Seedling Photomorphogenesis Research Articles

Photomorphogenesis and Photosynthetic Trait Changes in Melon Seedlings Responding to Red and Blue Light

Red and blue light have significant effects on plant growth; however, most of the current studies have focused on common horticultural crops such as cucumber and tomato, and there are fewer studies on how red and blue light affect the growth of melon seedlings. Therefore, in this study, we used melon (Cucumis melo L.) as the experimental material to investigate the effects of red and blue light ratios on the photomorphogenesis and photosynthesis of melon seedlings. Five red and blue light ratios were set at a fixed light intensity 200 μmol·m−2·s−1, including R:B = 1:9, R:B = 3:7, R:B = 1:1, R:B = 7:3, and R:B = 9:1. The results showed that with the increase in red light ratios, melon seedling height, stem diameter, total leaf area and stomatal pore size of melon seedlings increased, while the upper epidermis, palisade tissue, spongy tissue, leaf thickness, and stomatal density showed a decreasing trend. Under the R:B = 7:3 treatment, melon seedlings were optimized in all morphological indexes and had higher photosynthetic efficiency; these results indicated that the growth of melon seedlings could be regulated by adjusting the ratio of red to blue light, thus promoting the morphogenesis of melon seedlings.

Regulation of cryptochrome-mediated blue light signaling by the ABI4-PIF4 module.

ABSCISIC ACID-INSENSITIVE 4 (ABI4) is a pivotal transcription factor which coordinates multiple aspects of plant growth and development as well as plant responses to environmental stresses. ABI4 has been shown to be involved in regulating seedling photomorphogenesis; however, the underlying mechanism remains elusive. Here, we show that the role of ABI4 in regulating photomorphogenesis is generally regulated by sucrose, but ABI4 promotes hypocotyl elongation of Arabidopsis seedlings under blue (B) light under all tested sucrose concentrations. We further show that ABI4 physically interacts with PHYTOCHROME INTERACTING FACTOR 4 (PIF4), a well-characterized growth-promoting transcription factor, and post-translationally promotes PIF4 protein accumulation under B light. Further analyses indicate that ABI4 directly interacts with the B light photoreceptors cryptochromes (CRYs) and inhibits the interactions between CRYs and PIF4, thus relieving CRY-mediated repression of PIF4 protein accumulation. In addition, while ABI4 could directly activate its own expression, CRYs enhance, whereas PIF4 inhibits, ABI4-mediated activation of the ABI4 promoter. Together, our study demonstrates that the ABI4-PIF4 module plays an important role in mediating CRY-induced B light signaling in Arabidopsis.

The pleiotropic functions of GOLDEN2-LIKE transcription factors in plants.

The regulation of gene expression is crucial for biological plant growth and development, with transcription factors (TFs) serving as key switches in this regulatory mechanism. GOLDEN2-LIKE (GLK) TFs are a class of functionally partially redundant nuclear TFs belonging to the GARP superfamily of MYB TFs that play a key role in regulating genes related to photosynthesis and chloroplast biogenesis. Here, we summarized the current knowledge of the pleiotropic roles of GLKs in plants. In addition to their primary functions of controlling chloroplast biogenesis and function maintenance, GLKs have been proven to regulate the photomorphogenesis of seedlings, metabolite synthesis, flowering time, leaf senescence, and response to biotic and abiotic stress, ultimately contributing to crop yield. This review will provide a comprehensive understanding of the biological functions of GLKs and serve as a reference for future theoretical and applied studies of GLKs.

Reducing PHYTOENE SYNTHASE activity fine-tunes the abundance of a cis-carotene-derived signal that regulates the PIF3/HY5 module and plastid biogenesis.

PHYTOENE SYNTHASE (PSY) is a rate-limiting enzyme catalysing the first committed step of carotenoid biosynthesis, and changes in PSY gene expression and/or protein activity alter carotenoid composition and plastid differentiation in plants. Four genetic variants of PSY (psy-4, psy-90, psy-130, and psy-145) were identified using a forward genetics approach that rescued leaf virescence phenotypes and plastid abnormalities displayed by the Arabidopsis CAROTENOID ISOMERASE (CRTISO) mutant ccr2 (carotenoid and chloroplast regulation 2) when grown under a shorter photoperiod. The four non-lethal mutations affected alternative splicing, enzyme-substrate interactions, and PSY:ORANGE multi-enzyme complex binding, constituting the dynamic post-transcriptional fine-tuning of PSY levels and activity without changing localization to the stroma and protothylakoid membranes. psy genetic variants did not alter total xanthophyll or β-carotene accumulation in ccr2, yet they reduced specific acyclic linear cis-carotenes linked to the biosynthesis of a currently unidentified apocarotenoid signal regulating plastid biogenesis, chlorophyll biosynthesis, and photomorphogenic regulation. ccr2 psy variants modulated the PHYTOCHROME-INTERACTING FACTOR 3/ELONGATED HYPOCOTYL 5 (PIF3/HY5) ratio, and displayed a normal prolamellar body formation in etioplasts and chlorophyll accumulation during seedling photomorphogenesis. Thus, suppressing PSY activity and impairing PSY:ORANGE protein interactions revealed how cis-carotene abundance can be fine-tuned through holoenzyme-metabolon interactions to control plastid development.

Photomorphogenesis and Photosynthetic Traits Changes in Rice Seedlings Responding to Red and Blue Light.

The purpose of this study is to determine the effects of red and blue lights on the photomorphogenesis and photosynthetic traits of rice seedlings. The rice seedlings were cultured with red light (R), blue light (B), combined red and blue lights (R3B1/R1B1/R1B3), and white light (CK) as the control. The combined application of red and blue lights could promote the growth of rice seedlings to varying degrees; enhance photosynthesis by increasing the seedling leaf area, chlorophyll content, and chlorophyll fluorescence; improve root characteristics by increasing root number, root volume, and root activity; and thus increase the dry matter accumulation of rice seedlings. In addition, the combination of red and blue lights could regulate the expression of genes related to photosynthesis in rice leaves, affect the activity of the Rubisco enzyme, and then affect the photosynthesis of rice seedlings. These results indicate that red and blue lights have direct synergistic effects, which can regulate the growth of rice seedlings and promote the morphogenesis of rice seedlings. The combined application of red and blue lights can be used to supplement the light in rice-factory seedling raising.

MYB112 connects light and circadian clock signals to promote hypocotyl elongation in Arabidopsis.

Ambient light and the endogenous circadian clock play key roles in regulating Arabidopsis (Arabidopsis thaliana) seedling photomorphogenesis. PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) acts downstream of both light and the circadian clock to promote hypocotyl elongation. Several members of the R2R3-MYB transcription factor (TF) family, the most common type of MYB TF family in Arabidopsis, have been shown to be involved in regulating photomorphogenesis. Nonetheless, whether R2R3-MYB TFs are involved in connecting the light and clock signaling pathways during seedling photomorphogenesis remains unknown. Here, we report that MYB112, a member of the R2R3-MYB family, acts as a negative regulator of seedling photomorphogenesis in Arabidopsis. The light signal promotes the transcription and protein accumulation of MYB112. myb112 mutants exhibit short hypocotyls in both constant light and diurnal cycles. MYB112 physically interacts with PIF4 to enhance the transcription of PIF4 target genes involved in the auxin pathway, including YUCCA8 (YUC8), INDOLE-3-ACETIC ACID INDUCIBLE 19 (IAA19), and IAA29. Furthermore, MYB112 directly binds to the promoter of LUX ARRHYTHMO (LUX), the central component of clock oscillators, to repress its expression mainly in the afternoon and relieve LUX-inhibited expression of PIF4. Genetic evidence confirms that LUX acts downstream of MYB112 in regulating hypocotyl elongation. Thus, the enhanced transcript accumulation and transcriptional activation activity of PIF4 by MYB112 additively promotes the expression of auxin-related genes, thereby increasing auxin synthesis and signaling and fine-tuning hypocotyl growth under diurnal cycles.

The B-box transcription factor 4 regulates seedling photomorphogenesis and flowering in tomato

Tomato (Solanum lycopersicum L.) is an important solanaceous crop worldwide. Research in the model plant species Arabidopsis revealed that B-box family transcription factors (BBXs) play key roles in plant development, however, only very few tomato BBXs have been functionally characterized. In this work, we report that the transcription factor SlBBX4 (Solyc08g006530) plays critical roles in tomato seedling photomorphogenesis. The slbbx4 frame-shift mutants showed hyposensitive response to red light, while maintained normal responses to blue, far-red light and UV-B in hypocotyl length assays. The red light-specific function in seedling photomorphogenesis is analogous to Arabidopsis BBX4 (AtBBX4). Whereas atbbx4 mutant flowered earlier than wildtype, the slbbx4 mutants showed delayed flowering, under both long-day and short-day conditions. Thus, SlBBX4 and AtBBX4 play both shared and distinct functions in plant development. The E3 ligase CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) is known to play important functions in the regulation of seedling photomorphogenesis and flowering in Arabidopsis. SlBBX4 interacts with tomato COP1 (SlCOP1) in yeast and in planta, suggesting that SlBBX4 is likely regulated by SlCOP1. Taken together, our data suggest that SlBBX4 plays promotive roles in red light-induced photomorphogenesis and flowering, both of which are potentially beneficial in agriculture.

Identification of the Brassica Campestris ssp. Chinensis BcHY5 Gene Important for Hypocotyl Length

The primary domain/leucine zipper (bZIP) transcription factor, Elongated Hypocotyl (HY5), is crucial for the photomorphogenesis of seedlings. Here, we identified BcHY5 as a regulator of hypocotyl length from the non-heading Chinese cabbage (NHCC) variety ‘Suzhouqing’, which encoded a protein comprised 164 amino acid residues. Ectopic expression of BcHY5 in Arabidopsis shortens the length of the hypocotyl. Additionally, we discovered a protein called BcBBX24 containing the B-BOX (BBX) domain, which is the interacting partner of BcHY5. Yeast two-hybrid (Y2H), bimolecular fluorescence complementation (BiFC) and GST pull-down assays revealed that BcHY5 interacted with BcBBX24. Additionally, by physically binding to the promoter of BcHY5, BcBBX24 inhibited the transcriptional activity of BcHY5. Therefore, our findings reveal a transcriptional mechanism through which light response occurs in NHCC seedlings, where BcHY5 can interact with BcBBX24 and BcBBX24 can prevent BcHY5 transcription.

Transcriptional regulation of photomorphogenesis in seedlings of Brassica napus under different light qualities.

This study displayed the transcriptional regulation network of key regulators and downstream pathway in seedling morphogenesis of Brassica napus under different light quality. Plants undergo photomorphogenesis upon the presence of light, mediated by different light (e.g., blue, red, and far-red) signaling pathways. Although the light signaling pathway has been well documented in Arabidopsis, the underlying mechanisms were studied to a less extent in other plant species including Brassica napus. In this study, we investigated the effect of different light qualities (white, blue, red, and far-red light) on the hypocotyl elongation in B. napus, and performed the transcriptomic analysis of seedlings in response to different light qualities. The results showed that hypocotyl elongation was slightly inhibited by red light, while it was strongly inhibited by blue/far-red light. Transcriptome analysis identified 9748 differentially expressed genes (DEGs) among treatments. Gene ontology (GO) enrichment analysis of DEGs showed that light-responsive and photosynthesis-related genes were highly expressed in response to blue/far-red light rather than in red light. Furthermore, the key genes in light signaling (i.e., PHYB, HY5, HYH, HFR1, and PIF3) exhibited distinct expression patterns between blue/far-red and red light treatments. In addition, subgenome dominant expression of homoeologous genes were observed for some genes, such as PHYA, PHYB, HFR1, and BBXs. The current study displayed a comprehensive dissection of light-mediated transcriptional regulation network, including light signaling, phytohormone, and cell elongation/modification, which improved the understanding on the underlying mechanism of light-regulated hypocotyl growth in B. napus.

Identification, evolution, expression and protein interaction analysis of genes encoding B-box zinc-finger proteins in maize

The B-box (BBX) family of proteins consists of zinc-finger transcription factors with one or two highly conserved B-box motifs at their N-termini. BBX proteins play crucial roles in various aspects of plant growth and development, including seedling photomorphogenesis, shade avoidance, flowering time, and biotic and abiotic stress responses. Previous studies have identified many different BBXs from several plant species, although the BBX family members in maize are largely unknown. Genome-wide identification and comprehensive analysis of maize BBX (ZmBBX) expression and interaction networks would therefore provide valuable information for understanding their functions. In this study, 36 maize BBXs in three major clades were identified. The ZmBBXs within a given clade were found to share similar domains, motifs, and genomic structures. Gene duplication analyses revealed that the expansion of BBX proteins in maize has mainly occurred by segmental duplication. The expression levels of ZmBBXs were analyzed in various organs and tissues, and under different abiotic stress conditions. Protein–protein interaction networks of ZmBBXs were established using bioinformatic tools and verified by bimolecular fluorescence complementation (BiFC) assays. Our findings can facilitate a greater understanding of the complexity of the ZmBBX family and provide novel clues for unravelling ZmBBX protein functions.

Characterization of Cry2 genes (CRY2a and CRY2b) of B. napus and comparative analysis of BnCRY1 and BnCRY2a in regulating seedling photomorphogenesis.

Cryptochrome 2 (CRY2) perceives blue/UV-A light and regulates photomorphogenesis in plants. However, besides Arabidopsis, CRY2 has been functionally characterized only in native species of japonica rice and tomato. In the present study, the BnCRY2a, generating a relatively longer cDNA and harboring an intron in its 5'UTR, has been characterized in detail. Western blot analysis revealed that BnCRY2a is light labile and degraded rapidly by 26S proteasome when seedlings are irradiated with blue light. For functional analysis, BnCRY2a was over-expressed in Brassica juncea, a related species more amenable to transformation. The BnCRY2a over-expression (BnCRY2aOE) transgenics developed short hypocotyl and expanded cotyledons, accumulated more anthocyanin in light-grown seedlings, and displayed early flowering on maturity. Early flowering in BnCRY2aOE transgenics was coupled with the up-regulation of many flowering-related genes such as FT. The present study also highlights the differential light sensitivity of cry1 and cry2 in controlling hypocotyl elongation growth in Brassica. BnCRY2aOE seedlings developed much shorter hypocotyl under the low-intensity of blue light, while BnCRY1OE seedling hypocotyls were shorter under the high-intensity blue light, compared to untransformed seedlings.

Molecular cloning of two maize (<italic>Zea mays</italic>) <italic>ZmCOP1</italic> genes and their transcription abundances in response to different light treatments

<p id="C3">As an E3 ubiquitin ligase, CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) mediates ubiquitination and degradation of positive regulatory factors such as HY5, LAF1, HFR1, and CO of the light signal transduction in <italic>Arabidopsis thaliana</italic>, thus mediating seedling photomorphogenesis, anthocyanin synthesis, and flowering time. To explore functional differentiation of <italic>ZmCOP1</italic> genes, we cloned two <italic>ZmCOP1</italic> genes, designated as <italic>ZmCOP1a</italic> and <italic>ZmCOP1b</italic>, from maize inbred line B73 by reverse transcription PCR (RT-PCR). The physicochemical properties, domain prediction and phylogenetic tree of ZmCOP1 were analyzed by bioinformatics software and website. The transcription abundances of two <italic>ZmCOP1</italic> genes in different tissues and their responses to different light treatments at seedling stage were further analyzed by quantitative RT-PCR (qRT-PCR). We found that the open reading frames (ORFs) of <italic>ZmCOP1a</italic> and <italic>ZmCOP1b</italic> possessed 2082 and 2061 nucleotides, encoding 693 and 686 amino acids, respectively. The COP1 proteins of maize, rice, sorghum, millet, and <italic>Arabidopsis</italic> shared the same structural domains and high amino acid sequence identity, indicating that they may have similar functions. The two <italic>ZmCOP1</italic> genes were mainly expressed in the above-ground tissues. Both of them could rapidly respond to different light treatments, while the transcription abundances of <italic>ZmCOP1a</italic> were generally higher than those of <italic>ZmCOP1b</italic>. It might suggest that <italic>ZmCOP1a</italic> may play more important roles under different light conditions. Under long-day or short-day condition, the transcription abundances of both <italic>ZmCOP1a</italic> and <italic>ZmCOP1b</italic> during the dark phase were higher than those during the light phase. Another interesting case is that both genes kept similar expression patterns during the light phase, while, transcription abundances of <italic>ZmCOP1b</italic> were higher than those of <italic>ZmCOP1a</italic> during the dark phase. It might imply that <italic>ZmCOP1b</italic> may be more important than <italic>ZmCOP1a</italic> in response to long-day or short-day treatment. In conclusion, both <italic>ZmCOP1a</italic> and <italic>ZmCOP1b</italic> had got functional redundancy and differentiation. Both genes could participate in different light signal pathways, and regulate maize photomorphogenesis and flowering period. Our results also provide a research foundation for further exploration of two <italic>ZmCOP1</italic> genes and application in molecular breeding in maize.

SWELLMAP 2, a phyB-Interacting Splicing Factor, Negatively Regulates Seedling Photomorphogenesis in Arabidopsis.

Light-triggered transcriptome reprogramming is critical for promoting photomorphogenesis in Arabidopsis seedlings. Nonetheless, recent studies have shed light on the importance of alternative pre-mRNA splicing (AS) in photomorphogenesis. The splicing factors splicing factor for phytochrome signaling (SFPS) and reduced red-light responses in cry1cry2 background1 (RRC1) are involved in the phytochrome B (phyB) signaling pathway and promote photomorphogenesis by controlling pre-mRNA splicing of light- and clock-related genes. However, splicing factors that serve as repressors in phyB signaling pathway remain unreported. Here, we report that the splicing factor SWELLMAP 2 (SMP2) suppresses photomorphogenesis in the light. SMP2 physically interacts with phyB and colocalizes with phyB in photobodies after light exposure. Genetic analyses show that SMP2 antagonizes phyB signaling to promote hypocotyl elongation in the light. The homologs of SMP2 in yeast and human belong to second-step splicing factors required for proper selection of the 3' splice site (3'SS) of an intron. Notably, SMP2 reduces the abundance of the functional REVEILLE 8 a (RVE8a) form, probably by determining the 3'SS, and thereby inhibits RVE8-mediated transcriptional activation of clock genes containing evening elements (EE). Finally, SMP2-mediated reduction of functional RVE8 isoform promotes phytochrome interacting factor 4 (PIF4) expression to fine-tune hypocotyl elongation in the light. Taken together, our data unveil a phyB-interacting splicing factor that negatively regulates photomorphogenesis, providing additional information for further mechanistic investigations regarding phyB-controlled AS of light- and clock-related genes.

Karrikins control seedling photomorphogenesis and anthocyanin biosynthesis through a HY5-BBX transcriptional module.

The butenolide molecule, karrikin (KAR), emerging in smoke of burned plant material, enhances light responses such as germination, inhibition of hypocotyl elongation, and anthocyanin accumulation in Arabidopsis. The KAR signaling pathway consists of KARRIKIN INSENSITIVE 2 (KAI2) and MORE AXILLARY GROWTH 2 (MAX2), which, upon activation, act in an SCF E3 ubiquitin ligase complex to target the downstream signaling components SUPPRESSOR OF MAX2 1 (SMAX1) and SMAX1-LIKE 2 (SMXL2) for degradation. How degradation of SMAX1 and SMXL2 is translated into growth responses remains unknown. Although light clearly influences the activity of KAR, the molecular connection between the two pathways is still poorly understood. Here, we demonstrate that the KAR signaling pathway promotes the activity of a transcriptional module consisting of ELONGATED HYPOCOTYL 5 (HY5), B-BOX DOMAIN PROTEIN 20 (BBX20), and BBX21. The bbx20 bbx21 mutant is largely insensitive to treatment with KAR2 , similar to a hy5 mutant, with regards to inhibition of hypocotyl elongation and anthocyanin accumulation. Detailed analysis of higher order mutants in combination with RNA-sequencing analysis revealed that anthocyanin accumulation downstream of SMAX1 and SMXL2 is fully dependent on the HY5-BBX module. However, the promotion of hypocotyl elongation by SMAX1 and SMXL2 is, in contrast to KAR2 treatment, only partially dependent on BBX20, BBX21, and HY5. Taken together, these results suggest that light- and KAR-dependent signaling intersect at the HY5-BBX transcriptional module.

Beyond Arabidopsis: BBX Regulators in Crop Plants.

B-box proteins represent diverse zinc finger transcription factors and regulators forming large families in various plants. A unique domain structure defines them—besides the highly conserved B-box domains, some B-box (BBX) proteins also possess CCT domain and VP motif. Based on the presence of these specific domains, they are mostly classified into five structural groups. The particular members widely differ in structure and fulfill distinct functions in regulating plant growth and development, including seedling photomorphogenesis, the anthocyanins biosynthesis, photoperiodic regulation of flowering, and hormonal pathways. Several BBX proteins are additionally involved in biotic and abiotic stress response. Overexpression of some BBX genes stimulates various stress-related genes and enhanced tolerance to different stresses. Moreover, there is evidence of interplay between B-box and the circadian clock mechanism. This review highlights the role of BBX proteins as a part of a broad regulatory network in crop plants, considering their participation in development, physiology, defense, and environmental constraints. A description is also provided of how various BBX regulators involved in stress tolerance were applied in genetic engineering to obtain stress tolerance in transgenic crops.

Functional analysis of a novel cryptochrome gene (GbCRY1) from Ginkgo biloba

ABSTRACT Cryptochrome (CRY) is a blue light receptor that is widely distributed in animals, plants, and microorganisms. CRY as a coding gene of cryptochrome that regulates the organism gene expression and plays an important role in organism growth and development. In this study, we identified four photolyase/cryptochrome (PHR/CRY) members from the genome of Ginkgo biloba. Phylogenetic tree analysis showed that the Ginkgo PHR/CRY family members were closely related to Arabidopsis thaliana and Solanum lycopersicum. We isolated a cryptochrome gene, GbCRY1, from G. biloba and analyzed its structure and function. GbCRY1 shared high similarity with AtCRY1 from A. thaliana. GbCRY1 expression level was higher in stems and leaves and lower in roots, male strobili, female strobili. GbCRY1 expression level fluctuated periodically within 24 h, gradually increased in the dark, and decreased under blue light. The newly germinated ginkgo seedlings were cultured under dark, white light, and blue light conditions. The blue light normally induced photomorphogenesis of ginkgo seedlings, which included hypocotyl elongation inhibition, leaf expansion inhibition, and chlorophyll formation. Treating dark-adapted ginkgo leaves with blue light could induce stomatal opening. At the same time, blue light reduced the expression level of GbCRY1 in the process of inducing photomorphogenesis and stoma opening. Our results provide evidence that GbCRY1 expression is affected by space, circadian cycle and light, and also proves that GbCRY1 is related to ginkgo circadian clock, photomorphogenesis and stoma opening process.

HY5-COP1: the central module of light signaling pathway

Light acts catalytically to initiate a cascade of events to eventually regulate different aspects of plant development. The cascade of light signal transduction involves several components that can be broadly grouped as photoreceptors, early signaling factors, central integrators and downstream effectors. ELONGATED HYPOCOTYL 5 (HY5), a bZIP transcription factor, is the most well characterized downstream effector in Arabidopsis, which acts as a positive regulator of photomorphogenesis. HY5 acts as a master regulator and binds to promoters of close to 4000 genes, thereby, regulating wide diversity of photomorphogenic responses. It physically interacts with CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), a negative regulator of photomorphogenesis. HY5-COP1 interaction mediates cross-talk among multiple pathways, thereby, enhancing the phenotypic plasticity of plants. This interaction regulates not only the upstream components of light signaling pathway like photoreceptors but also regulates a number of downstream effectors and early signaling factors, which help to regulate a number of developmental processes in plants. This review highlights the key interaction between HY5 and COP1 and how it regulates seedling photomorphogenesis, shade avoidance, circadian clock, root architecture, flowering, thermomorphogenesis, hormone signaling and stress responses, in conjunction with other signaling components and transcription factors.

Self-transcriptional repression of the Arabidopsis NAC transcription factor ATAF2 and its genetic interaction with phytochrome A in modulating seedling photomorphogenesis.

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.

The cold response regulator CBF1 promotes Arabidopsis hypocotyl growth at ambient temperatures.

Light and temperature are two core environmental factors that coordinately regulate plant growth and survival throughout their entire life cycle. However, the mechanisms integrating light and temperature signaling pathways in plants remain poorly understood. Here, we report that CBF1, an AP2/ERF-family transcription factor essential for plant cold acclimation, promotes hypocotyl growth under ambient temperatures in Arabidopsis. We show that CBF1 increases the protein abundance of PIF4 and PIF5, two phytochrome-interacting bHLH-family transcription factors that play pivotal roles in modulating plant growth and development, by directly binding to their promoters to induce their gene expression, and by inhibiting their interaction with phyB in the light. Moreover, our data demonstrate that CBF1 promotes PIF4/PIF5 protein accumulation and hypocotyl growth at both 22°C and 17°C, but not at 4°C, with a more prominent role at 17°C than at 22°C. Together, our study reveals that CBF1 integrates light and temperature control of hypocotyl growth by promoting PIF4 and PIF5 protein abundance in the light, thus providing insights into the integration mechanisms of light and temperature signaling pathways in plants.

Red and blue light differentially impact retrograde signalling and photoprotection in rice.

Chloroplast-to-nucleus retrograde signalling (RS) is known to impact plant growth and development. In Arabidopsis, we and others have shown that RS affects seedling establishment by inhibiting deetiolation. In the presence of lincomycin, a chloroplast protein synthesis inhibitor that triggers RS, Arabidopsis light-grown seedlings display partial skotomorphogenesis with undeveloped plastids and closed cotyledons. By contrast, RS in monocotyledonous has been much less studied. Here, we show that emerging rice seedlings exposed to lincomycin do not accumulate chlorophyll but otherwise remain remarkably unaffected. However, by using high red (R) and blue (B) monochromatic lights in combination with lincomycin, we have uncovered a RS inhibition of length and a reduction in the B light-induced declination of the second leaf. Furthermore, we present data showing that seedlings grown in high B and R light display different non-photochemical quenching capacity. Our findings support the view that excess B and R light impact seedling photomorphogenesis differently to photoprotect and optimize the response to high-light stress. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.