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Although green light (GL) is located in the middle of the visible light spectrum and regulates a series of plant developmental processes, the mechanism by which it regulates seedling development is largely unknown. In this study, we demonstrated that GL promotes atypical photomorphogenesis in Arabidopsis thaliana via the dual regulations of phytochrome B (phyB) and phyA. Although the Pr-to-Pfr conversion rates of phyB and phyA under GL were lower than those under red light (RL) in a fluence rate-dependent and time-dependent manner, long-term treatment with GL induced high Pfr/Pr ratios of phyB and phyA. Moreover, GL induced the formation of numerous small phyB photobodies in the nucleus, resulting in atypical photomorphogenesis, with smaller cotyledon opening angles and longer hypocotyls in seedlings compared to RL. The abundance of phyA significantly decreased after short- and long-term GL treatments. We determined that four major PHYTOCHROME-INTERACTING FACTORs (PIFs: PIF1, PIF3, PIF4, and PIF5) act downstream of phyB in GL-mediated cotyledon opening. In addition, GL plays opposite roles in regulating different PIFs. For example, under continuous GL, the protein levels of all PIFs decreased, whereas the transcript levels of PIF4 and PIF5 strongly increased compared with dark treatment. Taken together, our work provides a detailed molecular framework for understanding the role of the antagonistic regulations of phyB and phyA in GL-mediated atypical photomorphogenesis., (© 2024 Institute of Botany, Chinese Academy of Sciences.)

Increasing planting density is a key strategy for enhancing maize yields 1-3 . An ideotype for dense planting requires a 'smart canopy' with leaf angles at different canopy layers differentially optimized to maximize light interception and photosynthesis 4-6 , among other features. Here we identified leaf angle architecture of smart canopy 1 (lac1), a natural mutant with upright upper leaves, less erect middle leaves and relatively flat lower leaves. lac1 has improved photosynthetic capacity and attenuated responses to shade under dense planting. lac1 encodes a brassinosteroid C-22 hydroxylase that predominantly regulates upper leaf angle. Phytochrome A photoreceptors accumulate in shade and interact with the transcription factor RAVL1 to promote its degradation via the 26S proteasome, thereby inhibiting activation of lac1 by RAVL1 and decreasing brassinosteroid levels. This ultimately decreases upper leaf angle in dense fields. Large-scale field trials demonstrate that lac1 boosts maize yields under high planting densities. To quickly introduce lac1 into breeding germplasm, we transformed a haploid inducer and recovered homozygous lac1 edits from 20 diverse inbred lines. The tested doubled haploids uniformly acquired smart-canopy-like plant architecture. We provide an important target and an accelerated strategy for developing high-density-tolerant cultivars, with lac1 serving as a genetic chassis for further engineering of a smart canopy in maize., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

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 (F v /F m ) with phyB1 exhibiting a decrease, while phyB2 exhibiting an increase in F v /F m . HL resulted in an increase in the efficient quantum yield of photosystem II (Φ PSII ) and a decrease in the non-photochemical quantum yields (Φ NPQ and Φ N0 ) solely in phyA. Under HL, phyA showed a significant decrease in the energy-dependent quenching component of NPQ (q E ), while phyB2 mutants showed an increase in the state transition (q T ) component. Furthermore, ΔΔF v /F m 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 F v /F m 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., (© 2024 Society for Experimental Biology and John Wiley & Sons Ltd.)

The involvement of the microRNA miR165a in the light-dependent mechanisms of regulation of target genes in maize ( Zea mays ) has been studied. The light-induced change in the content of free miR165a was associated with its binding by the AGO10 protein and not with a change in the rate of its synthesis from the precursor. The use of knockout Arabidopsis plants for the phytochrome A and B genes demonstrated that the presence of an active form of phytochrome B causes an increase in the level of the RNA-induced silencing miR165a complex, which triggers the degradation of target mRNAs. The two fractions of vesicles from maize leaves, P40 and P100 that bind miR165a, were isolated by ultracentrifugation. The P40 fraction consisted of larger vesicles of the size >0.170 µm, while the P100 fraction vesicles were <0.147 µm. Based on the quantitative PCR data, the predominant location of miR165a on the surface of extracellular vesicles of both fractions was established. The formation of the active form of phytochrome upon the irradiation of maize plants with red light led to a redistribution of miR165a, resulting in an increase in its proportion inside P40 vesicles and a decrease in P100 vesicles.

Phytochrome A (phyA) is the plant far-red (FR) light photoreceptor and plays an essential role in regulating photomorphogenic development in FR-rich conditions, such as canopy shade. It has long been observed that phyA is a phosphoprotein in vivo; however, the protein kinases that could phosphorylate phyA remain largely unknown. Here we show that a small protein kinase family, consisting of four members named PHOTOREGULATORY PROTEIN KINASES (PPKs) (also known as MUT9-LIKE KINASES), directly phosphorylate phyA in vitro and in vivo. In addition, TANDEM ZINC-FINGER/PLUS3 (TZP), a recently characterized phyA-interacting protein required for in vivo phosphorylation of phyA, is also directly phosphorylated by PPKs. We reveal that TZP contains two intrinsically disordered regions in its amino-terminal domain that undergo liquid-liquid phase separation (LLPS) upon light exposure. The LLPS of TZP promotes colocalization and interaction between PPKs and phyA, thus facilitating PPK-mediated phosphorylation of phyA in FR light. Our study identifies PPKs as a class of protein kinases mediating the phosphorylation of phyA and demonstrates that the LLPS of TZP contributes significantly to more production of the phosphorylated phyA form in FR light., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

The DNA damage response avoids mutations into dividing cells. Here, we analysed the role of photoreceptors on the restriction of root growth imposed by genotoxic agents and its relationship with cell viability and performance of meristems. Comparison of root growth of Arabidopsis WT, phyA-211, phyB-9, and phyA-211phyB-9 double mutants unveiled a critical role for phytochrome A (PhyA) in protecting roots from genotoxic stress, regeneration and cell replenishment in the meristematic zone. PhyA was located on primary root tips, where it influences genes related to the repair of DNA, including ERF115 and RAD51. Interestingly, phyA-211 mutants treated with zeocin failed to induce the expression of the repressor of cell cycle MYB3R3, which correlated with expression of the mitotic cyclin CycB1, suggesting that PhyA is required for safeguarding the DNA integrity during cell division. Moreover, the growth of the primary roots of PhyA downstream component HY5 and root growth analyses in darkness suggest that cell viability and DNA damage responses within root meristems may act independently from light and photomorphogenesis. These data support novel roles for PhyA as a key player for stem cell niche maintenance and DNA damage responses, which are critical for proper root growth., (© 2024 John Wiley & Sons Ltd.)

This study investigated the effect of light intensity and signaling on the regulation of far-red (FR)-induced alteration in photosynthesis. The low (LL: 440 μmol m -2 s -1 ) and high (HL: 1135 μmol m -2 s -1 ) intensity of white light with or without FR (LLFR: 545 μmol m -2 s -1 including 115 μmol m -2 s -1 ; HLFR: 1254 μmol m -2 s -1  + 140 μmol m -2 s -1 ) was applied on the tomato cultivar (Solanum Lycopersicon cv. Moneymaker) and mutants of phytochrome A (phyA) and phytochrome B (phyB1, and phyB2). Both light intensity and FR affected plant morphological traits, leaf biomass, and flowering time. Irrespective of genotype, flowering was delayed by LLFR and accelerated by HLFR compared to the corresponding light intensity without FR. In LLFR, a reduced energy flux through the electron transfer chain along with a reduced energy dissipation per reaction center improved the maximum quantum yield of PSII, irrespective of genotype. HLFR increased net photosynthesis and gas exchange properties in a genotype-dependent manner. FR-dependent regulation of hormones was affected by light signaling. It appeared that PHYB affected the levels of abscisic acid and salicylic acid while PHYA took part in the regulation of CK in FR-exposed plants. Overall, light intensity and signaling of FR influenced plants' photosynthesis and growth by altering electron transport, gas exchange, and changes in the level of endogenous hormones., Competing Interests: Declaration of competing interest Authors declare there is no conflict of interest., (Copyright © 2024 Elsevier Masson SAS. All rights reserved.)

In order to flower in the appropriate season, plants monitor light and temperature changes and alter downstream pathways that regulate florigen genes such as Arabidopsis (Arabidopsis thaliana) FLOWERING LOCUS T (FT). In Arabidopsis, FT messenger RNA levels peak in the morning and evening under natural long-day conditions (LDs). However, the regulatory mechanisms governing morning FT induction remain poorly understood. The morning FT peak is absent in typical laboratory LDs characterized by high red:far-red light (R:FR) ratios and constant temperatures. Here, we demonstrate that ZEITLUPE (ZTL) interacts with the FT repressors TARGET OF EATs (TOEs), thereby repressing morning FT expression in natural environments. Under LDs with simulated sunlight (R:FR = 1.0) and daily temperature cycles, which are natural LD-mimicking environmental conditions, FT transcript levels in the ztl mutant were high specifically in the morning, a pattern that was mirrored in the toe1 toe2 double mutant. Low night-to-morning temperatures increased the inhibitory effect of ZTL on morning FT expression by increasing ZTL protein levels early in the morning. Far-red light counteracted ZTL activity by decreasing its abundance (possibly via phytochrome A (phyA)) while increasing GIGANTEA (GI) levels and negatively affecting the formation of the ZTL-GI complex in the morning. Therefore, the phyA-mediated high-irradiance response and GI play pivotal roles in morning FT induction. Our findings suggest that the delicate balance between low temperature-mediated ZTL activity and the far-red light-mediated functions of phyA and GI offers plants flexibility in fine-tuning their flowering time by controlling FT expression in the morning., (© 2023 The Authors. Journal of Integrative Plant Biology published by John Wiley & Sons Australia, Ltd on behalf of Institute of Botany, Chinese Academy of Sciences.)

Exposure of dark-grown etiolated seedlings to light triggers the transition from skotomorphogenesis/etiolation to photomorphogenesis/de-etiolation. In the life cycle of plants, de-etiolation is essential for seedling development and plant survival. The mobilization of soluble sugars (glucose [Glc], sucrose, and fructose) derived from stored carbohydrates and lipids to target organs, including cotyledons, hypocotyls, and radicles, underpins de-etiolation. Therefore, dynamic carbohydrate biochemistry is a key feature of this phase transition. However, the molecular mechanisms coordinating carbohydrate status with the cellular machinery orchestrating de-etiolation remain largely opaque. Here, we show that the Glc sensor HEXOKINASE 1 (HXK1) interacts with GROWTH REGULATOR FACTOR5 (GRF5), a transcriptional activator and key plant growth regulator, in Arabidopsis (Arabidopsis thaliana). Subsequently, GRF5 directly binds to the promoter of phytochrome A (phyA), encoding a far-red light (FR) sensor/cotyledon greening inhibitor. We demonstrate that the status of Glc within dark-grown etiolated cotyledons determines the de-etiolation of seedlings when exposed to light irradiation by the HXK1-GRF5-phyA molecular module. Thus, following seed germination, accumulating Glc within dark-grown etiolated cotyledons stimulates a HXK1-dependent increase of GRF5 and an associated decrease of phyA, triggering the perception, amplification, and relay of HXK1-dependent Glc signaling, thereby facilitating the de-etiolation of seedlings following light irradiation. Our findings, therefore, establish how cotyledon carbohydrate signaling under subterranean darkness is sensed, amplified, and relayed, determining the phase transition from skotomorphogenesis to photomorphogenesis on exposure to light irradiation., Competing Interests: Conflict of interest statement. None declared., (© American Society of Plant Biologists 2023. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Soil salinity is one of the most detrimental abiotic stresses affecting plant survival, and light is a core environmental signal regulating plant growth and responses to abiotic stress. However, how light modulates the plant's response to salt stress remains largely obscure. Here, we show that Arabidopsis (Arabidopsis thaliana) seedlings are more tolerant to salt stress in the light than in the dark, and that the photoreceptors phytochrome A (phyA) and phyB are involved in this tolerance mechanism. We further show that phyA and phyB physically interact with the salt tolerance regulator SALT OVERLY SENSITIVE2 (SOS2) in the cytosol and nucleus, and enhance salt-activated SOS2 kinase activity in the light. Moreover, SOS2 directly interacts with and phosphorylates PHYTOCHROME-INTERACTING FACTORS PIF1 and PIF3 in the nucleus. Accordingly, PIFs act as negative regulators of plant salt tolerance, and SOS2 phosphorylation of PIF1 and PIF3 decreases their stability and relieves their repressive effect on plant salt tolerance in both light and dark conditions. Together, our study demonstrates that photoactivated phyA and phyB promote plant salt tolerance by increasing SOS2-mediated phosphorylation and degradation of PIF1 and PIF3, thus broadening our understanding of how plants adapt to salt stress according to their dynamic light environment., Competing Interests: Conflict of interest statement. The authors declare no conflict of interests., (© American Society of Plant Biologists 2023. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

In natural long days, the florigen gene FLOWERING LOCUS T (FT) shows a bimodal expression pattern with morning and dusk peaks in Arabidopsis. This pattern differs from the one observed in the laboratory, and little is known about underlying mechanisms. A red : far-red (R : FR) ratio difference between sunlight and fluorescent light causes this FT pattern mismatch. We showed that bimodal FT expression patterns were induced in a day longer than 14 h with sunlight R : FR (= c. 1) conditions. By circadian gating experiments, we found that cumulative exposure of R : FR-adjusted light (R : FR ratio was adjusted to 1 with FR supplement) spanning from the afternoon to the next morning required full induction of FT in the morning. Conversely, only 2 h of R : FR adjustment in the late afternoon was sufficient for FT induction at dusk. We identified that phytochrome A (phyA) is required for the morning FT expression in response to the R : FR adjustment on the previous day. As a part of this mechanism, we showed that PHYTOCHROME-INTERACTING FACTOR 7 contributes to FT regulation. Our results suggest that phyA-mediated high-irradiance response and the external coincidence mechanism contribute to morning FT induction under natural long-day conditions., (© 2023 The Authors. New Phytologist © 2023 New Phytologist Foundation.)

Plants can sense the shade from neighboring plants by detecting a reduction of the red:far-red light (R:FR) ratio. Phytochrome B (phyB) is the primary photoreceptor that perceives shade light and regulates jasmonic acid (JA) signaling. However, the molecular mechanisms underlying phyB and JA signaling integration in shade responses remain largely unknown. Here, we show the interaction of phyB and FAR-RED INSENSITIVE 219 (FIN219)/JASMONATE RESISTANT1 (JAR1) in a functional demand manner in Arabidopsis (Arabidopsis thaliana) seedling development. Genetic evidence and interaction studies indicated that phyB and FIN219 synergistically and negatively regulate shade-induced hypocotyl elongation. Moreover, phyB interacted with various isoforms of FIN219 under high and low R:FR light. Methyl jasmonate (MeJA) treatment, FIN219 mutation, and PHYBOE digalactosyldiacylglycerol synthase1-1 (dgd1-1) plants, which show increased levels of JA, altered the patterns of phyB-associated nuclear speckles under the same conditions. Surprisingly, PHYBOE dgd1-1 showed a shorter hypocotyl phenotype than its parental mutants under shade conditions. Microarray assays using PHYBOE and PHYBOE fin219-2 indicated that PHYB overexpression substantially affects defense response-related genes under shade light and coregulates expression of auxin-responsive genes with FIN219. Thus, our findings reveal that phyB substantially crosstalks with JA signaling through FIN219 to modulate seedling development under shade light., Competing Interests: Conflict of interest statement. The authors declare no competing interest., (© American Society of Plant Biologists 2023. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Integration of light and phytohormones is essential for plant growth and development. FAR-RED INSENSITIVE 219 (FIN219)/JASMONATE RESISTANT 1 (JAR1) participates in phytochrome A (phyA)-mediated far-red (FR) light signaling in Arabidopsis and is a jasmonate (JA)-conjugating enzyme for the generation of an active JA-isoleucine. Accumulating evidence indicates that FR and JA signaling integrate with each other. However, the molecular mechanisms underlying their interaction remain largely unknown. Here, the phyA mutant was hypersensitive to JA. The double mutant fin219-2phyA-211 showed a synergistic effect on seedling development under FR light. Further evidence revealed that FIN219 and phyA antagonized with each other in a mutually functional demand to modulate hypocotyl elongation and expression of light- and JA-responsive genes. Moreover, FIN219 interacted with phyA under prolonged FR light, and MeJA could enhance their interaction with CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) in the dark and FR light. FIN219 and phyA interaction occurred mainly in the cytoplasm, and they regulated their mutual subcellular localization under FR light. Surprisingly, the fin219-2 mutant abolished the formation of phyA nuclear bodies under FR light. Overall, these data identified a vital mechanism of phyA-FIN219-COP1 association in response to FR light, and MeJA may allow the photoactivated phyA to trigger photomorphogenic responses., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2023 Jiang et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Emission of scent volatiles by flowers is important for successful pollination and consequently, reproduction. Petunia (Petunia hybrida) floral scent is formed mainly by volatile products of the phenylpropanoid pathway. We identified and characterized a regulator of petunia scent production: the GRAS protein PHENYLPROPANOID EMISSION-REGULATING SCARECROW-LIKE (PES). Its expression increased in petals during bud development and was highest in open flowers. Overexpression of PES increased the production of floral volatiles, while its suppression resulted in scent reduction. We showed that PES upregulates the expression of genes encoding enzymes of the phenylpropanoid and shikimate pathways in petals, and of the core regulator of volatile biosynthesis ODORANT1 by activating its promoter. PES is an ortholog of Arabidopsis (Arabidopsis thaliana) PHYTOCHROME A SIGNAL TRANSDUCTION 1, involved in physiological responses to far-red (FR) light. Analyses of the effect of nonphotosynthetic irradiation (low-intensity FR light) on petunia floral volatiles revealed FR light as a scent-activating factor. While PHYTOCHROME A regulated scent-related gene expression and floral scent production under FR light, the influence of PES on volatile production was not limited by FR light conditions., Competing Interests: Conflict of interest statement. The authors declare no conflict of interest., (© American Society of Plant Biologists 2023. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Plant roots possess remarkable regenerative potential owing to their ability to replenish damaged or lost stem cells. ETHYLENE RESPONSE FACTOR 115 (ERF115), one of the key molecular elements linked to this potential, plays a predominant role in the activation of regenerative cell divisions. However, the downstream operating molecular machinery driving wound-activated cell division is largely unknown. Here, we biochemically and genetically identified the GRAS-domain transcription factor SCARECROW-LIKE 5 (SCL5) as an interaction partner of ERF115 in Arabidopsis thaliana. Although nonessential under control growth conditions, SCL5 acts redundantly with the related PHYTOCHROME A SIGNAL TRANSDUCTION 1 (PAT1) and SCL21 transcription factors to activate the expression of the DNA-BINDING ONE FINGER 3.4 (DOF3.4) transcription factor gene. DOF3.4 expression is wound-inducible in an ERF115-dependent manner and, in turn, activates D3-type cyclin expression. Accordingly, ectopic DOF3.4 expression drives periclinal cell division, while its downstream D3-type cyclins are essential for the regeneration of a damaged root. Our data highlight the importance and redundant roles of the SCL5, SCL21, and PAT1 transcription factors in wound-activated regeneration processes and pinpoint DOF3.4 as a key downstream element driving regenerative cell division., Competing Interests: Conflict of interest statement. The authors declare that they have no conflicts of interest., (© American Society of Plant Biologists 2023. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

In Arabidopsis, although studies have demonstrated that phytochrome A (phyA) and phyB are involved in blue light signaling, how blue light-activated phytochromes modulate the activity of the CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1)-SUPPRESSOR OF PHYA-105 (SPA1) E3 complex remains largely unknown. Here, we show that phyA responds to early and weak blue light, whereas phyB responds to sustainable and strong blue light. Activation of both phyA and phyB by blue light inhibits SPA1 activity. Specifically, blue light irradiation promoted the nuclear import of both phytochromes to stimulate their binding to SPA1, abolishing SPA1's interaction with LONG HYPOCOTYL 5 (HY5) to release HY5, which promoted seedling photomorphogenesis., (© 2022 The Authors. Journal of Integrative Plant Biology published by John Wiley & Sons Australia, Ltd on behalf of Institute of Botany, Chinese Academy of Sciences.)

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.

Drought stress is a severe environmental issue that threatens agriculture at a large scale. PHYTOCHROMES (PHYs) are important photoreceptors in plants that control plant growth and development and are involved in plant stress response. The aim of this study was to identify the role of PHYs in the tomato cv. 'Moneymaker' under drought conditions. The tomato genome contains five PHYs , among which mutant lines in tomato PHYA and PHYB ( B1 and B2 ) were used. Compared to the WT, phyA and phyB1B2 mutants exhibited drought tolerance and showed inhibition of electrolyte leakage and malondialdehyde accumulation, indicating decreased membrane damage in the leaves. Both phy mutants also inhibited oxidative damage by enhancing the expression of reactive oxygen species (ROS) scavenger genes, inhibiting hydrogen peroxide (H 2 O 2 ) accumulation, and enhancing the percentage of antioxidant activities via DPPH test. Moreover, expression levels of several aquaporins were significantly higher in phyA and phyB1B2 , and the relative water content (RWC) in leaves was higher than the RWC in the WT under drought stress, suggesting the enhancement of hydration status in the phy mutants. Therefore, inhibition of oxidative damage in phyA and phyB1B2 mutants may mitigate the harmful effects of drought by preventing membrane damage and conserving the plant hydrostatus.

Photoperiod is an important environmental cue. Plants can distinguish the seasons and flower at the right time through sensing the photoperiod. Soybean is a sensitive short-day crop, and the timing of flowering varies greatly at different latitudes, thus affecting yields. Soybean cultivars in high latitudes adapt to the long day by the impairment of two phytochrome genes, PHYA3 and PHYA2 , and the legume-specific flowering suppressor, E1 . However, the regulating mechanism underlying phyA and E1 in soybean remains largely unknown. Here, we classified the regulation of the E1 family by phyA2 and phyA3 at the transcriptional and posttranscriptional levels, revealing that phyA2 and phyA3 regulate E1 by directly binding to LUX proteins, the critical component of the evening complex, to regulate the stability of LUX proteins. In addition, phyA2 and phyA3 can also directly associate with E1 and its homologs to stabilize the E1 proteins. Therefore, phyA homologs control the core flowering suppressor E1 at both the transcriptional and posttranscriptional levels, to double ensure the E1 activity. Thus, our results disclose a photoperiod flowering mechanism in plants by which the phytochrome A regulates LUX and E1 activity.

To enhance plant fitness under natural conditions, the circadian clock is synchronized and entrained by light via photoreceptors. In turn, the circadian clock exquisitely regulates the abundance and activity of photoreceptors via largely uncharacterized mechanisms. Here we show that the clock regulator TIME FOR COFFEE (TIC) controls the activity of the far-red light photoreceptor phytochrome A (phyA) at multiple levels in Arabidopsis thaliana. Null mutants of TIC displayed dramatically increased sensitivity to light irradiation with respect to hypocotyl growth, especially to far-red light. RNA-sequencing demonstrated that TIC and phyA play largely opposing roles in controlling light-regulated gene expression at dawn. Additionally, TIC physically interacts with the transcriptional repressor TOPLESS (TPL), which was associated with the significantly increased PHYA transcript levels in the tic-2 and tpl-1 mutants. Moreover, TIC interacts with phyA in the nucleus, thereby affecting phyA protein turnover and the formation of phyA nuclear speckles following light irradiation. Genetically, phyA was found to act downstream of TIC in regulating far red light-inhibited growth. Taken together, these findings indicate that TIC acts as a major negative regulator of phyA by integrating transcriptional and post-translational mechanisms at multiple levels., (� American Society of Plant Biologists 2022. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Optogenetic technologies have transformed our ability to precisely control biological processes in time and space. Yet, current eukaryotic optogenetic systems are limited by large or complex optogenetic modules, long illumination times, low tissue penetration or slow activation and deactivation kinetics. Here, we report a red/far-red light-mediated and miniaturized Δphytochrome A (ΔPhyA)-based photoswitch (REDMAP) system based on the plant photoreceptor PhyA, which rapidly binds the shuttle protein far-red elongated hypocotyl 1 (FHY1) under illumination with 660-nm light with dissociation occurring at 730 nm. We demonstrate multiple applications of REDMAP, including dynamic on/off control of the endogenous Ras/Erk mitogen-activated protein kinase (MAPK) cascade and control of epigenetic remodeling using a REDMAP-mediated CRISPR-nuclease-deactivated Cas9 (CRISPR-dCas9) (REDMAP cas ) system in mice. We also demonstrate the utility of REDMAP tools for in vivo applications by activating the expression of transgenes delivered by adeno-associated viruses (AAVs) or incorporated into cells in microcapsules implanted into mice, rats and rabbits illuminated by light-emitting diodes (LEDs). Further, we controlled glucose homeostasis in type 1 diabetic (T1D) mice and rats using REDMAP to trigger insulin expression. REDMAP is a compact and sensitive tool for the precise spatiotemporal control of biological activities in animals with applications in basic biology and potentially therapy., (© 2021. The Author(s), under exclusive licence to Springer Nature America, Inc.)

Heat stress (HS) is a prevalent negative factor affecting plant growth and development, as it is predominant worldwide and threatens agriculture on a large scale. PHYTOCHROMES (PHYs) are photoreceptors that control plant growth and development, and the stress signaling response partially interferes with their activity. PHYA, B1, and B2 are the most well-known PHY types in tomatoes. Our study aimed to identify the role of tomato 'Money Maker' phyA and phyB1B2 mutants in stable and fluctuating high temperatures at different growth stages. In the seed germination and vegetative growth stages, the phy mutants were HS tolerant, while during the flowering stage the phy mutants revealed two opposing roles depending on the HS exposure period. The response of the phy mutants to HS during the fruiting stage showed similarity to WT. The most obvious stage that demonstrated phy mutants' tolerance was the vegetative growth stage, in which a high degree of membrane stability and enhanced water preservation were achieved by the regulation of stomatal closure. In addition, both mutants upregulated the expression of heat-responsive genes related to heat tolerance. In addition to lower malondialdehyde accumulation, the phyA mutant enhanced proline levels. These results clarified the response of tomato phyA and phyB1B2 mutants to HS.

Phytochrome A (phyA) is the far-red (FR) light photoreceptor in plants that is essential for seedling de-etiolation under FR-rich environments, such as canopy shade. TANDEM ZINC-FINGER/PLUS3 (TZP) was recently identified as a key component of phyA signal transduction in Arabidopsis thaliana; however, how TZP is integrated into the phyA signaling networks remains largely obscure. Here, we demonstrate that ELONGATED HYPOCOTYL5 (HY5), a well-characterized transcription factor promoting photomorphogenesis, mediates FR light induction of TZP expression by directly binding to a G-box motif in the TZP promoter. Furthermore, TZP physically interacts with CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1), an E3 ubiquitin ligase targeting HY5 for 26S proteasome-mediated degradation, and this interaction inhibits COP1 interaction with HY5. Consistent with those results, TZP post-translationally promotes HY5 protein stability in FR light, and in turn, TZP protein itself is destabilized by COP1 in both dark and FR light conditions. Moreover, tzp hy5 double mutants display an additive phenotype relative to their respective single mutants under high FR light intensities, indicating that TZP and HY5 also function in largely independent pathways. Together, our data demonstrate that HY5 and TZP mutually upregulate each other in transmitting the FR light signal, thus providing insights into the complicated but delicate control of phyA signaling networks., (© American Society of Plant Biologists 2021. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Photoactivated phytochrome B (PHYB) binds to antagonistically acting PHYTOCHROME-INTERACTING transcription FACTORs (PIFs) to regulate hundreds of light responsive genes in Arabidopsis by promoting PIF degradation. However, whether PHYB directly controls the transactivation activity of PIFs remains ambiguous. Here we show that the prototypic PIF, PIF3, possesses a p53-like transcription activation domain (AD) consisting of a hydrophobic activator motif flanked by acidic residues. A PIF3mAD mutant, in which the activator motif is replaced with alanines, fails to activate PIF3 target genes in Arabidopsis, validating the functions of the PIF3 AD in vivo. Intriguingly, the N-terminal photosensory module of PHYB binds immediately adjacent to the PIF3 AD to repress PIF3's transactivation activity, demonstrating a novel PHYB signaling mechanism through direct interference of the transactivation activity of PIF3. Our findings indicate that PHYB, likely also PHYA, controls the stability and activity of PIFs via structurally separable dual signaling mechanisms., (© 2021. The Author(s).)

The effects of high-intensity light (HIL, 4 and 24 h) and UV-B (1 h) on the net photosynthesis rate, activity of photosystem II (PSII), content of photosynthetic pigments, anthocyanin and UV-absorbing pigments as well as the expression of certain light-responsive genes (HY5,CAB1) chalcone synthase (CHS) and main antioxidants enzyme genes (APX1, GPX and GR) in the leaves of phyB and phyA mutant A. thaliana plants were studied. Both UV-B and 4 and 24 h HIL decreased the PSII maximum quantum yield (F v /F m ), PSII performance index (PI ABS ), photosynthesis and respiration rates in plants. Moreover, all stress treatments increased the dissipation of the absorbed energy (DI 0 /RC) as well as the flux of absorbed energy per RC (ABS/RC). The maximal changes in photosynthesis and chlorophyll fluorescence parameters were observed in the phyB mutant. The WT and the phyA mutant showed similar responses. In addition, the phyB mutant exhibited decreases in the expression of genes encoding enzyme CHS, the transcription factor HY5 and the antioxidant enzymes APX1 and GPX. One of the possible mechanisms protecting the photosynthetic apparatus from light excess or UV radiation is the elevated content of various pigments that can act as antioxidants or optical filters. We assume that the greater decrease in photosynthetic activity in the phyB mutant under HIL and UV-B conditions was due to the decreased content of carotenoids and UV-absorbing pigments in this mutant., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Light is a key environmental cue that fundamentally regulates plant growth and development, which is mediated by the multiple photoreceptors including the blue light (BL) photoreceptor cryptochrome 1 (CRY1). The signaling mechanism of Arabidopsis thaliana CRY1 involves direct interactions with CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1)/SUPPRESSOR OF PHYA-105 1 and stabilization of COP1 substrate ELONGATED HYPOCOTYL 5 (HY5). H2A.Z is an evolutionarily conserved histone variant, which plays a critical role in transcriptional regulation through its deposition in chromatin catalyzed by SWR1 complex. Here we show that CRY1 physically interacts with SWC6 and ARP6, the SWR1 complex core subunits that are essential for mediating H2A.Z deposition, in a BL-dependent manner, and that BL-activated CRY1 enhances the interaction of SWC6 with ARP6. Moreover, HY5 physically interacts with SWC6 and ARP6 to direct the recruitment of SWR1 complex to HY5 target loci. Based on previous studies and our findings, we propose that CRY1 promotes H2A.Z deposition to regulate HY5 target gene expression and photomorphogenesis in BL through the enhancement of both SWR1 complex activity and HY5 recruitment of SWR1 complex to HY5 target loci, which is likely mediated by interactions of CRY1 with SWC6 and ARP6, and CRY1 stabilization of HY5, respectively., (© American Society of Plant Biologists 2021. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Shade-avoiding plants can detect the presence of neighboring vegetation and evoke escape responses before canopy cover limits photosynthesis. Rapid stem elongation facilitates light foraging and enables plants to overtop competitors. A major regulator of this response is the phytochrome B photoreceptor, which becomes inactivated in light environments with a low ratio of red to far-red light (low R:FR), characteristic of vegetational shade. Although shade avoidance can provide plants with a competitive advantage in fast-growing stands, excessive stem elongation can be detrimental to plant survival. As such, plants have evolved multiple feedback mechanisms to attenuate shade-avoidance signaling. The very low R:FR and reduced levels of photosynthetically active radiation (PAR) present in deep canopy shade can, together, trigger phytochrome A (phyA) signaling, inhibiting shade avoidance and promoting plant survival when resources are severely limited. The molecular mechanisms underlying this response have not been fully elucidated. Here, we show that Arabidopsis thaliana phyA elevates early-evening expression of the central circadian-clock components TIMING OF CAB EXPRESSION 1 ( TOC1 ), PSEUDO RESPONSE REGULATOR 7 (PRR7), EARLY FLOWERING 3 (ELF3) , and ELF4 in photocycles of low R:FR and low PAR. These collectively suppress stem elongation, antagonizing shade avoidance in deep canopy shade., Competing Interests: The authors declare no competing interest.

Phytochromes are photoreceptors regulating growth and development in plants. Using the model plant Arabidopsis, we identified a novel signalling pathway downstream of the far-red light-sensing phytochrome, phyA, that depends on the highly conserved CCR4-NOT complex. CCR4-NOT is integral to RNA metabolism in yeast and animals, but its function in plants is largely unknown. NOT9B, an Arabidopsis homologue of human CNOT9, is a component of the CCR4-NOT complex, and acts as negative regulator of phyA-specific light signalling when bound to NOT1, the scaffold protein of the complex. Light-activated phyA interacts with and displaces NOT9B from NOT1, suggesting a potential mechanism for light signalling through CCR4-NOT. ARGONAUTE 1 and proteins involved in splicing associate with NOT9B and we show that NOT9B is required for specific phyA-dependent alternative splicing events. Furthermore, association with nuclear localised ARGONAUTE 1 raises the possibility that NOT9B and CCR4-NOT are involved in phyA-modulated gene expression., Competing Interests: PS, DS, JP, AS, KL, EL, KM, UH, EK, AV, AH No competing interests declared, (© 2021, Schwenk et al.)

Main Conclusion: The NAC transcription factor ATAF2 suppresses its own transcription via self-promoter binding. ATAF2 genetically interacts with the circadian regulator CCA1 and phytochrome A to modulate seedling photomorphogenesis in Arabidopsis thaliana. ATAF2 (ANAC081) is a NAC (NAM, ATAF and CUC) transcription factor (TF) that participates in the regulation of disease resistance, stress tolerance and hormone metabolism in Arabidopsis thaliana. We previously reported that ATAF2 promotes Arabidopsis hypocotyl growth in a light-dependent manner via transcriptionally suppressing the brassinosteroid (BR)-inactivating cytochrome P450 genes BAS1 (CYP734A1, formerly CYP72B1) and SOB7 (CYP72C1). Assays using low light intensities suggest that the photoreceptor phytochrome A (PHYA) may play a more critical role in ATAF2-regulated photomorphogenesis than phytochrome B (PHYB) and cryptochrome 1 (CRY1). In addition, ATAF2 is also regulated by the circadian clock. The core circadian TF CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) physically interacts with ATAF2 at the DNA-protein and protein-protein levels, and both differentially suppress BAS1- and SOB7-mediated BR catabolism. In this research, we show that ATAF2 can bind its own promoter as a transcriptional self-repressor. This self-feedback-suppression loop is a typical feature of multiple circadian-regulated genes. Additionally, ATAF2 and CCA1 synergistically suppress seedling photomorphogenesis as reflected by the light-dependent hypocotyl growth analysis of their single and double gene knock-out mutants. Similar fluence-rate response assays using ATAF2 and photoreceptor (PHYB, CRY1 and PHYA) knock-out mutants demonstrate that PHYA is required for ATAF2-regulated photomorphogenesis in a wide range of light intensities. Furthermore, disruption of PHYA can suppress the BR-insensitive hypocotyl-growth phenotype of ATAF2 loss-of-function seedlings in the light, but not in darkness. Collectively, our results provide a genetic interaction synopsis of the circadian-clock-photomorphogenesis-BR integration node involving ATAF2, CCA1 and PHYA.

Background: Flowering time is an important trait for productivity in legumes, which include many food and fodder plants. Medicago truncatula (Medicago) is a model temperate legume used to study flowering time pathways. Like Arabidopsis thaliana (Arabidopsis), its flowering is promoted by extended periods of cold (vernalization, V), followed by warm long day (LD) photoperiods. However, Arabidopsis flowering-time genes such as the FLOWERING LOCUS C (FLC)/ MADS AFFECTING FLOWERING (MAF) clade are missing and CONSTANS-LIKE (CO-LIKE) genes do not appear to have a role in Medicago or Pisum sativum (pea). Another photoperiodic regulator, the red/far red photoreceptor PHYTOCHROME A (PHYA), promotes Arabidopsis flowering by stabilizing the CO protein in LD. Interestingly, despite the absence of CO-LIKE function in pea, PsPHYA plays a key role in promoting LD photoperiodic flowering and plant architecture. Medicago has one homolog of PHYA, MtPHYA, but its function is not known., Results: Genetic analysis of two MtPHYA Tnt1 insertion mutant alleles indicates that MtPHYA has an important role in promoting Medicago flowering and primary stem elongation in VLD and LD and in perception of far-red wavelengths in seedlings. MtPHYA positively regulates the expression of MtE1-like (MtE1L), a homologue of an important legume-specific flowering time gene, E1 in soybean and other Medicago LD-regulated flowering-time gene homologues, including the three FLOWERING LOCUS T-LIKE (FT-LIKE) genes, MtFTa1, MtFTb1 and MtFTb2 and the two FRUITFULL-LIKE (FUL-LIKE) genes MtFULa and MtFULb. MtPHYA also modulates the expression of the circadian clock genes, GIGANTEA (GI) and TIMING OF CAB EXPRESSION 1a (TOC1a). Genetic analyses indicate that Mtphya-1 Mte1l double mutants flowered at the same time as the single mutants. However, Mtphya-1 Mtfta1 double mutants had a weak additive effect in delaying flowering and in reduction of primary axis lengths beyond what was conferred by either of the single mutants., Conclusion: MtPHYA has an important role in LD photoperiodic control of flowering, plant architecture and seedling de-etiolation under far-red wavelengths in Medicago. It promotes the expression of LD-induced flowering time genes and modulates clock-related genes. In addition to MtFTa1, MtPHYA likely regulates other targets during LD floral induction in Medicago.

In Arabidopsis, stamen elongation, which ensures male fertility, is controlled by the auxin response factor ARF8, which regulates the expression of the auxin repressor IAA19. Here, we uncover a role for light in controlling stamen elongation. By an extensive genetic and molecular analysis we show that the repressor of light signaling COP1, through its targets HY5 and HYH, controls stamen elongation, and that HY5 - oppositely to ARF8 - directly represses the expression of IAA19 in stamens. In addition, we show that in closed flower buds, when light is shielded by sepals and petals, the blue light receptors CRY1/CRY2 repress stamen elongation. Coherently, at flower disclosure and in subsequent stages, stamen elongation is repressed by the red and far-red light receptors PHYA/PHYB. In conclusion, different light qualities - sequentially perceived by specific photoreceptors - and the downstream COP1-HY5/HYH module finely tune auxin-induced stamen elongation and thus male fertility., (© 2020 Society for Experimental Biology and John Wiley & Sons Ltd.)

Phytochromes are red (R) and far-red (FR) light photoreceptors in plants, and PHYTOCHROME-INTERACTING FACTORS (PIFs) are a group of basic helix-loop-helix family transcription factors that play central roles in repressing photomorphogenesis. Here, we report that MYB30, an R2R3-MYB family transcription factor, acts as a negative regulator of photomorphogenesis in Arabidopsis ( Arabidopsis thaliana ). We show that MYB30 preferentially interacts with the Pfr (active) forms of the phytochrome A (phyA) and phytochrome B (phyB) holoproteins and that MYB30 levels are induced by phyA and phyB in the light. It was previously shown that phytochromes induce rapid phosphorylation and degradation of PIFs upon R light exposure. Our current data indicate that MYB30 promotes PIF4 and PIF5 protein reaccumulation under prolonged R light irradiation by directly binding to their promoters to induce their expression and by inhibiting the interaction of PIF4 and PIF5 with the Pfr form of phyB. In addition, our data indicate that MYB30 interacts with PIFs and that they act additively to repress photomorphogenesis. In summary, our study demonstrates that MYB30 negatively regulates Arabidopsis photomorphogenic development by acting to promote PIF4 and PIF5 protein accumulation under prolonged R light irradiation, thus providing new insights into the complicated but delicate control of PIFs in the responses of plants to their dynamic light environment., (© 2020 American Society of Plant Biologists. All rights reserved.)

In response to far-red light (FR), FAR-RED ELONGATED HYPOCOTYL 1 (FHY1) transports the photoactivated phytochrome A (phyA), the primary FR photoreceptor, into the nucleus, where it initiates FR signaling in plants. Light promotes the 26S proteasome-mediated degradation of FHY1, which desensitizes FR signaling, but the underlying regulatory mechanism remains largely unknown. Here, we show that reversible SUMOylation of FHY1 tightly regulates this process. Lysine K32 (K32) and K103 are major SUMOylation sites of FHY1. We found that FR exposure promotes the SUMOylation of FHY1, which accelerates its degradation. Furthermore, we discovered that ARABIDOPSIS SUMO PROTEASE 1 (ASP1) interacts with FHY1 in the nucleus under FR and facilitates its deSUMOylation. FHY1 was strongly SUMOylated and its protein level was decreased in the asp1-1 loss-of-function mutant compared with that in the wild type under FR. Consistently, asp1-1 seedlings exhibited a decreased sensitivity to FR, suggesting that ASP1 plays an important role in the maintenance of proper FHY1 levels under FR. Genetic analysis further revealed that ASP1 regulates FR signaling through an FHY1- and phyA-dependent pathway. Interestingly, We found that continuous FR inhibits ASP1 accumulation, perhaps contributing to the desensitization of FR signaling. Taken together, these results indicate that FR-induced SUMOylation and ASP1-dependent deSUMOylation of FHY1 represent a key regulatory mechanism that fine-tunes FR signaling., (Copyright © 2020 The Author. Published by Elsevier Inc. All rights reserved.)

Phytochrome A (phyA) is the primary photoreceptor mediating various plant responses to far-red (FR) light. The defense-related phytohorme jasmonic acid (JA) has been shown recently to play a role in regulating phyA-mediated FR signaling. However, the detailed molecular mechanisms governing phyA- and JA-mediated signaling cross talks are still not well understood. Here, we uncover a molecular cascade in which JAZ1 inactivates phyA signaling through repressing the transcriptional activity of FHY3 on FHY1 and FHL . Furthermore, we demonstrate that the expression levels of FHY1 and FHL , and some FR response genes are reduced in the coi1 mutant. These findings unveil a previously unrecognized mechanism whereby JA modulates phyA signaling through repressing the activities of FHY3 by JAZs.

Exposure of dark-grown (etiolated) seedlings to light induces the heterotrophic-to-photoautotrophic transition (de-etiolation) processes, including the formation of photosynthetic machinery in the chloroplast and cotyledon expansion. Phytochrome is a red (R)/far-red (FR) light photoreceptor that is involved in the various aspects of de-etiolation. However, how phytochrome regulates metabolic dynamics in response to light stimulus has remained largely unknown. In this study, to elucidate the involvement of phytochrome in the metabolic response during de-etiolation, we performed widely targeted metabolomics in Arabidopsis ( Arabidopsis thaliana ) wild-type and phytochrome A and B double mutant seedlings de-etiolated under R or FR light. The results revealed that phytochrome had strong impacts on the primary and secondary metabolism during the first 24 h of de-etiolation. Among those metabolites, sugar levels decreased during de-etiolation in a phytochrome-dependent manner. At the same time, phytochrome upregulated processes requiring sugars. Triacylglycerols are stored in the oil bodies as a source of sugars in Arabidopsis seedlings. Sugars are provided from triacylglycerols through fatty acid β-oxidation and the glyoxylate cycle in glyoxysomes. We examined if and how phytochrome regulates sugar production from oil bodies. Irradiation of the etiolated seedlings with R and FR light dramatically accelerated oil body mobilization in a phytochrome-dependent manner. Glyoxylate cycle-deficient mutants not only failed to mobilize oil bodies but also failed to develop thylakoid membranes and expand cotyledon cells upon exposure to light. Hence, phytochrome plays a key role in the regulation of metabolism during de-etiolation., (© 2020 American Society of Plant Biologists. All Rights Reserved.)

PHYTOCHROME INTERACTING FACTORs (PIFs) are a group of basic helix-loop-helix (bHLH) transcription factors that repress plant light responses. PIF8 is one of the less-characterized Arabidopsis ( Arabidopsis thaliana ) PIFs, whose putative orthologs are conserved in other plant species. PIF8 possesses a bHLH motif and an active phytochrome B motif but not an active phytochrome A motif. Consistent with this motif composition, PIF8 binds to G-box elements and interacts with the Pfr form of phyB but only very weakly, if at all, with that of phyA. PIF8 differs, however, from other PIFs in its protein accumulation pattern and functional roles in different light conditions. First, PIF8 inhibits phyA-induced seed germination, suppression of hypocotyl elongation, and randomization of hypocotyl growth orientation in far-red light, but it does not inhibit phyB-induced red light responses. Second, PIF8 protein accumulates more in far-red light than in darkness or red light. This is distinct from the pattern observed with PIF3, which accumulates more in darkness. This PIF8 accumulation pattern requires degradation of PIF8 by CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1) in darkness, inhibition of COP1 by phyA in far-red light, and promotion of PIF8 degradation by phyB in red light. Together, our results indicate that PIF8 is a genuine PIF that represses phyA-mediated light responses., (© 2020 American Society of Plant Biologists. All rights reserved.)

Photoreceptors of the phytochrome family control a multitude of responses in plants. Phytochrome A (phyA) is essential for far-red light perception, which is important for germination and seedling establishment in strong canopy shade. Translocation of phyA from the cytosol into nucleus is a key step in far-red light signaling and requires FAR-RED ELONGATED HYPOCOTYL 1 (FHY1) and FHY1-LIKE (FHL). FHY1/FHL bind to phyA downstream signaling components. Therefore, it has been suggested that FHY1/FHL also have a function in assembling phyA transcription factor complexes in the nucleus. Yet, in this study, we show that constitutively nuclear-localized phyA is active in the absence of FHY1 and FHL. Furthermore, an artificial FHY1, consisting of an SV40 NLS, a phyA binding site, and a YFP tag as spacer between them, complements the fhy1-3 fhl-1 double mutant. These findings show that FHY1 and FHL are not required for phyA downstream signaling in the nucleus. However, we found that lines expressing phyA-NLS-YFP are hypersensitive to red and far-red light and that slightly increased levels of constitutively nuclear-localized phyA result in photomorphogenic development in the dark. Thus, restricting phyA to the cytosol and inducing nuclear transport in light by interaction with FHY1/FHL might be important to suppress photomorphogenesis in the dark., (© 2019 The Author(s).)

Plants have evolved two major ways to deal with nearby vegetation or shade: avoidance and tolerance. Moreover, some plants respond to shade in different ways; for example, Arabidopsis ( Arabidopsis thaliana ) undergoes an avoidance response to shade produced by vegetation, but its close relative Cardamine hirsuta tolerates shade. How plants adopt opposite strategies to respond to the same environmental challenge is unknown. Here, using a genetic strategy, we identified the C. hirsuta slender in shade1 mutants, which produce strongly elongated hypocotyls in response to shade. These mutants lack the phytochrome A (phyA) photoreceptor. Our findings suggest that C. hirsuta has evolved a highly efficient phyA-dependent pathway that suppresses hypocotyl elongation when challenged by shade from nearby vegetation. This suppression relies, at least in part, on stronger phyA activity in C. hirsuta ; this is achieved by increased ChPHYA expression and protein accumulation combined with a stronger specific intrinsic repressor activity. We suggest that modulation of photoreceptor activity is a powerful mechanism in nature to achieve physiological variation (shade tolerance versus avoidance) for species to colonize different habitats., (© 2019 American Society of Plant Biologists. All rights reserved.)

Increasing crop yield per unit of area can be achieved by increasing planting density. However, high-density planting could trigger shade avoidance responses, which cause exaggerated growth and increased susceptibility to various diseases. Previous studies have shown that the rapid elongation of plants under shade (i.e., reduced red to far-red ratios) is regulated by phytochromes and various phytohormones. However, the detailed molecular mechanisms governing the interaction among these signaling pathways are not well understood. Here, we report that loss-of-function mutants of FAR-RED ELONGATED HYPOCOTYL3 ( FHY3 ) and FAR-RED-IMPAIRED RESPONSE1 ( FAR1 ), which encode two homologous transcription factors essential for phytochrome signaling, exhibit an exaggerated shade avoidance phenotype. We show that FHY3 and FAR1 repress plant growth through directly activating the expression of two atypical basic helix-loop-helix transcriptional cofactors, PHYTOCHROME RAPIDLY REGULATED1 ( PAR1 ) and PAR2 , and that this process is antagonized by a group of JASMONATE ZIM-DOMAIN proteins, key repressors of the jasmonic acid (JA) signaling pathway, through physical interactions. Furthermore, we show that FHY3 interacts with MYC2, a key transcriptional regulator of JA responses, coordinately regulating JA-responsive defense gene expression. Our results unveil a previously unrecognized mechanism whereby plants balance their growth and defense responses through convergence of the phytochrome signaling pathway and JA signaling pathway under shade conditions., (© 2019 American Society of Plant Biologists. All rights reserved.)

Red light promotes germination after activating phytochrome phyB, which destabilizes the germination repressor PIF1. Early upon seed imbibition, canopy light, unfavorable for photosynthesis, represses germination by stabilizing PIF1 after inactivating phyB. Paradoxically, later upon imbibition, canopy light stimulates germination after activating phytochrome phyA. phyA-mediated germination is poorly understood and, intriguingly, is inefficient, compared to phyB-mediated germination, raising the question of its physiological significance. A genetic screen identified polyamine uptake transporter 2 (put2) mutants that overaccumulate polyamines, a class of antioxidant polycations implicated in numerous cellular functions, which we found promote phyA-mediated germination. In WT seeds, our data suggest that canopy light represses polyamines accumulation through PIF1 while red light promotes polyamines accumulation. We show that canopy light also downregulates PIF1 levels, through phyA; however, PIF1 reaccumulates rapidly, which limits phyA-mediated germination. High polyamines levels in decaying seeds bypass PIF1 repression of germination and stimulate phyA-mediated germination, suggesting an adaptive mechanism promoting survival when viability is compromised., Competing Interests: The authors have declared that no competing interests exist.

Phytochrome A (phyA) is represented in plants by two species, phyA' and phyA'', with different properties and modes of action (Sineshchekov, Funct. Plant Biol., 2019, 46, 118-135). They differ by the modification of a serine(s) residue at the N-terminus, possibly, by phosphorylation. To verify if these serines could be the Ser8 and Ser18 (in Avena sativa phyA, AsphyA), whose autophosphorylation modulates AsphyA stability and sensitivity as shown with the use of the serine-to-alanine substitution AsphyA mutants (S8A, S18A and S8/18A) (Han et al., Plant Cell Physiol., 2010, 51, 596-609), we have undertaken low-temperature (85 K) fluorescence investigations of phyA in these transgenic lines. The content and proportion of phyA' and phyA'' were essentially the same in wild-type AsphyA and its mutants, and in endogenous Arabidopsis phyA. All the lines revealed a higher phyA'/phyA'' proportion upon longer germination-inducing preillumination (3 h vs. 15 min white light) supporting our earlier finding that the phyA differentiation into the subpools is light-regulated. These observations and our earlier data imply that this process involves N-terminal serine(s) different from the autophosphorylated Ser8 and Ser18 (in AsphyA) narrowing down the area of further search for the exact site(s) of the phyA modification.

The photosynthetic apparatus accomplishes two major functions in plants - solar energy conversion and protection of the plant from photodestruction. Its highly orchestrated formation includes coordinated biosynthesis of chlorophyll (Chl) and of its binding to matrix proteins. Light plays here the central role driving both metabolic and regulatory processes. The regulation is achieved via operation of sophisticated photoreceptor machinery with the phytochrome system as its main component. This review concentrates on Chl a biosynthesis and the role of phytochrome A (phyA) in this process. The mechanism of action of phyA and the specificity of its state in the plant has been described, in particular, the existence of two native types with different modes of action. This review touches upon the dependence of the effects of phyA on tissues and organs of the plant and its species, genetic modifications, and hormonal status.

Common bean (Phaseolus vulgaris L.) is an important grain legume domesticated independently in Mexico and Andean South America approximately 8000 years ago. Wild forms are obligate short-day plants, and relaxation of photoperiod sensitivity was important for expansion to higher latitudes and subsequent global spread. To better understand the nature and origin of this key adaptation, we examined its genetic control in progeny of a wide cross between a wild accession and a photoperiod-insensitive cultivar. We found that photoperiod sensitivity is under oligogenic control, and confirm a major effect of the Ppd locus on chromosome 1. The red/far-red photoreceptor gene PHYTOCHROME A3 (PHYA3) was identified as a strong positional candidate for Ppd, and sequencing revealed distinct deleterious PHYA3 mutations in photoperiod-insensitive Andean and Mesoamerican accessions. These results reveal the independent origins of photoperiod insensitivity within the two major common bean gene pools and demonstrate the conserved importance of PHYA genes in photoperiod adaptation of short-day legume species., (© The Author(s) 2019. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

During the transition from warm to cool seasons, plants experience decreased temperatures, shortened days, and decreased red/far-red (R/FR) ratios of light. The mechanism by which plants integrate these environmental cues to maintain plant growth and adaptation remains poorly understood. Here, we report that low temperature induced the transcription of PHYTOCHROME A and accumulation of LONG HYPOCOTYL5 (SlHY5, a basic Leu zipper transcription factor) in tomato ( Solanum lycopersicum ) plants, especially under short day conditions with low R/FR light ratios. Reverse genetic approaches and physiological analyses revealed that silencing of SlHY5 increased cold susceptibility in tomato plants, whereas overexpression of SlHY5 enhanced cold tolerance. SlHY5 directly bound to and activated the transcription of genes encoding a gibberellin-inactivation enzyme, namely GIBBERELLIN2-OXIDASE4 , and an abscisic acid biosynthetic enzyme, namely 9-CIS-EPOXYCAROTENOID DIOXYGENASE6 ( SlNCED6 ). Thus, phytochrome A-dependent SlHY5 accumulation resulted in an increased abscisic acid/gibberellin ratio, which was accompanied by growth cessation and induction of cold response. Furthermore, silencing of SlNCED6 compromises short day- and low R/FR-induced tomato resistance to cold stress. These findings provide insight into the molecular genetic mechanisms by which plants integrate environmental stimuli with hormones to coordinate their growth with impending cold temperatures. Moreover, this work reveals a molecular mechanism that plants have evolved for growth and survival in response to seasonal changes., (© 2019 American Society of Plant Biologists. All Rights Reserved.)

Seed germination assays consist of counting the number of germinated seeds, defined as seeds in which the radicle has ruptured the endosperm and emerged from the seed coat. In Arabidopsis seed germination assays, Arabidopsis seeds are surface-sterilized, plated on agar plates containing test compounds, and incubated at specific temperatures under specific light conditions, after which the germinated seeds are counted, either with the naked eye or under a microscope. This chapter describes step-by-step protocols for Arabidopsis seed germination assays under phytochrome-dependent conditions.

Phytochrome A (phyA) is the only plant photoreceptor that perceives far-red light and then mediates various responses to this signal. Phosphorylation and dephosphorylation of oat phyA have been extensively studied, and it was shown that phosphorylation of a serine residue in the hinge region of oat phyA could regulate the interaction of phyA with its signal transducers. However, little is known about the role of the hinge region of Arabidopsis phyA. Here, we report that three sites in the hinge region of Arabidopsis phyA (i.e., S590, T593, and S602) are essential in regulating phyA function. Mutating all three of these sites to either alanines or aspartic acids impaired phyA function, changed the interactions of mutant phyA with FHY1 and FHL, and delayed the degradation of mutant phyA upon light exposure. Moreover, the in vivo formation of a phosphorylated phyA form was greatly affected by these mutations, while our data indicated that the abundance of this phosphorylated phyA form correlated well with the extent of phyA function, thus suggesting a pivotal role of the phosphorylated phyA in inducing the far-red light response. Taking these data together, our study reveals the important role of the hinge region of Arabidopsis phyA in regulating phyA phosphorylation and function, thus linking specific residues in the hinge region to the regulatory mechanisms of phyA phosphorylation., Competing Interests: The authors declare no conflict of interest.

Deetiolation is an essential developmental process transforming young plant seedlings into the vegetative phase with photosynthetic activities. Light signals initiate this important developmental process by triggering massive reprogramming of the transcriptome and translatome. Compared with the wealth of knowledge of transcriptional regulation, the molecular mechanism underlying this light-triggered translational enhancement remains unclear. Here we show that light-enhanced translation is orchestrated by a light perception and signaling pathway composed of photoreceptors, CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1), the phytohormone auxin, target of rapamycin (TOR), and ribosomal protein S6 (RPS6). In deetiolating Arabidopsis seedlings, photoreceptors, including phytochrome A and cryptochromes, perceive far-red and blue light to inactivate the negative regulator COP1, which leads to activation of the auxin pathway for TOR-dependent phosphorylation of RPS6. Arabidopsis mutants defective in TOR, RPS6A, or RPS6B exhibited delayed cotyledon opening, a characteristic of the deetiolating process to ensure timely vegetative development of a young seedling. This study provides a mechanistic view of light-triggered translational enhancement in deetiolating Arabidopsis ., Competing Interests: The authors declare no conflict of interest., (Copyright © 2018 the Author(s). Published by PNAS.)

Phytochrome A (phyA) is a red and far-red (FR) sensing photoreceptor regulating plant growth and development. Its biologically active FR-absorbing form Pfr translocates into the nucleus and subsequently regulates gene expression. Two transport facilitators, FR elongated hypocotyl 1 (FHY1) and FHY1-like (FHL), are crucial for its cytoplasmic-nuclear translocation. FHY1 interacts preferentially with activated phyA (Pfr) in assays with recombinant phyA and FHY1 and in vivo. Nuclear translocation of the phyA-FHY1 complex depends on a nuclear localization signal (NLS) of FHY1, which is recognized by IMPαs independently of phyA. The complex is guided along the actin cytoskeleton. Additionally, FHY1 has the ability to exit the nucleus via the exportin route, thus is able to repeatedly transport phyA molecules to the nucleus, balancing the nucleo-cytoplasmic distribution. The direction of FHY1s transport appears to depend on its phosphorylation state in different compartments. Phosphorylated serins close to the NLS prevent FHY1 binding to IMPα. The work presented here elucidates key steps of the mechanism by which photoactivated phyA translocates to the nucleus., (© 2018 The Authors The Plant Journal © 2018 John Wiley & Sons Ltd.)

Although photoreceptors are expressed throughout all plant organs, most studies have focused on their function in aerial parts with laboratory-grown plants. Photoreceptor function in naturally dark-grown roots of plants in their native habitats is lacking. We characterized patterns of photoreceptor expression in field- and glasshouse-grown Nicotiana attenuata plants, silenced the expression of PhyB1/B2/A/Cry2 whose root transcripts levels were greater/equal to those of shoots, and by micrografting combined empty vector transformed shoots onto photoreceptor-silenced roots, creating chimeric plants with "blind" roots but "sighted" shoots. Micrografting procedure was robust in both field and glasshouse, as demonstrated by transcript accumulation patterns, and a spatially-explicit lignin visual reporter chimeric line. Field- and glasshouse-grown plants with PhyB1B2, but not PhyA or Cry2, -blind roots, were delayed in stalk elongation compared with control plants, robustly for two field seasons. Wild-type plants with roots directly exposed to FR phenocopied the growth of irPhyB1B2-blind root grafts. Additionally, root-expressed PhyB1B2 was required to activate the positive photomorphogenic regulator, HY5, in response to aboveground light. We conclude that roots of plants growing deep into the soil in nature sense aboveground light, and possibly soil temperature, via PhyB1B2 to control key traits, such as stalk elongation., (© 2018 John Wiley & Sons Ltd.)