Abstract:

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.