In addition, we also discovered that SlUVR8 promotes fruit chloroplast development through enhancing accumulation of transcription factor GOLDEN2-LIKE2 (SlGLK2) which determines chloroplast and chlorophyll levels. Furthermore, UV-B radiation could increase expression of SlGLK2 and its target genes in fruits and leaves. SlUVR8 is required for UVB-induced SlGLK2 expression. Together, our work not only identified the conserved functions of SlUVR8 gene in response to UV-B stress, but also uncovered a novel role that SlUVR8 could boost chloroplast development by accumulating SlGLK2 proteins.

Perception of Sunflecks by the UV-B Photoreceptor UV RESISTANCE LOCUS8.

Sunflecks, transient patches of light that penetrate through gaps in the canopy and transiently interrupt shade, are eco-physiologically and agriculturally important sources of energy for carbon gain, but our molecular understanding of how plant organs perceive and respond to sunflecks through photoreceptors remains limited. The UV-B photoreceptor UV RESISTANCE LOCUS8 (UVR8) is a recent addition to the list of plant photosensory receptors, and we have made considerable advances in our understanding of the physiology and molecular mechanisms of action of UVR8 and its signaling pathway. However, the function of UVR8 in the natural environment is poorly understood. Here, we show that the UVR8 dimer/monomer ratio responds quantitatively and reversibly to the intensity of sunflecks that interrupt shade in the field. Sunflecks reduced hypocotyl growth and increased CHALCONE SYNTHASE (CHS) and ELONGATED HYPOCOTYL5 gene expression and CHS protein abundance in wild-type Arabidopsis (Arabidopsis thaliana) seedlings, but the uvr8 mutant was impaired in these responses. UVR8 was also required for normal nuclear dynamics of CONSTITUTIVELY PHOTOMORPHOGENIC1. We propose that UVR8 plays an important role in the plant perception of and response to sunflecks.

Expression of succinate dehydrogenase and fumarase genes in maize leaves is mediated by cryptochrome.

Blue light inhibits succinate dehydrogenase and fumarase enzyme activity and gene expression in green leaves of maize (Zea mays L.). Irradiation of maize plants by blue light resulted in the transient decrease of transcripts of genes Sdh1-2 and Sdh2-3 encoding correspondingly the flavoprotein and iron-sulfur protein subunits of succinate dehydrogenase, and of Fum1 encoding the mitochondrial form of fumarase. The blue light effect was probably mediated by transcription factors COP1 and HY5, with the expression of the latter increased upon blue light treatment. This was accompanied by a decrease in the expression of COP1, presumably involved in proteasome degradation of HY5. It was also demonstrated that calcium ions do not participate in this process.