Plant UV‐B responses are mediated by the photoreceptor UV RESISTANCE LOCUS 8 (UVR8). In response to UV‐B irradiation, UVR8 homodimers dissociate into monomers that bind to the E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1). The interaction of the C27 domain in the C‐terminal tail of UVR8 with the WD40 domain of COP1 is critical for UV‐B signaling. However, the function of the last 17 amino acids (C17) of the C‐terminus of UVR8, which are adjacent to C27, is unknown, although they are largely conserved in land plants. In this study, we established that Arabidopsis thaliana UVR8 C17 binds to full‐length UVR8, but not to COP1, and reduces COP1 binding to the remaining portion of UVR8, including C27. We hypothesized that overexpression of C17 in a wild‐type background would have a dominant negative effect on UVR8 activity; however, C17 overexpression caused strong silencing of endogenous UVR8 , precluding a detailed analysis. We therefore generated YFP‐UVR8^N423 transgenic lines, in which C17 was deleted, to examine C17 function indirectly. YFP‐UVR8^N423 was more active than YFP‐UVR8, suggesting that C17 inhibits UV‐B signaling by attenuating binding between C27 and COP1. Our study reveals an inhibitory role for UVR8 C17 in fine‐tuning UVR8–COP1 interactions during UV‐B signaling.

Light is crucial for plants, not only because of photosynthesis, but also because of photomorphogenesis. As one of the most important environmental cues, light influences multiple responses in plants, including seed germination, seedling de‐etiolation, shade avoidance, phototropism, stomata and chloroplast movement, circadian rhythms, and flowering time. In model plant Arabidopsis thaliana, at least five types of photoreceptors are involved in the regulation of overlapping physiological functions essential to plant growth and development. The main photoreceptors include the UV‐B photoreceptor UV RESISTANCE LOCUS 8 (UVR8) (Rizzini et al. 2011), the blue light photoreceptors, known as cryptochromes (CRYs) (Lin 2002); the blue light/UV‐A photoreceptor phototropins (PHOTs) (Briggs and Christie 2002); the LOV‐domain/F‐box proteins ZEITLUPE (ZTL), FLAVIN BINDING, KELCH REPEAT, F‐BOX PROTEIN 1 (FKF), and LOV KELCH PROTEIN2 (LKP2) (Demarsy and Fankhauser 2009); and the red/far‐red light photoreceptors, called phytochromes (PHYs) (Quail 2002). How those photoreceptors transduce respective light signals are fundamental questions in plant biology.

In this Special Issue, we collected three reviews to summarize the recent progress in light signaling and five articles to show the latest research progress in photobiology from different perspectives and raise exciting new questions for future investigations.

The review by Yadav et al. (2020) summarized the current developments in light signaling with a major focus on UV‐B‐mediated plant growth regulation. They outlined the perception of far‐red, red, blue, and UV‐B signals and the central regulatory intermediates involved in their downstream signaling pathways. It further focuses on current understanding of the developmental changes shown by plants in response to UV‐B radiation. It also discusses the diverse strategies plants have adapted at molecular, biochemical, and metabolic levels to protect themselves from UV‐B mediated damages.

Transcription regulation is critical for light signaling. B‐box proteins are a class of zinc‐coordinated transcription factors or regulators that not only directly mediate the transcription of target genes, but also interact with various other factors to create a complex regulatory network involved in the precise control of plant growth and development. A group of B‐box proteins (BBXs) function as important players in light‐mediated developmental processes. Song et al. (2020) summarized and highlighted the recent findings concerning the critical regulatory functions of BBXs in photoperiodic flowering, light signal transduction and light‐induced pigment accumulation and their molecular modes of action at the transcriptional and post‐translational levels in plants.

Seed dormancy is an evolved trait that determines the timing of germination, thereby playing essential roles in ensuring plant survival and agricultural production. Seed dormancy and the subsequent germination are controlled by both the internal cues, mainly hormones and several dormancy proteins, and the environmental signals, including light. Yang et al. (2020b) provided an overview of the molecular mechanism by which seed light signal modulates the induction, maintenance and release of seed dormancy, as well as seed germination, and further summarize/discuss the interaction between light and the internal hormones and dormancy‐specific regulators.

CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) is a RING finger E3 ubiquitin ligase that acts downstream of the PHYs, CRYs, and UVR8 (Ang and Deng 1994; Christie et al. 2012). In response to UV‐B irradiation, UVR8 homodimers dissociate into monomers that bind to the E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1). The interaction of the C27 domain in the C‐terminal tail of UVR8 with the WD40 domain of COP1 is critical for UV‐B signaling. Lin et al. (2020) report an inhibitory role for UVR8 C17 in fine‐tuning UVR8–COP1 interactions during U2020V‐B signaling. They established that Arabidopsis UVR8 C17 binds to full‐length UVR8, but not to COP1, and reduces COP1 binding to the remaining portion of UVR8, including C27.

Being shaded is a common environmental stress for plants, especially for densely planted crops. Shaded plants display shade avoidance syndrome (SAS): elongated hypocotyls, internodes, and petioles, hyponastic leaves, early flowering and are inhibited in branching. Shade decreases red: far‐red (R:FR) ratios that inactivate phytochrome B (PHYB) and subsequently release phytochrome interaction factors (PIFs). ZTL is a blue light photoreceptor and circadian clock component, which is also involved in floral rhythms and plant defense in Nicotiana attenuata. Zou et al. (2019) show that ZTL may regulate PHYB‐ and the auxin‐mediated signaling pathway, which functions in the SAS of N. attenuata.

Light regulates the distribution pattern of chloroplasts in photosynthesizing plant cells (Wada et al. 2003). Mitochondria are frequently observed in the vicinity of chloroplasts in photosynthesizing cells, and this association is considered necessary for their metabolic interactions. In leaf palisade cells of Arabidopsis, mitochondria exhibit blue‐light‐dependent redistribution together with chloroplasts, which conduct accumulation and avoidance responses under the control of blue‐light receptor phototropins. Islam et al. (2020) further demonstrate that the physical interaction between mitochondria and chloroplasts is cooperatively mediated by phototropin 2‐ and photosynthesis‐dependent signals.

The phyB photoreceptor plays a major role that inputs light signals to regulate seed dormancy and germination. PIF1 is a key transcription factor repressing phyB‐mediated seed germination, while REVEILLE1 (RVE1) factor functions as a curial regulator in controlling both seed dormancy and germination. Yang et al. (2020a) found that PIF1 physically interacts with RVE1. They formed a transcriptional feedback loop that coordinately inhibits seed germination, providing a mechanistic understanding of how phyB‐mediated light signal is transduced to the seeds.

The transition to flowering is the most dramatic phase change in flowering plants and is crucial for reproductive success. Plants integrate environmental cues with endogenous signals to regulate flowering time. The amount of FLOWERING LOCUS T (FT), which encodes a mobile stimulus largely determines the flowering time. Liu et al. (2020) demonstrate that ambient temperatures regulate both FT messenger RNA expression and FT protein trafficking to prevent precocious flowering at low temperatures and ensure plant reproductive success under favorable environmental conditions.

This Special Issue covers a selected range of topics and directions in photobiology. In recent years, significant progress has been made in plant photobiology research, from the understanding of light signal transduction, to novel functions of photoreceptors. Knowledge gained from these studies will be important not only to the understanding of light signal transduction, but also to agricultural efforts for better crop yield and performance.

Gamma‐aminobutyric acid (GABA) is an important metabolite which functions in plant growth, development, and stress responses. However, its role in plant defense and how it is regulated are largely unknown. Here, we report a detailed analysis of GABA induction during the resistance response to Pseudomonas syringae in Arabidopsis thaliana. While searching for the mechanism underlying the pathogen‐responsive mitogen‐activated protein kinase (MPK)3/MPK6 signaling cascade in plant immunity, we found that activation of MPK3/MPK6 greatly induced GABA biosynthesis, which is dependent on the glutamate decarboxylase genes GAD1 and GAD4. Inoculation with Pseudomonas syringae pv tomato DC3000 (Pst) and Pst‐avrRpt2 expressing the avrRpt2 effector gene induced GAD1 and GAD4 gene expression and increased the levels of GABA. Genetic evidence revealed that GAD1, GAD2, and GAD4 play important roles in both GABA biosynthesis and plant resistance in response to Pst‐avrRpt2 infection. The gad1/2/4 triple and gad1/2/4/5 quadruple mutants, in which the GABA levels were extremely low, were more susceptible to both Pst and Pst‐avrRpt2. Functional loss of MPK3/MPK6, or their upstream MKK4/MKK5, or their downstream substrate WRKY33 suppressed the induction of GAD1 and GAD4 expression after Pst‐avrRpt2 treatment. Our findings shed light on both the regulation and role of GABA in the plant immunity to a bacterial pathogen.

FLAVIN‐BINDING KELCH REPEAT F‐BOX 1 (FKF1) encodes an F‐box protein that regulates photoperiod flowering in Arabidopsis under long‐day conditions (LDs). Gibberellin (GA) is also important for regulating flowering under LDs. However, how FKF1 and the GA pathway work in concert in regulating flowering is not fully understood. Here, we showed that the mutation of FKF1 could cause accumulation of DELLA proteins, which are crucial repressors in GA signaling pathway, thereby reducing plant sensitivity to GA in flowering. Both in vitro and in vivo biochemical analyses demonstrated that FKF1 directly interacted with DELLA proteins. Furthermore, we showed that FKF1 promoted ubiquitination and degradation of DELLA proteins. Analysis of genetic data revealed that FKF1 acted partially through DELLAs to regulate flowering under LDs. In addition, DELLAs exerted a negative feedback on FKF1 expression. Collectively, these findings demonstrate that FKF1 promotes flowering partially by negatively regulating DELLA protein stability under LDs, and suggesting a potential mechanism linking the FKF1 to the GA signaling DELLA proteins.

The phytochrome B (phyB) photoreceptor plays a major role that inputs light signals to regulate seed dormancy and germination. PHYTOCHROME‐INTERACTING FACTOR1 (PIF1) is a key transcription factor repressing phyB‐mediated seed germination, while REVEILLE1 (RVE1) factor functions as a curial regulator in controlling both seed dormancy and germination. However, the relationship between the PIF1‐ and RVE1‐modulated signaling pathways remains mostly unknown. Here, we find that PIF1 physically interacts with RVE1. Genetic analysis indicates that RVE1 inhibition on seed germination requires PIF1; reciprocally, the repressive effect of PIF1 is partially dependent on RVE1. Strikingly, PIF1 and RVE1 directly bind to the promoter and activate the expression of each other. Furthermore, PIF1 and RVE1 coordinately regulate the transcription of many downstream genes involved in abscisic acid and gibberellin pathways. Moreover, PIF1 enhances the DNA‐binding ability and transcriptional repression activity of RVE1 in regulating GIBBERELLIN 3‐OXIDASE2, and RVE1 promotes PIF1's DNA‐binding ability in modulating ABSCISIC ACID‐INSENSITIVE3 expression. Thus, this study demonstrates that PIF1 and RVE1 form a transcriptional feedback loop that coordinately inhibits seed germination, providing a mechanistic understanding of how phyB‐mediated light signal is transduced to the seeds.

Light signals mediate a number of physiological and developmental processes in plants, such as flowering, photomorphogenesis, and pigment accumulation. Emerging evidence has revealed that a group of B‐box proteins (BBXs) function as central players in these light‐mediated developmental processes. B‐box proteins are a class of zinc‐coordinated transcription factors or regulators that not only directly mediate the transcription of target genes but also interact with various other factors to create a complex regulatory network involved in the precise control of plant growth and development. This review summarizes and highlights the recent findings concerning the critical regulatory functions of BBXs in photoperiodic flowering, light signal transduction and light‐induced pigment accumulation and their molecular modes of action at the transcriptional and post‐translational levels in plants

Light plays an important role in plants’ growth and development throughout their life cycle. Plants alter their morphological features in response to light cues of varying intensity and quality. Dedicated photoreceptors help plants to perceive light signals of different wavelengths. Activated photoreceptors stimulate the downstream signaling cascades that lead to extensive gene expression changes responsible for physiological and developmental responses. Proteins such as ELONGATED HYPOCOTYL5 (HY5) and CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) act as important factors which modulate light‐regulated gene expression, especially during seedling development. These factors function as central regulatory intermediates not only in red, far‐red, and blue light pathways but also in the UV‐B signaling pathway. UV‐B radiation makes up only a minor fraction of sunlight, yet it imparts many positive and negative effects on plant growth. Studies on UV‐B perception, signaling, and response in plants has considerably surged in recent times. Plants have developed different strategies to use UV‐B as a developmental cue as well as to withstand high doses of UV‐B radiation. Plants’ responses to UV‐B are an integration of its cross‐talks with both environmental factors and phytohormones. This review outlines the current developments in light signaling with a major focus on UV‐B‐mediated plant growth regulation.

Mitochondria are frequently observed in the vicinity of chloroplasts in photosynthesizing cells, and this association is considered necessary for their metabolic interactions. We previously reported that, in leaf palisade cells of Arabidopsis thaliana, mitochondria exhibit blue‐light‐dependent redistribution together with chloroplasts, which conduct accumulation and avoidance responses under the control of blue‐light receptor phototropins. In this study, precise motility analyses by fluorescent microscopy revealed that the individual mitochondria in palisade cells, labeled with green fluorescent protein, exhibit typical stop‐and‐go movement. When exposed to blue light, the velocity of moving mitochondria increased in 30 min, whereas after 4 h, the frequency of stoppage of mitochondrial movement markedly increased. Using different mutant plants, we concluded that the presence of both phototropin1 and phototropin2 is necessary for the early acceleration of mitochondrial movement. On the contrary, the late enhancement of stoppage of mitochondrial movement occurs only in the presence of phototropin2 and only when intact photosynthesis takes place. A plasma‐membrane ghost assay suggested that the stopped mitochondria are firmly adhered to chloroplasts. These results indicate that the physical interaction between mitochondria and chloroplasts is cooperatively mediated by phototropin2‐ and photosynthesis‐dependent signals. The present study might add novel regulatory mechanism for light‐dependent plant organelle interactions.

The COP9 signalosome (CSN) is a conserved protein complex, typically composed of eight subunits (designated as CSN1 to CSN8) in higher eukaryotes such as plants and animals, but of fewer subunits in some lower eukaryotes such as yeasts. The CSN complex is originally identified in plants from a genetic screen for mutants that mimic light‐induced photomorphogenic development when grown in the dark. The CSN complex regulates the activity of cullin‐RING ligase (CRL) families of E3 ubiquitin ligase complexes, and play critical roles in regulating gene expression, cell proliferation, and cell cycle. This review aims to summarize the discovery, composition, structure, and function of CSN in the regulation of plant development in response to external (light and temperature) and internal cues (phytohormones).

Being shaded is a common environmental stress for plants, especially for densely planted crops. Shade decreases red: far‐red (R:FR) ratios that inactivate phytochrome B (PHYB) and subsequently release p̱hytochrome i̱nteraction f̱actors (PIFs). Shaded plants display elongated hypocotyls, internodes, and petioles, hyponastic leaves, early flowering and are inhibited in branching: traits collectively called the shade avoidance syndrome (SAS). ZEITLUPE (ZTL) is a circadian clock component and blue light photoreceptor, which is also involved in floral rhythms and plant defense in Nicotiana attenuata. ztl mutants are hypersensitive to red light and ZTL physically interacts with PHYB, suggesting the involvement of ZTL in R:FR light signaling. Here, we show that N. attenuata ZTL‐silenced plants display a phenotype opposite to that of the SAS under normal light. After simulated shade, the normally induced transcript levels of the SAS marker gene, ATHB2 are attenuated in ZTL‐silenced plants. The auxin signaling pathway, known to be involved in SAS, was also significantly attenuated. Furthermore, NaZTL directly interacts with NaPHYBs, and regulates the transcript levels of PHYBs, PIF3a, PIF7 and PIF8 under shade. Our results suggest that ZTL may regulate PHYB‐ and the auxin‐mediated signaling pathway, which functions in the SAS of N. attenuata.

Leaf shape has important implications for optimizing plant architecture for grain crops and horticultural crops. Examination of the cucumber (Cucumis sativus L.) round leaf (rl) mutant by Song et al. (2019) revealed that the PINOID protein kinase affects leaf shape by altering auxin biosynthesis, transport, and signaling.

Rice (Oryza sativa L.) is a major staple food crop for over half of the world's population. As a crop species originated from the subtropics, rice production is hampered by chilling stress. The genetic mechanisms of rice responses to chilling stress have attracted much attention, focusing on chilling‐related gene mining and functional analyses. Plants have evolved sophisticated regulatory systems to respond to chilling stress in coordination with light signaling pathway and internal circadian clock. However, in rice, information about light‐signaling pathways and circadian clock regulation and their roles in chilling tolerance remains elusive. Further investigation into the regulatory network of chilling tolerance in rice is needed, as knowledge of the interaction between temperature, light, and circadian clock dynamics is limited. Here, based on phenotypic analysis of transgenic and mutant rice lines, we delineate the relevant genes with important regulatory roles in chilling tolerance. In addition, we discuss the potential coordination mechanism among temperature, light, and circadian clock in regulating chilling response and tolerance of rice, and provide perspectives for the ongoing chilling signaling network research in rice.

Both phototropins (phot1 and phot2) and cryptochromes (cry1 and cry2) were proven as the Arabidopsis thaliana blue light receptors. Phototropins predominately function in photomovement, and cryptochromes play a role in photomorphogenesis. Although cryptochromes have been proposed to serve as positive modulators of phototropic responses, the underlying mechanism remains unknown. Here, we report that depleting sucrose from the medium or adding gibberellic acids (GAs) can partially restore the defects in phototropic curvature of the phot1 phot2 double mutants under high‐intensity blue light; this restoration does not occur in phot1 phot2 cry1 cry2 quadruple mutants and nph3 (nonphototropic hypocotyl 3) mutants which were impaired phototropic response in sucrose‐containing medium. These results indicate that GAs and sucrose antagonistically regulate hypocotyl phototropism in a cryptochromes dependent manner, but it showed a crosstalk with phototropin signaling on NPH3. Furthermore, cryptochromes activation by blue light inhibit GAs synthesis, thus stabilizing DELLAs to block hypocotyl growth, which result in the higher GAs content in the shade side than the lit side of hypocotyl to support the asymmetric growth of hypocotyl. Through modulation of the abundance of DELLAs by sucrose depletion or added GAs, it revealed that cryptochromes have a function in mediating phototropic curvature.