Volume 228, Issue 6 p. 1748-1753
Tansley insight
Free Access

COP1 and BBXs-HY5-mediated light signal transduction in plants

Dongqing Xu

Corresponding Author

Dongqing Xu

State Key Laboratory of Crop Genetics and Germplasm Enhancement, College of Agriculture, Nanjing Agricultural University, Nanjing, 210095 China

Author for correspondence:

Dongqing Xu

Tel: +86 25 84395813

Email:[email protected]

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First published: 30 October 2019
Citations: 85
Dongqing Xu is a finalist of the 2019 New Phytologist Tansley Medal competition for excellence in plant science. See Lennon & Dolan (2020, 228: 1697) for more details.

Summary

Light is one of the most essential environmental factors affecting many aspects of growth and developmental processes in plants. Plants undergo skotomorphogenic or photomorphogenic development dependent on the absence or presence of light. These two developmental programs enable a germinated seed to become a healthy seedling at the early stage of the plant life cycle. CULLIN 4-DNA DAMAGE-BINDING PROTEIN 1 (DDB1)-based CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1)-SUPPRESSOR OF PHYA and COP10-DEETIOLATED 1-DDB1 E3 ubiquitin ligase complexes promote the skotomorphogenesis by ubiquitinating and degrading a number of photomorphogenic-promoting factors in darkness. Photoreceptors sense and transduce light information to downstream signaling, thereby initiating a set of molecular events and subsequent photomorphogenesis. These processes are precisely modulated by a group of components including various photoreceptors, E3 ubiquitin ligase, and transcription factors at the molecular level. This review provides an overview of the current understanding of the COP1, ELONGATED HYPOCOTYL 5, and B-BOX CONTAINING PROTEINs-mediated light signal transduction pathway and highlights still open questions in the field.

Contents
  Summary 1748
I. Introduction 1748
II. Photoreceptors 1749
III. The COP/DET/FUS system 1749
IV. The BBXs-HY5 regulatory network 1750
V. Conclusion 1751
  Acknowledgements 1751
  References 1752

I. Introduction

Under natural conditions, most seeds, which are covered by soil, are under dark or weak light conditions. Once the seeds germinate in suitable seasons, they develop dramatically elongated hypocotyls, closed cotyledons, and apical hooks to promptly penetrate the covered soil to reach light. Upon light exposure, hypocotyl elongation is inhibited and the cotyledons are rapidly expanded. These two distinct developmental processes are termed skotomorphogenesis (etiolation) and photomorphogenesis (de-etiolation), respectively. The light signal transduction pathway plays a critical role in these processes (X. Xu et al., 2015; Paik & Huq, 2019). Numerous genetic and biochemical studies have established a complicated but delicate light signal transduction pathway involving photoreceptors, E3 ubiquitin ligases, and transcription factors in plants (Lau & Deng, 2012; Huang et al., 2014; Hoecker, 2017). In this review, I predominantly summarize recent advances on CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1), ELONGATED HYPOCOTYL 5 (HY5), and B-BOX CONTAINING PROTEINs (BBXs)-mediated light signal transduction pathways. For details on the roles of other key components of light signaling, readers are directed to excellent recent reviews (Lee & Choi, 2017; Paik et al., 2017; Martínez et al. 2018; Liang et al., 2019).

II. Photoreceptors

Various wavelength-specific light signals are perceived by at least five classes of photoreceptors, including phytochromes (phys) perceiving red and far-red light (600–750 nm), cryptochromes (CRYs), phototropins (PHOTs), and ZEITLUPE family members (ZTL, FKF1, and LKP2) sensing blue and UV-A light (320–500 nm), and UV RESISTANCE LOCUS 8 (UVR8) absorbing UV-B light (282–320 nm) (Galvao & Fankhauser, 2015). Upon light illumination, these photoreceptors are responsible for perceiving different wavelength-specific light signals and rapidly convert to biologically active isoforms, which initiate a set of biochemical regulatory events in the plant cells. Photoexcited phys, CRYs, and UVR8 interfere with the COP1-SUPPRESSOR OF PHYA (SPA) E3 ubiquitin ligase complex by direct protein–protein interactions, thereby disrupting COP1-SPA activity to promote photomorphogenesis (Podolec & Ulm, 2018). Moreover, photoactivated phys also directly interact with a group of basic helix–loop–helix transcription factors, PHYTOCHROME-INTERACTING FACTORS (PIFs), repressors of photomorphogenesis, and promote their phosphorylation and degradation (Pham et al., 2018a). In addition, CRYs associate with PIF4 and PIF5 to modulate their activities, which in turn serve to control their target gene expression under blue light conditions (Ma et al., 2016; Pedmale et al., 2016; Fig. 1). COP1 negatively controls the abundance of phys and CRYs, and it is also required for the nuclear accumulation of UVR8 upon UV-B light exposure, implying feedback regulation between COP1 and photoreceptors (Podolec & Ulm, 2018).

Details are in the caption following the image
A simplified model showing phytochromes (phys) and cryptochromes (CRYs)-mediated light signal transduction pathways in plants. phyA and phyB perceive far-red light and red light, respectively, and CRY1 and CRY2 sense blue light. On the one hand, upon far-red and red-light illumination, phyA and phyB interact with PIFs and trigger their rapid phosphorylation and degradation within minutes. CRY1 and CRY2 associate with PIF4 and PIF5 to repress their action under blue light conditions. These events consequently lead to release of the repression of light-controlled gene expression by PIFs. On the other hand, photoactivated phyA, phyB, CRY1, and CRY2 negatively control the activity of two E3 ubiquitin ligase complexes: CUL4-DDB1-COP1-SPA and CUL4-DDB1-COP10-DET1, which promote the ubiquitination and degradation of a group of transcriptional factors, including, for example, HY5, BBXs, HYH, and HFR1, in darkness. These transcription factors synergistically and/or additively mediate a large group of gene expression which promotes the photomorphogenic development. PIFs, PHYTOCHROME-INTERACTING FACTORS; CUL4, CULLIN4; COP1-SPA, CONSTITUTIVELY PHOTOMORPHOGENIC 1-SUPPRESSOR OF PHYA; COP10-DET1-DDB1, CONSTITUTIVELY PHOTOMORPHOGENIC 1-DEETIOLATED 1-DNA DAMAGE BINDING PROTEIN 1; HY5, ELONGATED HYPOCOTYL 5; HYH, HY5 homologue; BBX, B-BOX CONTAINING PROTEIN; HFR1, LONG HYPOCOTYL IN FAR-RED; LAF1, LONG AFTER FAR-RED LIGHT1. Solid lines indicate direct regulation, and dotted lines indicate indirect regulation.

III. The COP/DET/FUS system

A group of COP/DEETIOLATED (DET)/FUSCA (FUS) proteins constitutes three distinct biochemical entities: COP1-SPA, COP10-DET1-DNA DAMAGE-BINDING PROTEIN 1 (DDB1; CDD), and COP9 signalosome (Lau & Deng, 2012; Huang et al., 2014). All of them work synergistically and are functionally linked by CULLIN4 (CUL4) in the regulation of seedling development (Chen et al., 2006, 2010; Huang et al., 2014). They are biologically active in the nucleus of plant cells to promote skotomorphogenesis in darkness. Recessive mutations in an individual COP/DET/FUS result in a constitutively photomorphogenic phenotype even in darkness, suggesting these gene products are essential and required for etiolation. Two COP1 and two SPA proteins form a tetramer complex through their respective coiled-coil domains. COP1 is evolutionally and functionally conserved from plants to animals, whereas SPA proteins are specific to the green lineage and have diverged in function (Artz et al., 2019; Han et al., 2019). The CDD complex consists of COP10, DET1, and DDB1, in which DET1 and COP10 act as the substrate receptor of CUL4-DDB1-COP10-DET1 E3 ubiquitin ligase. This complex also promotes the action of CUL4-DDB1-COP1-SPA E3 ubiquitin ligase (Yanagawa et al., 2004; Chen et al., 2006, 2010; Shi et al., 2015). The COP9 signalosome is composed of eight subunits that regulates all of the CULLIN-RING E3 ligases through its NEDD8/RUB1 isopeptidase activity (Schwechheimer, 2018). Both CUL4-DDB1-based COP1-SPA and CDD complexes not only target numerous photomorphogenetic-promoting factors (like HY5, LONG AFTER FAR-RED LIGHT1, and LONG HYPOCOTYL IN FAR-RED) for ubiquitination and degradation, but also promote the accumulation of PIFs, a group of repressors (Osterlund et al., 2000; Seo et al., 2003; Jang et al., 2005; Dong et al., 2014; Shi et al., 2015; Pham et al., 2018b) (Fig. 1). Consequently, these molecular regulatory events maintain an extremely low abundance of positive regulators (like HY5) but high accumulation of repressors (such as PIFs), which lead to skotomorphogenetic development in darkness. COP/DET/FUS proteins control massive light-regulated gene expression, which is required for light-mediated seedling development (Ma et al., 2003). Light not only induces the gene expression but also activates the translation of thousands of messenger RNAs in plants (Liu et al., 2013). COP1 represses the TOR-S6K-RPS6 pathway, which is required for light-enhanced translation, thereby leading to the inhibition of de novo protein synthesis in etiolated seedlings. This might also contribute to the COP1-mediated seedling development (Chen et al., 2018). Hence, COP/DET/FUS proteins promote skotomorphogenic development via multiple regulatory mechanisms at transcriptional, translational, and post-translational levels.

Ubiquitin-mediated protein degradation is a central theme in the light signal transduction pathway, in which CUL4-DDB1-COP1-SPA and CUL4-DDB1-COP10-DET1 function as the major E3 ubiquitin ligase complexes. Of these, COP1 is the most extensively studied. Being a RING-finger-type E3 ubiquitin ligase, COP1 targets numerous substrates for ubiquitination and promotes their proteolysis via the 26S proteasome system (Hoecker, 2017). Thus, the regulatory mechanisms on COP1 are critical steps for COP1-mediated protein degradation and developmental processes. Light signals inactivate COP1 through multiple distinct regulatory mechanisms. Upon light irradiation, photoreceptors, including phys, CRYs, and UVR8, interfere with COP1-SPA1 activity through direct protein–protein interaction (Podolec & Ulm, 2018). phyA contributes to the rapid degradation of SPA2 in the light, in which COP1 ubiquitinates SPA2 and promotes its degradation via the 26S proteasome system, thereby forming a negative feedback loop for repressing COP1-SPA2 activity (Chen et al., 2015). Photoactivated CRYs and UVR8 compete for COP1 binding through Val-Pro motifs, thus preventing its interaction with downstream substrates like HY5 (Lau et al., 2019). Light inactivates COP1 by triggering its translocation from the nucleus to the cytoplasm through an as yet unknown mechanism (Pacín et al., 2014). In addition, other biochemical regulatory mechanisms act on COP1 for precisely controlling its abundance and activity in etiolated seedlings. COP1 shows self-ubiquitination activity in vitro and mediates its own abundance in vivo, implying that COP1 itself is likely controlled by self-ubiquitination (Seo et al., 2003). A RING-figure domain containing E3 ubiquitin ligase COP1 SUPPRESSOR 1 (CSU1) targets COP1 for ubiquitination and degradation in maintaining its homeostasis (D. Xu et al., 2014). CSU2 associates with COP1 through their respective coiled-coil domains and CSU3/PINOID phosphorylates COP1 at the site of Ser382, both of which contribute to the repression of COP1 activity (D. Xu et al., 2015; Lin et al., 2017). PIF1 directly interacts with the COP1-SPA1 complex, thus enhancing the substrate recruitment and autoubiquitylation and transubiquitylation activities of COP1 (Xu et al., 2014). The SUMO E3 ligase SIZ1 targets COP1 at Lys193 for sumoylation, which results in enhancement of COP1 activity (Lin et al., 2016). As COP1 is a key regulator in controlling the abundance of a number of substrates, it is therefore necessary that multiple molecular regulatory circuits on COP1 occur in fine-tuning its abundance and biological activity in response to changing light conditions.

IV. The BBXs-HY5 regulatory network

Light inhibits the biochemical activity of the COP/DET/FUS system through multiple regulatory mechanisms, allowing the accumulation of downstream targets, including transcription factors HY5 and BBXs. HY5, which is a central regulator of light signaling, directly or indirectly controls approximately one-third of gene expression throughout the Arabidopsis genome (Lee et al., 2007; Gangappa & Botto, 2016). Recent studies have revealed that a group of BBXs promotes or represses photomorphogenic development. There are 32 BBXs members in Arabidopsis; namely, BBX1–BBX32 (Gangappa & Botto, 2014). Among these, BBX4, BBX21, BBX22 and BBX23 promote photomorphogenesis (Datta et al., 2006, 2008; Zhang et al., 2017; Job et al., 2018; Xu et al., 2016, 2018), whereas BBX24, BBX25, BBX28, BBX30, BBX31 and BBX32 are negative regulators of light signaling (Holtan et al., 2011; Gangappa et al., 2013; Job et al., 2018; Lin et al., 2018; Heng et al., 2019; Yadav et al., 2019). All of these BBXs converge on HY5 in the regulation of photomorphogenesis through distinct regulatory mechanisms. BBX21, BBX22, BBX24, BBX25 and BBX28 interact with HY5. BBX21, BBX22 and BBX23 enhance HY5 action (Datta et al., 2008; Zhang et al., 2017; Job et al., 2018), whereas BBX24, BBX25 and BBX28 repress HY5 transcriptional activity towards target genes (Gangappa et al., 2013; Job et al., 2018; Lin et al., 2018). In addition, BBX21 and HY5 itself directly bind to the T/G-box cis-element present in the HY5 promoter to activate its expression (Abbas et al., 2014; Binkert et al., 2014; Xu et al., 2016, 2018). HY5 acts upstream of BBX30 and BBX31, both of which repress photomorphogenesis, and negatively regulates their transcription (Heng et al., 2019; Yadav et al., 2019). Therefore, BBXs and HY5 constitute a transcriptionally regulatory network that controls a larger number of gene expressions. This regulatory module may represent a key node of convergence through which light regulates gene expression for promoting photomorphogenesis (Fig. 2).

Details are in the caption following the image
ELONGATED HYPOCOTYL 5 (HY5) at the center of a light-mediated transcriptional network. The transcription factor HY5 mediates one-third of genes expression throughout the Arabidopsis genome. B-BOX CONTAINING PROTEINs (BBXs) BBX21–BBX25, BBX28, and BBX32 interact directly with HY5. Subsequently, BBX21, BBX22 and BBX23 enhance the action of HY5, whereas the BBX24 , BBX25, BBX28 and BBX32 inhibit the transcriptional activity of HY5 towards downstream target genes. BBX21 and HY5 itself directly bind to the T/G-box present in the HY5 promoter to activate its transcription and promote photomorphogenesis. Meanwhile, HY5 associates with G-box within the promoters of BBX30 and BBX31, both of which negatively control photomorphogenesis.

In addition to BBXs, other proteins also modulate HY5 action at the transcriptional or protein levels. PIF1, PIF3, HY5 homologue (HYH), CALMODULIN 7, and TEOSINTE-LIKE1, CYCLOIDEA, and PROLIFERATING CELL FACTOR 2 upregulate HY5 expression, whereas WRKY36 represses its transcription (Abbas et al., 2014; He et al., 2016; Zhang et al., 2017; Yang et al., 2018). SPA proteins interact with and facilitate HY5 degradation by COP1 in darkness (Saijo et al., 2003). Both HYH and G-BOX BINDING FACTOR1 associate with HY5 to enhance or repress HY5 action, respectively (Holm et al., 2002; Singh et al., 2012). Together, all of this evidence suggests that a complicated regulatory network converges on the master regulator HY5 to regulate downstream gene expression and subsequent plant development and growth.

V. Conclusion

Although extensive studies have revealed a comprehensive map of the light signal transduction pathway in the last three decades, much remains to be learned. Various photoreceptors sense different wavelength-specific light signals and mediate distinct downstream signaling pathways. Plants are exposed to sunlight, which possesses the entire wavelength spectrum, including far-red, red, blue, UV-A and UV-B, and all of the photoreceptors function in parallel and coordinately in regulating seedling development. The exact molecular actions and regulatory mechanisms for photoreceptors under natural conditions are largely unclear, so further studies are necessary to explore how photoreceptor-mediated signaling interconnects in response to sunlight. COP/DET/FUS proteins are central repressors of light signaling and control numerous light-signaling components for degradation (Hoecker, 2017). Thus, the control of COP/DET/FUS abundance and activity is critical for the accumulation of a number of downstream substrates that affect massive gene reprogramming in plants. Despite a few studies revealing multiple post-translational modification of COP1, the exact regulatory network of COP1/DET/FUS awaits further investigation. Although BBXs-HY5 may largely contribute to the light-initiated transcriptomic reprogramming, additional transcription factors are also involved in these processes. It is intriguing to explore how the BBXs-HY5 module works in concert with other transcription factors in controlling transcriptomic alteration in response to dynamic changing light conditions.

Overwhelming studies in Arabidopsis have characterized and identified a number of key regulators of light signaling. The light signal transduction pathway and regulators of light signaling are highly conserved in various plant species (Han et al., 2019). Therefore, knowledge from light signal transduction in Arabidopsis can be quickly and efficiently applied to improve agricultural traits of crops. Previous studies have shown that overexpression of key components of light signaling positively affect the productivity in different crops. Overexpression of Arabidopsis phyA in basmati rice results in an approximately 5–20% increase in yield (Garg et al., 2006). Ectopic expression of Arabidopsis phyB and BBX21 in potato resulted in increased yield, both in gross weight and in number of tubers (Thiele et al., 1999; Crocco et al., 2018). These results suggest that the approach by refining the expression of key regulators of light signaling is viable and could yield improved crops as well as a better understanding of the mechanisms of light signaling.

Acknowledgements

This work was supported by grants from the National Natural Science Foundation of China (31970258) and Nanjing Agricultural University. I apologize to colleagues whose work could not be included owing to space constraints.