The transcriptional regulator BBX24 impairs DELLA activity to promote shade avoidance in Arabidopsis thaliana

Plant Morphology and a Phytochrome B Model Reveal That the Effects of Far-Red Light on Shade-Avoidance-Like Responses Persist Under High Light Intensity

Several studies have concluded that high photon flux density (PFD) attenuates the effects of the red (R; 600-699 nm) to far-red (FR; 700-750 nm) light ratio on morphology. However, the suppressive effects can depend on individual wavebands that modulate photoreceptor activity. We postulated that morphological responses of shade-avoiding plants to the FR fraction (FR-PFD divided by R + FR PFD) act independent of total PFD (TPFD; 400-750 nm) when TPFD increases are only from R and FR light. We grew kale and lettuce under three FR fractions and four TPFDs while maintaining a constant blue (B; 400-499 nm) PFD. An increase in the R + FR PFD reduced leaf elongation and specific leaf area (SLA). However, higher light did not suppress the FR-fraction effects on leaf elongation and SLA. We estimated PHYB activity with a three-state PHYB model to mechanistically explain the suppressive effects of high light on leaf elongation and SLA but not on FR-mediated leaf elongation and SLA increase. PHYB model predictions were in accordance with the morphological responses of kale and lettuce. This study is the first to apply the three-state PHYB model to explain photon-spectrum-induced morphology of light-grown whole plants, demonstrating its potential use to crops and for applications.

The Combination of Physiological and Transcriptomic Approaches Reveals New Insights into the Molecular Mechanisms of Leymus chinensis Growth Under Different Shading Intensities

Leymus chinensis is a grass species in the family Triticeae that is found in the Eurasian grassland region and is known for its outstanding ecological advantages and economic value. However, the increasing adoption of photovoltaic agriculture has modified the light environment for the grass, markedly inhibiting its photosynthesis, growth, and yield. This study used physiological and transcriptomic analyses to investigate the complex response mechanisms of two L. chinensis genotypes (Zhongke No. 3 [Lc3] and Zhongke No. 5 [Lc5]) under shading stress. Growth phenotype analysis revealed the superior growth performance of Lc3 under shading stress, evidenced by enhanced plant height and photosynthetic parameters. Additionally, differentially expressed genes (DEGs) were predominantly enriched in starch and sucrose metabolism and glycolysis/gluconeogenesis pathways, which were the most consistently enriched in both L. chinensis genotypes. However, the flavonoid biosynthesis and galactose metabolism pathways were more enriched in Lc3. Weighted gene co-expression network analysis identified the LcGolS2 gene, which encodes galactinol synthase, as a potential hub gene for resistance to shade stress in comparisons across different cultivars and shading treatments. The use of qRT-PCR analysis further validated the genes involved in these pathways, suggesting that they may play critical roles in regulating the growth and development of L. chinensis under shading conditions. These findings provide new insights into the molecular mechanisms underlying the growth and development of L. chinensis under different shading stress conditions.

DBB2 regulates plant height and shade avoidance responses in maize

Increasing plant density has been recognized as an effective strategy for boosting maize yields over the past few decades. However, dense planting significantly reduces the internal light intensity and the red to far-red (R:FR) light ratio in the canopy, which subsequently triggers shade avoidance responses (SAR) that limit further yield enhancements, particularly under high-density conditions. In this study, we identified double B-box containing protein DBB2, a member of the ZmBBX family that is rapidly induced by shade, as a crucial regulator of plant height and SAR. Disruption of DBB2 resulted in shorter internodes, reduced plant height, decreased cell elongation, and diminished sensitivity to shade in maize, effects that can be largely alleviated by external treatment with gibberellins (GA). Furthermore, we discovered that DBB2 physically interacted with the transcription factor HY5, inhibiting its transcriptional activation of ZmGA2ox4, a gene encoding a GA2 oxidase that can deactivate GA. This interaction positively influences maize plant height through the GA pathway. Additionally, we found that the induction of ZmDBB2 by shade is mediated by the transcription factor PIF4. Interestingly, DBB2 then interacted with PIF4 to enhance the transcriptional activation of cell elongation-related genes, such as ZmEXPA1, thereby establishing a positive feedback loop promoting cell elongation under canopy shade conditions. Our findings highlight the critical role of BBX proteins in modulating plant height and SAR, presenting them as key genetic targets for developing maize varieties suited to high-density planting conditions. This study also provides new insights into the molecular mechanisms underlying SAR and offers potential strategies for the genetic improvement of maize plant architecture and grain yield.

The BBX7/8-CCA1/LHY transcription factor cascade promotes shade avoidance by activating PIF4

Sun-loving plants undergo shade avoidance syndrome (SAS) to compete with their neighbors for sunlight in shade conditions. Phytochrome B (phyB) plays a dominant role in sensing the shading signals (low red to far-red ratios) and triggering SAS. Shade drives phyB conversion to inactive form, consequently leading to the accumulation of PHYTOCHROMEINTERACTING FACTOR 4 (PIF4) that promotes plant growth. Here, we show B-box PROTEIN 7 (BBX7)/BBX8 and CIRCADIAN CLOCK ASSOCIATED 1 (CCA1)/LATE ELONGATED HYPOCOTYL (LHY) positively regulate the low R : FR-induced PIF4 expression and promote the low R : FR-triggered hypocotyl growth in Arabidopsis. Shade interferes the interactions of phyB with BBX7 or BBX8 and triggers the accumulation of BBX7 and BBX8 independent of phyB. BBX7 and BBX8 associate with CCA1 and LHY to activate their transcription, the gene produces of which subsequently upregulate the expression of PIF4 in shade. Genetically, BBX7 and BBX8 act upstream of CCA1, LHY, and PIF4 with respect to hypocotyl growth in shade conditions. Our study reveals the BBX7/8-CCA1/LHY transcription factor cascade upregulates PIF4 expression and increases its abundance to promote plant growth and development in response to shade.

BBX22 enhances the accumulation of antioxidants to inhibit DNA damage and promotes DNA repair under high UV-B

Under changing climatic conditions, plant exposure to high-intensity UV-B can be a potential threat to plant health and all plant-derived human requirements, including food. It's crucial to understand how plants respond to high UV-B radiation so that proper measures can be taken to enhance tolerance towards high UV-B stress. We found that BBX22, a B-box protein-coding gene, is strongly induced within one hour of exposure to high-intensity UV-B. Our metabolomics data indicated that BBX22 promotes the accumulation of antioxidants like ascorbic acid and proline. These antioxidants play a vital role in shielding plants exposed to high UV-B from the detrimental effects of Reactive Oxygen Species (ROS), including DNA damage. Additionally, BBX22 promotes DNA damage repair by inducing the expression of DNA repair genes like UVR1 and UVR3. BBX22 directly binds to the promoter of UVR1 to regulate its expression. Furthermore, BBX22 indirectly induces the expression of UVR1 and UVR3 by enhancing the binding of HY5 to their promoters. Together, these results suggest a multi-pronged role of BBX22 in protection against high-intensity UV-B. Enhancing BBX22 levels or its orthologs in different plant species can potentially offer DNA damage protection and tolerance against intense UV radiation.

BBX21 Integrates Brassinosteroid Biosynthesis and Signaling in the Inhibition of Hypocotyl Growth under Shade

B-Box-containing zinc finger transcription factors (BBX) are involved in light-mediated growth, affecting processes such as hypocotyl elongation in Arabidopsis thaliana. However, the molecular and hormonal framework that regulates plant growth through BBX proteins is incomplete. Here, we demonstrate that BBX21 inhibits the hypocotyl elongation through the brassinosteroid (BR) pathway. BBX21 reduces the sensitivity to 24-epiBL, a synthetic active BR, principally at very low concentrations in simulated shade. The biosynthesis profile of BRs showed that two active BR-brassinolide and 28-homobrassinolide-and 8 of 11 intermediates can be repressed by BBX21 under white light (WL) or simulated shade. Furthermore, BBX21 represses the expression of CYTOCHROME P450 90B1 (DWF4/CYP90B1), BRASSINOSTEROID-6-OXIDASE 1 (BR6OX1, CYP85A1) and BR6OX2 (CYP85A2) genes involved in the BR biosynthesis in WL while specifically promoting DWF4 and PHYB ACTIVATION TAGGED SUPPRESSOR 1 (CYP2B1/BAS1) expression in WL supplemented with far-red (WL + FR), a treatment that simulates shade. In addition, BBX21 represses BR signaling genes, such as PACLOBUTRAZOL RESISTANCE1 (PRE1), PRE3 and ARABIDOPSIS MYB-LIKE 2 (MYBL2), and auxin-related and expansin genes, such as INDOLE-3-ACETIC ACID INDUCIBLE 1 (IAA1), IAA4 and EXPANSIN 11 in short-term shade. By a genetic approach, we found that BBX21 acts genetically upstream of BRASSINAZOLE-RESISTANT 1 (BZR1) for the promotion of DWF4 and BAS1 gene expression in shade. We propose that BBX21 integrates the BR homeostasis and shade-light signaling, allowing the fine-tuning of hypocotyl elongation in Arabidopsis.

B-BOX proteins:Multi-layered roles of molecular cogs in light-mediated growth and development in plants

B-box containing proteins (BBXs) are a class of zinc-ligating transcription factors or regulators that play essential roles in various physiological and developmental processes in plants. They not only directly associate with target genes to regulate their transcription, but also interact with other transcription factors to mediate target genes' expression, thus forming a complex transcriptional network ensuring plants' adaptation to dynamically changing light environments. This review summarizes and highlights the molecular and biochemical properties of BBXs, as well as recent advances with a focus on their critical regulatory functions in photomorphogenesis (de-etiolation), shade avoidance, photoperiodic-mediated flowering, and secondary metabolite biosynthesis and accumulation in plants.

Environmental Control of Hypocotyl Elongation

The hypocotyl is the embryonic stem connecting the primary root to the cotyledons. Hypocotyl length varies tremendously depending on the conditions. This developmental plasticity and the simplicity of the organ explain its success as a model for growth regulation. Light and temperature are prominent growth-controlling cues, using shared signaling elements. Mechanisms controlling hypocotyl elongation in etiolated seedlings reaching the light differ from those in photoautotrophic seedlings. However, many common growth regulators intervene in both situations. Multiple photoreceptors including phytochromes, which also respond to temperature, control the activity of several transcription factors, thereby eliciting rapid transcriptional reprogramming. Hypocotyl growth often depends on sensing in green tissues and interorgan communication comprising auxin. Hypocotyl auxin, in conjunction with other hormones, determines epidermal cell elongation. Plants facing cues with opposite effects on growth control hypocotyl elongation through intricate mechanisms. We discuss the status of the field and end by highlighting open questions.

B-Box transcription factor BBX28 requires CONSTITUTIVE PHOTOMORPHOGENESIS1 to induce shade-avoidance response in Arabidopsis thaliana

Shade avoidance syndrome is an important adaptive strategy. Under shade, major transcriptional rearrangements underlie the reallocation of resources to elongate vegetative structures and redefine the plant architecture to compete for photosynthesis. BBX28 is a B-box transcription factor involved in seedling de-etiolation and flowering in Arabidopsis (Arabidopsis thaliana), but its function in shade-avoidance response is completely unknown. Here, we studied the function of BBX28 using two mutant and two transgenic lines of Arabidopsis exposed to white light and simulated shade conditions. We found that BBX28 promotes hypocotyl growth under shade through the phytochrome system by perceiving the reduction of red photons but not the reduction of photosynthetically active radiation or blue photons. We demonstrated that hypocotyl growth under shade is sustained by the protein accumulation of BBX28 in the nuclei in a CONSTITUTIVE PHOTOMORPHOGENESIS1 (COP1)-dependent manner at the end of the photoperiod. BBX28 up-regulates the expression of transcription factor- and auxin-related genes, thereby promoting hypocotyl growth under prolonged shade. Overall, our results suggest the role of BBX28 in COP1 signaling to sustain the shade-avoidance response and extend the well-known participation of other members of BBX transcription factors for fine-tuning plant growth under shade.

A Novel 10-Base Pair Deletion in the First Exon of GmHY2a Promotes Hypocotyl Elongation, Induces Early Maturation, and Impairs Photosynthetic Performance in Soybean

Plants photoreceptors perceive changes in light quality and intensity and thereby regulate plant vegetative growth and reproductive development. By screening a γ irradiation-induced mutant library of the soybean (Glycine max) cultivar "Dongsheng 7", we identified Gmeny, a mutant with elongated nodes, yellowed leaves, decreased chlorophyll contents, altered photosynthetic performance, and early maturation. An analysis of bulked DNA and RNA data sampled from a population segregating for Gmeny, using the BVF-IGV pipeline established in our laboratory, identified a 10 bp deletion in the first exon of the candidate gene Glyma.02G304700. The causative mutation was verified by a variation analysis of over 500 genes in the candidate gene region and an association analysis, performed using two populations segregating for Gmeny. Glyma.02G304700 (GmHY2a) is a homolog of AtHY2a in Arabidopsis thaliana, which encodes a PΦB synthase involved in the biosynthesis of phytochrome. A transcriptome analysis of Gmeny using the Kyoto Encyclopedia of Genes and Genomes (KEGG) revealed changes in multiple functional pathways, including photosynthesis, gibberellic acid (GA) signaling, and flowering time, which may explain the observed mutant phenotypes. Further studies on the function of GmHY2a and its homologs will help us to understand its profound regulatory effects on photosynthesis, photomorphogenesis, and flowering time.

Arabidopsis ERD15 regulated by BBX24 plays a positive role in UV-B signaling

Ultraviolet-B (UV-B, 280-315 nm) is a minor component of solar radiation, but it has a major regulatory impact on plant growth and development. Solar UV-B regulates numerous aspects of plant metabolism, morphology and physiology through altering the expression of hundreds of genes. EARLY RESPONSIVE TO DEHYDRATION 15 (ERD15) is a drought-induced rapid response gene, formerly known as a negative regulator of the abscisic acid (ABA) signaling pathway. It is unclear whether ERD15 is involved in UV-B-induced photomorphogenesis. Previously, we reported that the BBX24 transcriptional factor negatively regulated UV-B signaling. In the present study, we identified that ERD15 is involved in UV-B photomorphogenesis as a positive regulator at phenotypic, physiological and molecular levels. Our results indicated that ERD15 expression is suppressed by UV-B, inhibited the elongation of Arabidopsis hypocotyls in a UV-B-dependent manner, promoted the expression of related UV-B signaling genes and increased the total antioxidant capacity of Arabidopsis under UV-B. Genetic hybridization results show that ERD15 acts downstream of BBX24, and BBX24 protein mediated the expression of ERD15 by binding to its promoter. Thus, ERD15 is a novel positive regulator of the UV-B signaling pathway, which is downstream of BBX24 and regulated by BBX24 protein to participate in UV-B photomorphogenesis.

Highlights in gibberellin research: A tale of the dwarf and the slender

It has been almost a century since biologically active gibberellin (GA) was isolated. Here, we give a historical overview of the early efforts in establishing the GA biosynthesis and catabolism pathway, characterizing the enzymes for GA metabolism, and elucidating their corresponding genes. We then highlight more recent studies that have identified the GA receptors and early GA signaling components (DELLA repressors and F-box activators), determined the molecular mechanism of DELLA-mediated transcription reprograming, and revealed how DELLAs integrate multiple signaling pathways to regulate plant vegetative and reproductive development in response to internal and external cues. Finally, we discuss the GA transporters and their roles in GA-mediated plant development.

Molecular mechanisms underlying coordinated responses of plants to shade and environmental stresses

Shade avoidance syndrome (SAS) is triggered by a low ratio of red (R) to far-red (FR) light (R/FR ratio), which is caused by neighbor detection and/or canopy shade. In order to compete for the limited light, plants elongate hypocotyls and petioles by deactivating phytochrome B (phyB), a major R light photoreceptor, thus releasing its inhibition of the growth-promoting transcription factors PHYTOCHROME-INTERACTING FACTORs. Under natural conditions, plants must cope with abiotic stresses such as drought, soil salinity, and extreme temperatures, and biotic stresses such as pathogens and pests. Plants have evolved sophisticated mechanisms to simultaneously deal with multiple environmental stresses. In this review, we will summarize recent major advances in our understanding of how plants coordinately respond to shade and environmental stresses, and will also discuss the important questions for future research. A deep understanding of how plants synergistically respond to shade together with abiotic and biotic stresses will facilitate the design and breeding of new crop varieties with enhanced tolerance to high-density planting and environmental stresses.

Arabidopsis transcription factor TCP13 promotes shade avoidance syndrome-like responses by directly targeting a subset of shade-responsive gene promoters

TCP13 belongs to a subgroup of TCP transcription factors implicated in the shade avoidance syndrome (SAS), but its exact role remains unclear. Here, we show that TCP13 promotes the SAS-like response by enhancing hypocotyl elongation and suppressing flavonoid biosynthesis as a part of the incoherent feed-forward loop in light signaling. Shade is known to promote the SAS by activating PHYTOCHROME-INTERACTING FACTOR (PIF)-auxin signaling in plants, but we found no evidence in a transcriptome analysis that TCP13 activates PIF-auxin signaling. Instead, TCP13 mimics shade by activating the expression of a subset of shade-inducible and cell elongation-promoting SAUR genes including SAUR19, by direct targeting of their promoters. We also found that TCP13 and PIF4, a molecular proxy for shade, repress the expression of flavonoid biosynthetic genes by directly targeting both shared and distinct sets of biosynthetic gene promoters. Together, our results indicate that TCP13 promotes the SAS-like response by directly targeting a subset of shade-responsive genes without activating the PIF-auxin signaling pathway.

Fine mapping and analysis of candidate genes for qBT2 and qBT7.2 locus controlling bolting time in radish (Raphanus sativus L.)

KEY MESSAGE

Two major QTLs for bolting time in radish were mapped to chromosome 02 and 07 in a 0.37 Mb and 0. 52 Mb interval, RsFLC1 and RsFLC2 is the critical genes. Radish (Raphanus sativus L.) is an important vegetable crop of Cruciferae. The premature bolting and flowering reduces the yield and quality of the fleshy root of radish. However, the molecular mechanism underlying bolting and flowering in radish remains unknown. In YZH (early bolting) × XHT (late bolting) F2 population, a high-density genetic linkage map was constructed with genetic distance of 2497.74 cM and an average interval of 2.31 cM. A total of nine QTLs for bolting time and two QTLs for flowering time were detected. Three QTLs associated with bolting time in radish were identified by QTL-seq using radish GDE (early bolting) × GDL (late bolting) F2 population. Fine mapping narrowed down qBT2 and qBT7.2 to an 0.37 Mb and 0.52 Mb region on chromosome 02 and 07, respectively. RNA-seq and qRT-PCR analysis showed that RsFLC1 and RsFLC2 were the candidate gene for qBT7.2 and qBT2 locus, respectively. Subcellular localization exhibited that RsFLC1 and RsFLC2 were mainly expressed in the nucleus. A 1856-bp insertion in the first intron of RsFLC1 was responsible for bolting time. Overexpression of RsFLC2 in Arabidopsis was significantly delayed flowering. These findings will provide new insights into the exploring the molecular mechanism of late bolting and promote the marker-assisted selection for breeding late-bolting varieties in radish.

Genome-wide analysis of blueberry B-box family genes and identification of members activated by abiotic stress

BACKGROUND

B-box (BBX) proteins play important roles in regulating plant growth, development, and abiotic stress responses. BBX family genes have been identified and functionally characterized in many plant species, but little is known about the BBX family in blueberry (Vaccinium corymbosum).

RESULT

In this study, we identified 23 VcBBX genes from the Genome Database for Vaccinium (GDV). These VcBBXs can be divided into five clades based on gene structures and conserved domains in their encoded proteins. The prediction of cis-acting elements in the upstream sequences of VcBBX genes and protein-protein interactions indicated that VcBBX proteins are likely involved in phytohormone signaling pathways and abiotic stress responses. Analysis of transcriptome deep sequencing (RNA-seq) data showed that VcBBX genes exhibited organ-specific expression pattern and 11 VcBBX genes respond to ultraviolet B (UV-B) radiation. The co-expression analysis revealed that the encoded 11 VcBBX proteins act as bridges integrating UV-B and phytohormone signaling pathways in blueberry under UV-B radiation. Reverse-transcription quantitative PCR (RT-qPCR) analysis showed that most VcBBX genes respond to drought, salt, and cold stress. Among VcBBX proteins, VcBBX24 is highly expressed in all the organs, not only responds to abiotic stress, but it also interacts with proteins in UV-B and phytohormone signaling pathways, as revealed by computational analysis and co-expression analysis, and might be an important regulator integrating abiotic stress and phytohormone signaling networks.

CONCLUSIONS

Twenty-three VcBBX genes were identified in blueberry, in which, 11 VcBBX genes respond to UV-B radiation, and act as bridges integrating UV-B and phytohormone signaling pathways according to RNA-seq data. The expression patterns under abiotic stress suggested that the functional roles of most VcBBX genes respose to drought, salt, and cold stress. Our study provides a useful reference for functional analysis of VcBBX genes and for improving abiotic stress tolerance in blueberry.

SlMPK1- and SlMPK2-mediated SlBBX17 phosphorylation positively regulates CBF-dependent cold tolerance in tomato

B-box (BBX) proteins are an important class of zinc finger transcription factors that play a critical role in plant growth and stress response. However, the mechanisms of how BBX proteins participate in the cold response in tomato remain unclear. Here, using approaches of reverse genetics, biochemical and molecular biology we characterized a BBX transcription factor, SlBBX17, which positively regulates cold tolerance in tomato (Solanum lycopersicum). Overexpressing SlBBX17 enhanced C-repeat binding factor (CBF)-dependent cold tolerance in tomato plants, whereas silencing SlBBX17 increased plant susceptibility to cold stress. Crucially, the positive role of SlBBX17 in CBF-dependent cold tolerance was dependent on ELONGATED HYPOCOTYL5 (HY5). SlBBX17 physically interacted with SlHY5 to directly promote the protein stability of SlHY5 and subsequently increased the transcriptional activity of SlHY5 on SlCBF genes under cold stress. Further experiments showed that cold-activated mitogen-activated protein kinases, SlMPK1 and SlMPK2, also physically interact with and phosphorylate SlBBX17 to enhance the interaction between SlBBX17 and SlHY5, leading to enhanced CBF-dependent cold tolerance. Collectively, the study unveiled a mechanistic framework by which SlMPK1/2-SlBBX17-SlHY5 regulated transcription of SlCBFs to enhance cold tolerance, thereby shedding light on the molecular mechanisms of how plants respond to cold stress via multiple transcription factors.

ELONGATED HYPOCOTYL5 (HY5) and HY5 HOMOLOGUE (HYH) maintain shade avoidance suppression in UV-B

Reductions in red to far-red ratio (R:FR) provide plants with an unambiguous signal of vegetational shade and are monitored by phytochrome photoreceptors. Plants integrate this information with other environmental cues to determine the proximity and density of encroaching vegetation. Shade-sensitive species respond to reductions in R:FR by initiating a suite of developmental adaptations termed shade avoidance. These include the elongation of stems to facilitate light foraging. Hypocotyl elongation is driven by increased auxin biosynthesis promoted by PHYTOCHROME INTERACTING FACTORs (PIF) 4, 5 and 7. UV-B perceived by the UV RESISTANCE LOCUS 8 (UVR8) photoreceptor rapidly inhibits shade avoidance, in part by suppressing PIF4/5 transcript accumulation and destabilising PIF4/5 protein. Here, we show that longer-term inhibition of shade avoidance is sustained by ELONGATED HYPOCOTYL 5 (HY5) and HY5 HOMOLOGUE (HYH), which regulate transcriptional reprogramming of genes involved in hormone signalling and cell wall modification. HY5 and HYH are elevated in UV-B and suppress the expression of XYLOGLUCAN ENDOTRANSGLUCOSYLASE/HYDROLASE (XTH) genes involved in cell wall loosening. They additionally increase expression GA2-OXIDASE1 (GA2ox1) and GA2ox2, encoding gibberellin catabolism enzymes that act redundantly to stabilise the PIF-inhibiting DELLA proteins. UVR8 therefore regulates temporally distinct signalling pathways to first rapidly inhibit and subsequently maintain suppression of shade avoidance following UV-B exposure.

Transcriptomic and physiological shade avoidance responses in potato (Solanum tuberosum) plants

Plants detect competitors in shaded environments by perceiving a reduction in photosynthetically active radiation (PAR) and the reduction between the red and far-red light (R:FR) ratio and blue photons. These light signals are detected by phytochromes and cryptochromes, which trigger shade avoidance responses such as shoot and petiole elongation and lead to increased susceptibility to pathogen attack. We studied morphological, anatomical, and photosynthesis differences in potato plants (Solanum tuberosum var. Spunta) exposed to sunlight or simulated shade in a greenhouse. We found that simulated shade strongly induced stem and internode elongation with a higher production of free auxin in stems and a lower production of tubers. The mesophyll thickness of the upper leaves of plants grown in simulated shade was lower, but the epidermis was wider compared with the leaves of plants cultivated in sunlight. In addition, the photosynthesis rate was lower in the upper leaves exposed to nonsaturated irradiances and higher in the basal leaves at saturated irradiances compared with control plants. RNA-seq analysis showed that 146 and 155 genes were up- and downregulated by shade, respectively. By quantitative reverse transcription polymerase chain reaction, we confirmed that FLOWERING LOCUS T (FT), WRKY-like, and PAR1b were induced, while FLAVONOL 4-SULFOTRANSFERASE was repressed under shade. In shaded plants, leaves and tubers were more susceptible to the necrotrophic fungus Botrytis cinerea attack. Overall, our work demonstrates configurational changes between growth and defense decisions in potato plants cultivated in simulated shade.

BBX24 Increases Saline and Osmotic Tolerance through ABA Signaling in Arabidopsis Seeds

Seed germination is a critical stage for survival during the life cycle of an individual plant. Genetic and environmental cues are integrated by individual seeds to determine germination, mainly achieved through regulation of the metabolism and signaling of gibberellins (GA) and abscisic acid (ABA), two phytohormones with antagonistic roles. Saline and drought conditions can arrest the germination of seeds and limit the seedling emergence and homogeneity of crops. This work aimed to study the function of BBX24, a B-Box transcription factor, in the control of germination of Arabidopsis thaliana seeds imbibed in saline and osmotic conditions. Seeds of mutant and reporter GUS lines of BBX24 were incubated at different doses of NaCl and polyethylene-glycol (PEG) solutions and with ABA, GA and their inhibitors to evaluate the rate of germination. We found that BBX24 promotes seed germination under moderated stresses. The expression of BBX24 is inhibited by NaCl and PEG. In addition, ABA suppresses BBX24-induced seed germination. Additional experiments suggest that BBX24 reduces ABA sensitivity, improving NaCl tolerance, and increases GA sensitivity in seeds imbibed in ABA. In addition, BBX24 inhibits the expression of ABI3 and ABI5 and genetically interacts upstream of HY5 and ABI5. This study demonstrates the relevance of BBX24 to induce drought and salinity tolerance in seed germination to ensure seedling emergence in sub-optimal environments.

BBX24 Interacts with JAZ3 to Promote Growth by Reducing DELLA Activity in Shade Avoidance

Shade avoidance syndrome (SAS) is a strategy of major adaptive significance and typically includes elongation of the stem and petiole, leaf hyponasty, reduced branching and phototropic orientation of the plant shoot toward canopy gaps. Both cryptochrome 1 and phytochrome B (phyB) are the major photoreceptors that sense the reduction in the blue light fluence rate and the low red:far-red ratio, respectively, and both light signals are associated with plant density and the resource reallocation when SAS responses are triggered. The B-box (BBX)-containing zinc finger transcription factor BBX24 has been implicated in the SAS as a regulator of DELLA activity, but this interaction does not explain all the observed BBX24-dependent regulation in shade light. Here, through a combination of transcriptional meta-analysis and large-scale identification of BBX24-interacting transcription factors, we found that JAZ3, a jasmonic acid signaling component, is a direct target of BBX24. Furthermore, we demonstrated that joint loss of BBX24 and JAZ3 function causes insensitivity to DELLA accumulation, and the defective shade-induced elongation in this mutant is rescued by loss of DELLA or phyB function. Therefore, we propose that JAZ3 is part of the regulatory network that controls the plant growth in response to shade, through a mechanism in which BBX24 and JAZ3 jointly regulate DELLA activity. Our results provide new insights into the participation of BBX24 and JA signaling in the hypocotyl shade avoidance response in Arabidopsis.

SlBBX20 attenuates JA signalling and regulates resistance to Botrytis cinerea by inhibiting SlMED25 in tomato

Jasmonic acid (JA) plays an important role in regulating plant growth and defence responses. Here, we show that a transcription factor that belongs to the B-box (BBX) family named SlBBX20 regulates resistance to Botrytis cinerea in tomato by modulating JA signalling. The response to JA was significantly suppressed when SlBBX20 was overexpressed in tomato. By contrast, the JA response was enhanced in SlBBX20 knockout lines. RNA sequencing analysis provided more evidence that SlBBX20 modulates the expression of genes that are involved in JA signalling. We found that SlBBX20 interacts with SlMED25, a subunit of the Mediator transcriptional co-activator complex, and prevents the accumulation of the SlMED25 protein and transcription of JA-responsive genes. JA contributes to the defence response against necrotrophic pathogens. Knocking out SlBBX20 or overexpressing SlMED25 enhanced tomato resistance to B. cinerea. The resistance was impaired when SlBBX20 was overexpressed in plants that also overexpressed SlMED25. These data show that SlBBX20 attenuates JA signalling by regulating SlMED25. Interestingly, in addition to developing enhanced resistance to B. cinerea, SlBBX20-KO plants also produced higher fruit yields. SlBBX20 is a potential target gene for efforts that aim to develop elite crop varieties using gene editing technologies.

AtBBX29 integrates photomorphogenesis and defense responses in Arabidopsis

Light is an environmental signal that modulates plant defenses against attackers. Recent research has focused on the effects of light on defense hormone signaling; however, the connections between light signaling pathways and the biosynthesis of specialized metabolites involved in plant defense have been relatively unexplored. Here, we show that Arabidopsis BBX29, a protein that belongs to the B-Box transcription factor (TF) family, integrates photomorphogenic signaling with defense responses by promoting flavonoid, sinapate and glucosinolate accumulation in Arabidopsis leaves. AtBBX29 transcript levels were up regulated by light, through photoreceptor signaling pathways. Genetic evidence indicated that AtBBX29 up-regulates MYB12 gene expression, a TF known to induce genes related to flavonoid biosynthesis in a light-dependent manner, and MYB34 and MYB51, which encode TFs involved in the regulation of glucosinolate biosynthesis. Thus, bbx29 knockout mutants displayed low expression levels of key genes of the flavonoid biosynthetic pathway, and the opposite was true in BBX29 overexpression lines. In agreement with the transcriptomic data, bbx29 mutant plants accumulated lower levels of kaempferol glucosides, sinapoyl malate, indol-3-ylmethyl glucosinolate (I3M), 4-methylsulfinylbutyl glucosinolate (4MSOB) and 3-methylthiopropyl glucosinolate (3MSP) in rosette leaves compared to the wild-type, and showed increased susceptibility to the necrotrophic fungus Botrytis cinerea and to the herbivore Spodoptera frugiperda. In contrast, BBX29 overexpressing plants displayed increased resistance to both attackers. In addition, we found that AtBBX29 plays an important role in mediating the effects of ultraviolet-B (UV-B) radiation on plant defense against B. cinerea. Taken together, these results suggest that AtBBX29 orchestrates the accumulation of specific light-induced metabolites and regulates Arabidopsis resistance against pathogens and herbivores.

Multi-layered roles of BBX proteins in plant growth and development

Light and phytohormone are external and internal cues that regulate plant growth and development throughout their life cycle. BBXs (B-box domain proteins) are a group of zinc finger proteins that not only directly govern the transcription of target genes but also associate with other factors to create a meticulous regulatory network to precisely regulate numerous aspects of growth and developmental processes in plants. Recent studies demonstrate that BBXs play pivotal roles in light-controlled plant growth and development. Besides, BBXs have been documented to regulate phytohormone-mediated physiological procedures. In this review, we summarize and highlight the multi-faced role of BBXs, with a focus in photomorphogenesis, photoperiodic flowering, shade avoidance, abiotic stress, and phytohormone-mediated growth and development in plant.

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.

BBX24 Interacts with DELLA to Regulate UV-B-Induced Photomorphogenesis in Arabidopsis thaliana

UV-B radiation, sensed by the photoreceptor UVR8, induces signal transduction for plant photomorphogenesis. UV-B radiation affects the concentration of the endogenous plant hormone gibberellin (GA), which in turn triggers DELLA protein degradation through the 26S proteasome pathway. DELLA is a negative regulator in GA signaling, partially relieving the inhibition of hypocotyl growth induced by UV-B in Arabidopsis thaliana. However, GAs do usually not work independently but integrate in complex networks linking to other plant hormones and responses to external environmental signals. Until now, our understanding of the regulatory network underlying GA-involved UV-B photomorphogenesis had remained elusive. In the present research, we investigate the crosstalk between the GA and UV-B signaling pathways in UV-B-induced photomorphogenesis of Arabidopsis thaliana. Compared with wild type Landsberg erecta (Ler), the abundance of HY5, CHS, FLS, and UF3GT were found to be down-regulated in rga-24 and gai-t6 mutants under UV-B radiation, indicating that DELLA is a positive regulator in UV-B-induced photomorphogenesis. Our results indicate that BBX24 interacts with RGA (one of the functional DELLA family members). Furthermore, we also found that RGA interacts with HY5 (the master regulator in plant photomorphogenesis). Collectively, our findings suggest that the HY5−BBX24−DELLA module serves as an important signal regulating network, in which GA is involved in UV-B signaling to regulate hypocotyl inhibition.

Arabidopsis B-box transcription factors BBX20-22 promote UVR8 photoreceptor-mediated UV-B responses

Plants undergo photomorphogenic development in the presence of light. Photomorphogenesis is repressed by the E3 ubiquitin ligase CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1), which binds to substrates through their valine-proline (VP) motifs. The UV RESISTANCE LOCUS 8 (UVR8) photoreceptor senses UV-B and inhibits COP1 through the cooperative binding of its own VP motif and photosensing core to COP1, thereby preventing COP1 binding to substrates, including the basic leucine zipper (bZIP) transcriptional regulator ELONGATED HYPOCOTYL 5 (HY5). As a key promoter of visible light and UV-B photomorphogenesis, HY5 requires coregulators for its function. The B-box family transcription factors BBX20-BBX22 were recently described as HY5 rate-limiting coactivators under red light, but their role in UVR8 signaling was unknown. Here we describe a hypermorphic bbx21-3D mutant with enhanced photomorphogenesis, carrying a proline-to-leucine mutation at position 314 in the VP motif that impairs the interaction with and regulation by COP1. We show that BBX21 and BBX22 are UVR8-dependently stabilized after UV-B exposure, which is counteracted by a repressor induced by HY5/BBX activity. bbx20 bbx21 bbx22 mutants under UV-B are impaired in hypocotyl growth inhibition, photoprotective pigment accumulation and the expression of several HY5-dependent genes under continuous UV-B, but the immediate induction of marker genes after exposure to UV-B remains surprisingly rather unaffected. We conclude that BBX20-BBX22 contribute to HY5 activity in a subset of UV-B responses, but that additional, presently unknown, coactivators for HY5 are functional in early UVR8 signaling.

The Genetic and Hormonal Inducers of Continuous Flowering in Orchids: An Emerging View

Orchids are the flowers of magnetic beauty. Vivid and attractive flowers with magnificent shapes make them the king of the floriculture industry. However, the long-awaited flowering is a drawback to their market success, and therefore, flowering time regulation is the key to studies about orchid flower development. Although there are some rare orchids with a continuous flowering pattern, the molecular regulatory mechanisms are yet to be elucidated to find applicable solutions to other orchid species. Multiple regulatory pathways, such as photoperiod, vernalization, circadian clock, temperature and hormonal pathways are thought to signalize flower timing using a group of floral integrators. This mini review, thus, organizes the current knowledge of floral time regulators to suggest future perspectives on the continuous flowering mechanism that may help to plan functional studies to induce flowering revolution in precious orchid species.

B-box containing protein 1 from Malus domestica (MdBBX1) is involved in the abiotic stress response

B-box proteins (BBXs), which act as transcription factors, mainly regulate photomorphogenesis. However, the molecular functions underlying the activity of plant BBXs in response to abiotic stress remain largely unclear. In this investigation, we found that a BBX from Malus domestica (MdBBX1) was involved in the response to various abiotic stresses. The expression of MdBBX1 was significantly upregulated in response to abiotic stresses and abscisic acid (ABA). Recombinant MdBBX1 increased stress tolerance in Escherichia coli cells. In addition, overexpression of MdBBX1 in Arabidopsis decreased sensitivity to exogenous ABA, resulting in a germination rate and root length that were greater and longer, respectively, than those of wild-type (WT) plants. Moreover, the expression of ABI5 was decreased in MdBBX1-overexpressing lines under ABA treatment. After salt and drought treatments, compared with the WT plants, the MdBBX1 transgenic plants displayed enhanced tolerance and had a higher survival rate. Furthermore, under salt stress, increased proline (PRO) contents, decreased levels of malondialdehyde (MDA), increased activity of antioxidant enzymes (superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX)) and decreased accumulation of reactive oxygen species (ROS) were observed in the MdBBX1-overexpressing plants. Overall, our results provide evidence that MdBBX1 might play a critical role in the regulation of abiotic stress tolerance by reducing the generation of ROS.

The BBX gene family in Moso bamboo (Phyllostachys edulis): identification, characterization and expression profiles

BACKGROUND

The BBX (B-box) family are zinc finger protein (ZFP) transcription factors that play an essential role in plant growth, development and response to abiotic stresses. Although BBX genes have been characterized in many model organisms, genome-wide identification of the BBX family genes have not yet been reported in Moso bamboo (Phyllostachys edulis), and the biological functions of this family remain unknown.

RESULT

In the present study, we identified 27 BBX genes in the genome of Moso bamboo, and analysis of their conserved motifs and multiple sequence alignments revealed that they all shared highly similar structures. Additionally, phylogenetic and homology analyses indicated that PeBBX genes were divided into three clusters, with whole-genome duplication (WGD) events having facilitated the expansion of this gene family. Light-responsive and stress-related cis-elements were identified by analyzing cis-elements in the promoters of all PeBBX genes. Short time-series expression miner (STEM) analysis revealed that the PeBBX genes had spatiotemporal-specific expression patterns and were likely involved in the growth and development of bamboo shoots. We further explored the downstream target genes of PeBBXs, and GO/KEGG enrichment analysis predicted multiple functions of BBX target genes, including those encoding enzymes involved in plant photosynthesis, pyruvate metabolism and glycolysis/gluconeogenesis.

CONCLUSIONS

In conclusion, we analyzed the PeBBX genes at multiple different levels, which will contribute to further studies of the BBX family and provide valuable information for the functional validation of this family.

Genome-wide identification of Brassicaceae B-BOX genes and molecular characterization of their transcriptional responses to various nutrient stresses in allotetraploid rapeseed

BACKGROUND

B-box (BBX) genes play important roles in plant growth regulation and responses to abiotic stresses. The plant growth and yield production of allotetraploid rapeseed is usually hindered by diverse nutrient stresses. However, no systematic analysis of Brassicaceae BBXs and the roles of BBXs in the regulation of nutrient stress responses have not been identified and characterized previously.

RESULTS

In this study, a total of 536 BBXs were identified from nine brassicaceae species, including 32 AtBBXs, 66 BnaBBXs, 41 BoBBXs, 43 BrBBXs, 26 CrBBXs, 81 CsBBXs, 52 BnBBXs, 93 BjBBXs, and 102 BcBBXs. Syntenic analysis showed that great differences in the gene number of Brassicaceae BBXs might be caused by genome duplication. The BBXs were respectively divided into five subclasses according to their phylogenetic relationships and conserved domains, indicating their diversified functions. Promoter cis-element analysis showed that BBXs probably participated in diverse stress responses. Protein-protein interactions between BnaBBXs indicated their functions in flower induction. The expression profiles of BnaBBXs were investigated in rapeseed plants under boron deficiency, boron toxicity, nitrate limitation, phosphate shortage, potassium starvation, ammonium excess, cadmium toxicity, and salt stress conditions using RNA-seq data. The results showed that different BnaBBXs showed differential transcriptional responses to nutrient stresses, and some of them were simultaneously responsive to diverse nutrient stresses.

CONCLUSIONS

Taken together, the findings investigated in this study provided rich resources for studying Brassicaceae BBX gene family and enriched potential clues in the genetic improvement of crop stress resistance.

Warm nights disrupt transcriptome rhythms in field-grown rice panicles

In rice, a small increase in nighttime temperature reduces grain yield and quality. How warm nighttime temperatures (WNT) produce these detrimental effects is not well understood, especially in field conditions where the typical day-to-night temperature fluctuation exceeds the mild increase in nighttime temperature. We observed genome-wide disruption of gene expression timing during the reproductive phase in field-grown rice panicles acclimated to 2 to 3 °C WNT. Transcripts previously identified as rhythmically expressed with a 24-h period and circadian-regulated transcripts were more sensitive to WNT than were nonrhythmic transcripts. The system-wide perturbations in transcript levels suggest that WNT disrupt the tight temporal coordination between internal molecular events and the environment, resulting in reduced productivity. We identified transcriptional regulators whose predicted targets are enriched for sensitivity to WNT. The affected transcripts and candidate regulators identified through our network analysis explain molecular mechanisms driving sensitivity to WNT and identify candidates that can be targeted to enhance tolerance to WNT.

Integration of light and hormone signaling pathways in the regulation of plant shade avoidance syndrome

As sessile organisms, plants are unable to move or escape from their neighboring competitors under high-density planting conditions. Instead, they have evolved the ability to sense changes in light quantity and quality (such as a reduction in photoactive radiation and drop in red/far-red light ratios) and evoke a suite of adaptative responses (such as stem elongation, reduced branching, hyponastic leaf orientation, early flowering and accelerated senescence) collectively termed shade avoidance syndrome (SAS). Over the past few decades, much progress has been made in identifying the various photoreceptor systems and light signaling components implicated in regulating SAS, and in elucidating the underlying molecular mechanisms, based on extensive molecular genetic studies with the model dicotyledonous plant Arabidopsis thaliana. Moreover, an emerging synthesis of the field is that light signaling integrates with the signaling pathways of various phytohormones to coordinately regulate different aspects of SAS. In this review, we present a brief summary of the various cross-talks between light and hormone signaling in regulating SAS. We also present a perspective of manipulating SAS to tailor crop architecture for breeding high-density tolerant crop cultivars.

Two bHLH transcription factors, bHLH48 and bHLH60, associate with phytochrome interacting factor 7 to regulate hypocotyl elongation in Arabidopsis

The basic helix-loop-helix (bHLH) transcription factor PHYTOCHROME-INTERACTING FACTOR 7 (PIF7) is a central regulator that promotes stem growth by activating growth-related gene expression during shade-avoidance responses. Studying the co-factors of PIF7 can facilitate understanding of the mechanism of PIFs and light signal transduction. Here, we describe the identification of two bHLH transcription factors, bHLH48 and bHLH60 (bHLH48/bHLH60), as essential partners for PIF7-dependent modulation of hypocotyl elongation and function downstream of phytochrome B. These two bHLH factors display DNA binding activity and interact with PIF7. Genetic analysis indicated that bHLH48/bHLH60 and PIF7 are interdependent in promoting hypocotyl elongation. Chromatin immunoprecipitation sequencing (ChIP-seq) analysis identified the substantially overlapping downstream targets of bHLH60 and PIF7. Biochemical analysis revealed that bHLH48/bHLH60 enhance the DNA binding ability of PIF7. These results provide evidence that bHLH48/bHLH60 act as positive partners of PIF7 for mutual benefit in the regulation of hypocotyl elongation.

Shade Avoidance: Expanding the Color and Hormone Palette

Major strides have been made over the past decade in elucidating the mechanisms that mediate shade-avoidance responses. The canonical PHYTOCHROME INTERACTING FACTOR (PIF)-auxin pathway that begins with inactivation of phytochrome B (phyB) by a low red:far-red (R:FR) ratio, and that leads to increased elongation, has been thoroughly characterized in arabidopsis (Arabidopsisthaliana) seedlings. Nevertheless, studies in other life stages and plant species have demonstrated the role of other wavelengths, photoreceptors, and hormones in the orchestration of shade-avoidance responses. We highlight recent developments that illustrate how canopy light cues regulate signaling through auxin, gibberellins (GAs), jasmonic acid (JA), salicylic acid (SA), abscisic acid (ABA), and strigolactones (SLs) to modulate key aspects of plant growth, metabolism, and defense.

Apple B-box protein BBX37 regulates jasmonic acid mediated cold tolerance through the JAZ-BBX37-ICE1-CBF pathway and undergoes MIEL1-mediated ubiquitination and degradation

The plant hormone jasmonic acid (JA) is involved in the cold stress response, and the inducer of CBF expression 1 (ICE1)- C-repeat binding factor (CBF) regulatory cascade plays a key role in the regulation of cold stress tolerance. In this study, we showed that a novel B-box (BBX) protein MdBBX37 positively regulates JA-mediated cold-stress resistance in apple. We found that MdBBX37 bound to the MdCBF1 and MdCBF4 promoters to activate their transcription, and also interacted with MdICE1 to enhance the transcriptional activity of MdICE1 on MdCBF1, thus promoting its cold tolerance. Two JA signaling repressors, MdJAZ1 and MdJAZ2 (JAZ, JAZMONATE ZIM-DOMAIN), interacted with MdBBX37 to repress the transcriptional activity of MdBBX37 on MdCBF1 and MdCBF4, and also interfered with the interaction between MdBBX37 and MdICE1, thus negatively regulating JA-mediated cold tolerance. E3 ligase MdMIEL1 (MIEL1, MYB30-Interacting E3 Ligase1) reduced MdBBX37-improved cold resistance by mediating ubiquitination and degradation of the MdBBX37 protein. The data reveal that MIEL1 and JAZ proteins co-regulate JA-mediated cold stress tolerance through the BBX37-ICE1-CBF module in apple. These results will aid further examination of the post-translational modification of BBX proteins and the regulatory mechanism of JA-mediated cold stress tolerance.

OsRE1 interacts with OsRIP1 to regulate rice heading date by finely modulating Ehd1 expression

Heading date is a key agronomic trait affecting crop yield. In rice, Early heading date 1 (Ehd1) is an important B-type response regulator in determination of heading date. Although many regulatory factors of Ehd1 expression have been functionally characterized, the direct regulators of Ehd1 largely remain to be identified. Here, we identified a new regulator of Ehd1, OsRE1, that directly binds to the A-box motif in the Ehd1 promoter. Osre1 confers an early heading phenotype due to elevated expression levels of Ehd1. OsRE1 is a nucleus-localized bZIP transcription factor with a diurnal rhythmic expression pattern. Furthermore, we identified an OsRE1-interacting protein, OsRIP1, and demonstrated that OsRIP1 can repress the transcript expression of Ehd1 in an OsRE1-dependent manner. Our genetic data showed that OsRE1 and OsRIP1 may function upstream of Ehd1 in regulating heading date. Together, our results suggest that OsRE1 functions cooperatively with OsRIP1 to regulate heading date through finely modulating the expression of Ehd1. In addition, OsRE1 and OsRIP1 are two minor heading date regulators, which are more desirable for fine-tuning heading date to improve rice regional adaptability.

Control of Plant Growth and Defense by Photoreceptors: From Mechanisms to Opportunities in Agriculture

Plants detect and respond to the proximity of competitors using light signals perceived by photoreceptor proteins. A low ratio of red to far-red radiation (R:FR ratio) is a key signal of competition that is sensed by the photoreceptor phytochrome B (phyB). Low R:FR ratios increase the synthesis of growth-related hormones, including auxin and gibberellins, promoting stem elongation and other shade-avoidance responses. Other photoreceptors that help plants to optimize their developmental configuration and resource allocation patterns in the canopy include blue light photoreceptors, such as cryptochromes and phototropins, and UV receptors, such as UVR8. All photoreceptors act by directly or indirectly controlling the activity of two major regulatory nodes for growth and development: the COP1/SPA ubiquitin E3 ligase complex and the PIF transcription factors. phyB is also an important modulator of hormonal pathways that regulate plant defense against herbivores and pathogens, including the jasmonic acid signaling pathway. In this Perspective, we discuss recent advances on the studies of the mechanisms that link photoreceptors with growth and defense. Understanding these mechanisms is important to provide a functional platform for breeding programs aimed at improving plant productivity, stress tolerance, and crop health in species of agronomic interest, and to manipulate the light environments in protected agriculture.

Setting Up an Arabidopsis LED Culture Module that Simulates Plant Neighbor Proximity

Competition for light between neighboring plants has important consequences for plant fitness and crop productivity. Studies on the molecular mechanisms of plant responses to neighbor proximity have been largely based on the model species Arabidopsis thaliana grown under controlled light environments. These controlled conditions commonly use fluorescent tubes for the main light source for photosynthesis and filtered light form incandescent bulbs to adjust the ratio of red (R) to far-red (FR) radiation. However, both of these types of bulbs are being discontinued and replaced by more efficient sources based on light emitting diodes (LEDs). For that reason, there is a need to evaluate alternative light sources, which can phenocopy the physiological and molecular results obtained with traditional lighting systems. Here we evaluate a custom-made LED culture module that can be used to effectively evaluate shade-avoidance responses, yielding results that, in Arabidopsis, are comparable to those obtained using traditional lighting systems.

A HY5-COL3-COL13 regulatory chain for controlling hypocotyl elongation in Arabidopsis

CONSTANS-LIKE (COL) family members are commonly implicated in light signal transduction during early photomorphogenesis. However, some of their functions remain unclear. Here, we propose a role for COL13 in hypocotyl elongation in Arabidopsis thaliana. We found that COL13 RNA accumulates at high levels in hypocotyls and that a disruption in the COL13 function via a T-DNA insertion or RNAi led to the formation of longer hypocotyls of Arabidopsis seedlings under red light. On the contrary, overexpression of COL13 resulted in the formation of shorter hypocotyls. Using various genetic, genomic, and biochemical assays, we proved that another COL protein, COL3, directly binds to the promoter of COL13, and the promoter region of COL3 was targeted by the transcription factor LONG HYPOCOTYL 5 (HY5), to form an HY5-COL3-COL13 regulatory chain for regulating hypocotyl elongation under red light. Additionally, further study demonstrated that COL13 interacts with COL3, and COL13 promotes the interaction between COL3 and CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1), suggesting a possible COP1-dependent COL3-COL13 feedback pathway. Our results provide new information regarding the gene network in mediating hypocotyl elongation.

B-box proteins: Pivotal players in light-mediated development in plants

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.

COP1 destabilizes DELLA proteins in Arabidopsis

DELLA transcriptional regulators are central components in the control of plant growth responses to the environment. This control is considered to be mediated by changes in the metabolism of the hormones gibberellins (GAs), which promote the degradation of DELLAs. However, here we show that warm temperature or shade reduced the stability of a GA-insensitive DELLA allele in Arabidopsis thaliana Furthermore, the degradation of DELLA induced by the warmth preceded changes in GA levels and depended on the E3 ubiquitin ligase CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1). COP1 enhanced the degradation of normal and GA-insensitive DELLA alleles when coexpressed in Nicotiana benthamiana. DELLA proteins physically interacted with COP1 in yeast, mammalian, and plant cells. This interaction was enhanced by the COP1 complex partner SUPRESSOR OF phyA-105 1 (SPA1). The level of ubiquitination of DELLA was enhanced by COP1 and COP1 ubiquitinated DELLA proteins in vitro. We propose that DELLAs are destabilized not only by the canonical GA-dependent pathway but also by COP1 and that this control is relevant for growth responses to shade and warm temperature.

Origin and evolution of gibberellin signaling and metabolism in plants

Gibberellins modulate multiple aspects of plant behavior. The molecular mechanism by which these hormones are perceived and how this information is translated into transcriptional changes has been elucidated in vascular plants: gibberellins are perceived by the nuclear receptor GID1, which then interacts with the DELLA nuclear proteins and promote their degradation, resulting in the modification of the activity of transcription factors with which DELLAs interact physically. However, several important questions are still pending: how does a single molecule perform such a vast array of functions along plant development? What property do gibberellins add to plant behavior? A closer look at gibberellin action from an evolutionary perspective can help answer these questions. DELLA proteins are conserved in all land plants, and predate the emergence of a full gibberellin metabolic pathway and the GID1 receptor in the ancestor of vascular plants. The origin of gibberellin signaling is linked to the exaptation by GID1 of the N-terminal domain in DELLA, which already acted as a transcriptional coactivator domain in the ancestral DELLA proteins. At least the ability to control plant growth seems to be encoded already in the ancestral DELLA protein too, suggesting that gibberellins' functional diversity is the direct consequence of DELLA protein activity. Finally, comparative network analysis suggests that gibberellin signaling increases the coordination of transcriptional responses, providing a theoretical framework for the role of gibberellins in plant adaptation at the evolutionary scale, which further needs experimental testing.

Apple B-box factors regulate light-responsive anthocyanin biosynthesis genes

Environmentally-responsive genes can affect fruit red colour via the activation of MYB transcription factors. The apple B-box (BBX) gene, BBX33/CONSTANS-like 11 (COL11) has been reported to influence apple red-skin colour in a light- and temperature-dependent manner. To further understand the role of apple BBX genes, other members of the BBX family were examined for effects on colour regulation. Expression of 23 BBX genes in apple skin was analysed during fruit development. We investigated the diurnal rhythm of expression of the BBX genes, the anthocyanin biosynthetic genes and a MYB activator, MYB10. Transactivation assays on the MYB10 promoter, showed that BBX proteins 1, 17, 15, 35, 51, and 54 were able to directly function as activators. Using truncated versions of the MYB10 promoter, a key region was identified for activation by BBX1. BBX1 enhanced the activation of MYB10 and MdbHLH3 on the promoter of the anthocyanin biosynthetic gene DFR. In transformed apple lines, over-expression of BBX1 reduced internal ethylene content and altered both cyanidin concentration and associated gene expression. We propose that, along with environmental signals, the control of MYB10 expression by BBXs in 'Royal Gala' fruit involves the integration of the expression of multiple BBXs to regulate fruit colour.

Plant Networks as Traits and Hypotheses: Moving Beyond Description

Biology relies on the central thesis that the genes in an organism encode molecular mechanisms that combine with stimuli and raw materials from the environment to create a final phenotypic expression representative of the genomic programming. While conceptually simple, the genotype-to-phenotype linkage in a eukaryotic organism relies on the interactions of thousands of genes and an environment with a potentially unknowable level of complexity. Modern biology has moved to the use of networks in systems biology to try to simplify this complexity to decode how an organism's genome works. Previously, biological networks were basic ways to organize, simplify, and analyze data. However, recent advances are allowing networks to move beyond description and become phenotypes or hypotheses in their own right. This review discusses these efforts, like mapping responses across biological scales, including relationships among cellular entities, and the direct use of networks as traits or hypotheses.

The BAP Module: A Multisignal Integrator Orchestrating Growth

Coordination of cell proliferation, cell expansion, and differentiation underpins plant growth. To maximise reproductive success, growth needs to be fine-tuned in response to endogenous and environmental cues. This developmental plasticity relies on a cellular machinery that integrates diverse signals and coordinates the downstream responses. In arabidopsis, the BAP regulatory module, which includes the BRASSINAZOLE RESISTANT 1 (BZR1), AUXIN RESPONSE FACTOR 6 (ARF6), and PHYTOCHROME INTERACTING FACTOR 4 (PIF4) transcription factors (TFs), has been shown to coordinate growth in response to multiple growth-regulating signals. In this Opinion article, we provide an integrative view on the BAP module control of cell expansion and discuss whether its function is conserved or diversified, thus providing new insights into the molecular control of growth.

Comparative analysis of B-BOX genes and their expression pattern analysis under various treatments in Dendrobium officinale

BACKGROUND

Studies have demonstrated that BBX (B-BOX) genes play crucial roles in regulatory networks controlling plant growth, developmental processes and stress response. Nevertheless, comprehensive study of BBX genes in orchids (Orchidaceae) is not well studied. The newly released genome sequences of Dendrobium officinale and Phalaenopsis equestris have allowed a systematic analysis of these important BBX genes in orchids.

RESULTS

Here we identified 19 (DoBBX01-19) and 16 (PeBBX01-16) BBX genes from D. officinale and P. equestris, respectively, and clustered into five clades (I-V) according to phylogenetic analysis. Thirteen orthologous, two DoBBXs paralogous and two PeBBXs paralogous gene pairs were validated. This gene family mainly underwent purifying selection, but five domains experienced positive selection during evolution. Noteworthy, the expression patterns of root, root_tips, stem, leaf, speal, column, lip, and flower_buds revealed that they might contribution to the formation of these tissues. According to the cis-regulatory elements analysis of BBX genes, qRT-PCR experiments were carried out using D. officinale PLBs (protocorm-like bodies) and displayed that these BBX genes were differentially regulated under AgNO₃, MeJA (Methyl Jasmonate), ABA (abscisic acid) and SA (salicylic acid) treatments.

CONCLUSIONS

Our analysis exposed that DoBBX genes play significant roles in plant growth and development, and response to different environmental stress conditions of D. officinale, which provide aid in the selection of appropriate candidate genes for further functional characterization of BBX genes in plants.

Transcriptome Profiling of the Elongating Internode of Cotton (Gossypium hirsutum L.) Seedlings in Response to Mepiquat Chloride

The plant growth retardant mepiquat chloride (MC) has been extensively used to produce compact plant canopies and increase yield in cotton (Gossypium hirsutum L.). Previous studies showed that MC reduced plant height and internode length by inhibiting GA biosynthesis and cell elongation. However, whether there are other molecular mechanisms underlying MC-induced growth retardation has remained largely unknown. In the present study, we conducted histological, transcriptomic, and phytohormone analyses of the second elongating internodes of cotton seedlings treated with MC. Histological analysis revealed that the MC shortened the internodes through suppressing both cell division and cell elongation. Consistent with the observed phenotype, many genes related to cell growth were significantly downregulated by MC. Transcriptome profiling showed that the expression of genes related not only to GA, but also to auxin, brassinosteroid (BR), and ethylene metabolism and signaling was remarkably suppressed, whereas that of genes related to cytokinin (CK) and abscisic acid (ABA) metabolism was induced by MC. Consistent with the expression pattern, significant decrease of endogenous GA, auxin, and BR content, but an increase in CK content was observed after MC treatment. Most of these hormone related genes displayed opposite regulation pattern by exogenous GA₃ treatment compared to MC; simultaneous application of MC and GA₃ could alleviate the genes expression changes induced by MC treatment, indicating MC does not directly affect other plant hormones, but through the inhibition of the GA biosynthesis. In addition, the expression of genes related to secondary metabolism and many transcription factors (TFs) were differentially regulated by MC. In summary, we confirmed the important role of GA in MC-induced growth inhibition of cotton, and further found that other hormones were also involved in this process in a GA-dependent manner. This study provides novel insights into the molecular mechanism underlying the MC-mediated inhibition of internode elongation in cotton seedlings.

The B-box bridge between light and hormones in plants

Plant development is meticulously modulated by interactions between the surrounding environment and the endogenous phytohormones. Light, as an external signal coordinates with the extensive networks of hormones inside the plant to execute its effects on growth and development. Several proteins in plants have been identified for their crucial roles in mediating light regulated development. Among these are the B-box (BBX) family of transcription factors characterized by the presence of zinc-finger B-box domain in their N-terminal region. In Arabidopsis there are 32 BBX proteins that are divided into five structural groups on the basis of the domains present. Several BBX proteins play important roles in seedling photomorphogenesis, neighbourhood detection and photoperiodic regulation of flowering. There is increasing evidence that besides light signaling BBX proteins also play integral roles in several hormone signaling pathways in plants. Here we attempt to comprehensively integrate the roles of multiple BBX proteins in various light and hormone signaling pathways. We further discuss the role of the BBX proteins in mediating crosstalk between the two signaling pathways to harmonize plant growth and development. Finally, we try to analyse the conservation of BBX genes across species and discuss the role of BBX proteins in regulating economically important traits in crop plants.

Regulation of xylem fiber differentiation by gibberellins through DELLA-KNAT1 interaction

The thickening of plant organs is supported by secondary growth, a process by which new vascular tissues (xylem and phloem) are produced. Xylem is composed of several cell types, including xylary fibers, parenchyma and vessel elements. In Arabidopsis, it has been shown that fibers are promoted by the class-I KNOX gene KNAT1 and the plant hormones gibberellins, and are repressed by a small set of receptor-like kinases; however, we lack a mechanistic framework to integrate their relative contributions. Here, we show that DELLAs, negative elements of the gibberellin signaling pathway, physically interact with KNAT1 and impair its binding to KNAT1-binding sites. Our analysis also indicates that at least 37% of the transcriptome mobilized by KNAT1 is potentially dependent on this interaction, and includes genes involved in secondary cell wall modifications and phenylpropanoid biosynthesis. Moreover, the promotion by constitutive overexpression of KNAT1 of fiber formation and the expression of genes required for fiber differentiation were still reverted by DELLA accumulation, in agreement with post-translational regulation of KNAT1 by DELLA proteins. These results suggest that gibberellins enhance fiber development by promoting KNAT1 activity.