Regulation of COP1 Function by Brassinosteroid Signaling

How Do Arabidopsis Seedlings Sense and React to Increasing Ambient Temperatures?

Plants respond to higher ambient temperatures by modifying their growth rate and habitus. This review aims to summarize the accumulated knowledge obtained with Arabidopsis seedlings grown at normal and elevated ambient temperatures. Thermomorphogenesis in the shoot and the root is overviewed separately, since the experiments indicate differences in key aspects of thermomorphogenesis in the two organs. This includes the variances in thermosensors and key transcription factors, as well as the predominance of cell elongation or cell division, respectively, even though auxin plays a key role in regulating this process in both organs. Recent findings also highlight the role of the root and shoot meristems in thermomorphogenesis and suggest that the cell cycle inhibitor RETINOBLASTOMA-RELATED protein may balance cell division and elongation at increased temperatures.

ZmCOP1 Regulates Maize Mesocotyl Length and Plant Height through the Phytohormone Pathways

The morphogenesis of crops is critical to their yield performance. COP1 (constitutively photomorphogenic1) is one of the core regulators in plant morphogenesis and has been deeply studied in Arabidopsis thaliana. However, the function of COP1 in maize is still unclear. Here, we found that the mesocotyl lengths of zmcop1 loss-of-function mutants were shorter than those of wild-type B73 in darkness, while the mesocotyl lengths of lines with ZmCOP1 overexpression were longer than those of wild-type B104. The plant height with zmcop1 was shorter than that of B73 in both short- and long-day photoperiods. Using transcriptome RNA sequencing technology, we identified 33 DEGs (differentially expressed genes) between B73's etiolated seedlings and those featuring zmcop1, both in darkness. The DEGs were mainly enriched in the plant phytohormone pathways. Our results provide direct evidence that ZmCOP1 functions in the elongation of etiolated seedlings in darkness and affects plant height in light. Our data can be applied in the improvement of maize plant architecture.

HOS15 represses flowering by promoting GIGANTEA degradation in response to low temperature in Arabidopsis

Flowering is the primary stage of the plant developmental transition and is tightly regulated by environmental factors such as light and temperature. However, the mechanisms by which temperature signals are integrated into the photoperiodic flowering pathway are still poorly understood. Here, we demonstrate that HOS15, which is known as a GI transcriptional repressor in the photoperiodic flowering pathway, controls flowering time in response to low ambient temperature. At 16°C, the hos15 mutant exhibits an early flowering phenotype, and HOS15 acts upstream of photoperiodic flowering genes (GI, CO, and FT). GI protein abundance is increased in the hos15 mutant and is insensitive to the proteasome inhibitor MG132. Furthermore, the hos15 mutant has a defect in low ambient temperature-mediated GI degradation, and HOS15 interacts with COP1, an E3 ubiquitin ligase for GI degradation. Phenotypic analyses of the hos15 cop1 double mutant revealed that repression of flowering by HOS15 is dependent on COP1 at 16°C. However, the HOS15-COP1 interaction was attenuated at 16°C, and GI protein abundance was additively increased in the hos15 cop1 double mutant, indicating that HOS15 acts independently of COP1 in GI turnover at low ambient temperature. This study proposes that HOS15 controls GI abundance through multiple modes as an E3 ubiquitin ligase and transcriptional repressor to coordinate appropriate flowering time in response to ambient environmental conditions such as temperature and day length.

COP1 dynamics integrate conflicting seasonal light and thermal cues in the control of Arabidopsis elongation

As the summer approaches, plants experience enhanced light inputs and warm temperatures, two environmental cues with an opposite morphogenic impact. Key components of this response are PHYTOCHROME B (phyB), EARLY FLOWERING 3 (ELF3), and CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1). Here, we used single and double mutant/overexpression lines to fit a mathematical model incorporating known interactions of these regulators. The fitted model recapitulates thermal growth of all lines used and correctly predicts thermal behavior of others not used in the fit. While thermal COP1 function is accepted to be independent of diurnal timing, our model shows that it acts at temperature signaling only during daytime. Defective response of cop1-4 mutants is epistatic to phyB-9 and elf3-8, indicating that COP1 activity is essential to transduce phyB and ELF3 thermosensory function. Our thermal model provides a unique toolbox to identify best allelic combinations enhancing climate change resilience of crops adapted to different latitudes.

Recent advances in understanding thermomorphogenesis signaling

Plants show remarkable phenotypic plasticity and are able to adjust their morphology and development to diverse environmental stimuli. Morphological acclimation responses to elevated ambient temperatures are collectively termed thermomorphogenesis. In Arabidopsis thaliana, morphological changes are coordinated to a large extent by the transcription factor PHYTOCHROME-INTERACTING FACTOR 4 (PIF4), which in turn is regulated by several thermosensing mechanisms and modulators. Here, we review recent advances in the identification of factors that regulate thermomorphogenesis of Arabidopsis seedlings by affecting PIF4 expression and PIF4 activity. We summarize newly identified thermosensing mechanisms and highlight work on the emerging topic of organ- and tissue-specificity in the regulation of thermomorphogenesis.

PIF4: Integrator of light and temperature cues in plant growth

Plants are sessile and lack behavioural responses to avoid extreme environmental changes linked to annual seasons. For survival, they have evolved elaborate sensory systems coordinating their architecture and physiology with fluctuating diurnal and seasonal temperatures. PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) was initially identified as a key component of the Arabidopsis thaliana phytochrome signalling pathway. It was then identified as playing a central role in promoting plant hypocotyl growth via the activation of auxin synthesis and signalling-related genes. Recent studies expanded its known regulatory functions to thermomorphogenesis and defined PIF4 as a central molecular hub for the integration of environmental light and temperature cues. The present review comprehensively summarizes recent progress in our understanding of PIF4 function in Arabidopsis thaliana, including PIF4-mediated photomorphogenesis and thermomorphogenesis, and the contribution of PIF4 to plant growth via the integration of environmental light and temperature cues. Remaining questions and possible directions for future research on PIF4 are also discussed.

Constitutive Photomorphogenic 1 Enhances ER Stress Tolerance in Arabidopsis

Interaction between light signaling and stress response has been recently reported in plants. Here, we investigated the role of CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), a key regulator of light signaling, in endoplasmic reticulum (ER) stress response in Arabidopsis. The cop1-4 mutant Arabidopsis plants were highly sensitive to ER stress induced by treatment with tunicarmycin (Tm). Interestingly, the abundance of nuclear-localized COP1 increased under ER stress conditions. Complementation of cop1-4 mutant plants with the wild-type or variant types of COP1 revealed that the nuclear localization and dimerization of COP1 are essential for its function in plant ER stress response. Moreover, the protein amount of ELONGATED HYPOCOTYL 5 (HY5), which inhibits bZIP28 to activate the unfolded protein response (UPR), decreased under ER stress conditions in a COP1-dependent manner. Accordingly, the binding of bZIP28 to the BIP3 promoter was reduced in cop1-4 plants and increased in hy5 plants compared with the wild type. Furthermore, introduction of the hy5 mutant locus into the cop1-4 mutant background rescued its ER stress-sensitive phenotype. Altogether, our results suggest that COP1, a negative regulator of light signaling, positively controls ER stress response by partially degrading HY5 in the nucleus.

McMYB4 improves temperature adaptation by regulating phenylpropanoid metabolism and hormone signaling in apple

Temperature changes affect apple development and production. Phenylpropanoid metabolism and hormone signaling play a crucial role in regulating apple growth and development in response to temperature changes. Here, we found that McMYB4 is induced by treatment at 28 °C and 18 °C, and McMYB4 overexpression results in flavonol and lignin accumulation in apple leaves. Yeast one-hybrid (Y1H) assays and electrophoretic mobility shift assays (EMSAs) further revealed that McMYB4 targets the promoters of the flavonol biosynthesis genes CHS and FLS and the lignin biosynthesis genes CAD and F5H. McMYB4 expression resulted in higher levels of flavonol and lignin biosynthesis in apple during growth at 28 °C and 18 °C than during growth at 23 °C. At 28 °C and 18 °C, McMYB4 also binds to the AUX/ARF and BRI/BIN promoters to activate gene expression, resulting in acceleration of the auxin and brassinolide signaling pathways. Taken together, our results demonstrate that McMYB4 promotes flavonol biosynthesis and brassinolide signaling, which decreases ROS contents to improve plant resistance and promotes lignin biosynthesis and auxin signaling to regulate plant growth. This study suggests that McMYB4 participates in the abiotic resistance and growth of apple in response to temperature changes by regulating phenylpropanoid metabolism and hormone signaling.

Illuminating the COP1/SPA Ubiquitin Ligase: Fresh Insights Into Its Structure and Functions During Plant Photomorphogenesis

CONSTITUTIVE PHOTOMORPHOGENIC 1 functions as an E3 ubiquitin ligase in plants and animals. Discovered originally in Arabidopsis thaliana, COP1 acts in a complex with SPA proteins as a central repressor of light-mediated responses in plants. By ubiquitinating and promoting the degradation of several substrates, COP1/SPA regulates many aspects of plant growth, development and metabolism. In contrast to plants, human COP1 acts as a crucial regulator of tumorigenesis. In this review, we discuss the recent important findings in COP1/SPA research including a brief comparison between COP1 activity in plants and humans.