The regulation of DWARF4 expression is likely a critical mechanism in maintaining the homeostasis of bioactive brassinosteroids in Arabidopsis

Tea green leafhopper infestations affect tea plant growth by altering the synthesis of brassinolide

Tea green leafhoppers are insects widely distributed in major tea-growing areas. At present, less attention has been paid to the study on effect of tea green leafhopper infestation on tea growth phenotype. In this study, tea green leafhoppers were used to treat tea branches in laboratory and co-treated with brassinolide (BL), the highest bioactivity of brassinosteroids (BRs), in tea garden. The results showed that the expression of genes related to BRs synthesis was inhibited and BL content was reduced in tea shoots after infestation by tea green leafhoppers. In addition, area of each leaf position, length and diameter of internodes, and the biomass of the tender shoots of tea plant were decreased after infestation by tea green leafhoppers. The number of trichomes, leaf thickness, palisade tissue thickness and cuticle thickness of tea shoots were increased after tea green leafhoppers infestation. BL spraying could partially recover the phenotypic changes of tea branches caused by tea green leafhoppers infestation. Further studies showed that tea green leafhoppers infestation may regulate the expression of CsDWF4 (a key gene for BL synthesis) through transcription factors CsFP1 and CsTCP1a, which finally affect the BL content. Moreover, BL was applied to inhibit the tea green leafhoppers infestation on tea shoots. In conclusion, our study revealed the effect of plant hormone BL-mediated tea green leafhoppers infestation on the growth phenotype of tea plants.

Brassinosteroid decreases cadmium accumulation via regulating gibberellic acid accumulation and Cd fixation capacity of root cell wall in rice (Oryza sativa)

The precise mechanism behind the association between plants' reactions to cadmium (Cd) stress and brassinosteroid (BR) remains unclear. In the current investigation, Cd stress quickly increased the endogenous BR concentration in the rice roots. Exogenous BR also increased the hemicellulose level in the root cell wall, which in turn increased its capacity to bind Cd. Simultaneously, the transcription level of genes responsible for root Cd absorption was decreased, including Natural Resistance-Associated Macrophage Protein 1/5 (OsNRAMP1/5) and a major facilitator superfamily gene called OsCd1. Ultimately, the increased expression of Heavy Metal ATPase 3 (OsHMA3) and the decreased expression of OsHMA2, which was in charge of separating Cd into vacuoles and translocating Cd to the shoots, respectively, led to a decrease in the amount of Cd that accumulated in the rice shoots. In contrast, transgenic rice lines overexpressing OsGSK2 (a negative regulator in BR signaling) accumulated more Cd, while OsGSK2 RNA interference (RNAi) rice line accumulated less Cd. Furthermore, BR increased endogenous Gibberellic acid (GA) level, and applying GA could replicate its alleviative effect. Taken together, BR decreased Cd accumulation in rice by mediating the cell wall's fixation capacity to Cd, which might relied on the buildup of the GA.

Structure and function of the Arabidopsis ABC transporter ABCB19 in brassinosteroid export

Brassinosteroids are steroidal phytohormones that regulate plant development and physiology, including adaptation to environmental stresses. Brassinosteroids are synthesized in the cell interior but bind receptors at the cell surface, necessitating a yet to be identified export mechanism. Here, we show that a member of the ATP-binding cassette (ABC) transporter superfamily, ABCB19, functions as a brassinosteroid exporter. We present its structure in both the substrate-unbound and the brassinosteroid-bound states. Bioactive brassinosteroids are potent activators of ABCB19 ATP hydrolysis activity, and transport assays showed that ABCB19 transports brassinosteroids. In Arabidopsis thaliana, ABCB19 and its close homolog, ABCB1, positively regulate brassinosteroid responses. Our results uncover an elusive export mechanism for bioactive brassinosteroids that is tightly coordinated with brassinosteroid signaling.

Wood of trees: Cellular structure, molecular formation, and genetic engineering

Wood is an invaluable asset to human society due to its renewable nature, making it suitable for both sustainable energy production and material manufacturing. Additionally, wood derived from forest trees plays a crucial role in sequestering a significant portion of the carbon dioxide fixed during photosynthesis by terrestrial plants. Nevertheless, with the expansion of the global population and ongoing industrialization, forest coverage has been substantially decreased, resulting in significant challenges for wood production and supply. Wood production practices have changed away from natural forests toward plantation forests. Thus, understanding the underlying genetic mechanisms of wood formation is the foundation for developing high-quality, fast-growing plantation trees. Breeding ideal forest trees for wood production using genetic technologies has attracted the interest of many. Tremendous studies have been carried out in recent years on the molecular, genetic, and cell-biological mechanisms of wood formation, and considerable progress and findings have been achieved. These studies and findings indicate enormous possibilities and prospects for tree improvement. This review will outline and assess the cellular and molecular mechanisms of wood formation, as well as studies on genetically improving forest trees, and address future development prospects.

Plasmodesmata mediate cell-to-cell transport of brassinosteroid hormones

Brassinosteroids (BRs) are steroidal phytohormones that are essential for plant growth, development and adaptation to environmental stresses. BRs act in a dose-dependent manner and do not travel over long distances; hence, BR homeostasis maintenance is critical for their function. Biosynthesis of bioactive BRs relies on the cell-to-cell movement of hormone precursors. However, the mechanism of the short-distance BR transport is unknown, and its contribution to the control of endogenous BR levels remains unexplored. Here we demonstrate that plasmodesmata (PD) mediate the passage of BRs between neighboring cells. Intracellular BR content, in turn, is capable of modulating PD permeability to optimize its own mobility, thereby manipulating BR biosynthesis and signaling. Our work uncovers a thus far unknown mode of steroid transport in eukaryotes and exposes an additional layer of BR homeostasis regulation in plants.

Brassinosteroid catabolic enzyme CYP734A129 regulates the morphologies of leaves and floral organs in woodland strawberry

Brassinosteroids (BRs) play critical roles in plant growth and development and regulate many important agronomic traits. However, the functions of BRs in strawberry are unclear. This study identified two mutants, named P6 and R87, in woodland strawberry (Fragaria vesca) from EMS mutagenesis populations that exhibit narrow leaves, petals and sepals. Mapping by sequencing and genetic studies revealed that the F. vesca CYP734A129, encoding a putative BR catabolic enzyme, is the causative gene for both P6 and R87. Overexpression of CYP734A129 in both F. vesca and Arabidopsis causes a severe dwarf phenotype, and the BRI1-EMS-SUPPRESSOR 1 (BES1) protein is less abundant in the CYP734A129-overexpressing Arabidopsis seedlings. This suggests that CYP734A129 is functionally conserved with CYP734A1, as a BR-inactivating enzyme. Transcriptome analysis of young leaves revealed that four BR biosynthetic genes were significantly downregulated in P6 (cyp734a129), and photosynthesis-related genes were highly enriched among the up-regulated genes in P6 compared to the wild type. This further supports that CYP734A129 inactivates BRs in F. vesca. Furthermore, we showed that mutations in CYP734A129 do not affect fruit shape and color during ripening in strawberry. Overall, our results suggest that F. vesca CYP734A129 is a BR catabolic enzyme, and provide insights into the roles of CYP734A129 in strawberry.

The brassinosteroid-responsive protein OCTOPUS is a novel regulator of Arabidopsis thaliana immune signaling

Phloem is a critical tissue for transport of photosynthates and extracellular signals in vascular plants. However, it also represents an ideal environment for pathogens seeking access to valuable host nutrients. Although many vascular pathogens induce economically relevant crop damage, there is still little known about the mechanisms by which immune signaling operates through the phloem. An existing phosphoproteomic dataset was mined to identify proteins that were both phosphorylated in response to the defense-elicitor flagellin (flg22) and expressed in vascular cells. A single candidate, OCTOPUS (OPS), is polarly associated with the plasma membrane of sieve element cells and has been characterized as an inhibitor of brassinosteroid insensitive-2 in promotion of brassinosteroid-related phytohormone signaling. The observation that OPS is differentially phosphorylated in response to flg22 led us to the examine whether OPS may also regulate flg22-induced immune signaling. Two independent alleles of ops exhibited enhanced immunity outputs across multiple signaling branches of PAMP-triggered immunity (PTI), constitutively and in response to flg22 treatment. Together with our observation that interactions between OPS and brassinosteroid insensitive-2 were disrupted by induction of salicylic acid and depletion of brassinosteriod, these data support a model whereby OPS modulates brassinolide and immune signaling to control downstream responses. We present OPS as a novel addition to the list of proteins with documented roles in PAMP-PTI signaling. These results further indicate that immune signaling in the phloem may be a significant and unique component of the host detection and response to pathogens in vascular plants.

Contents of endogenous brassinosteroids and the response to drought and/or exogenously applied 24-epibrassinolide in two different maize leaves

Exogenously applied brassinosteroids (BRs) improve plant response to drought. However, many important aspects of this process, such as the potential differences caused by different developmental stages of analyzed organs at the beginning of drought, or by BR application before or during drought, remain still unexplored. The same applies for the response of different endogenous BRs belonging to the C₂₇, C₂₈-and C₂₉- structural groups to drought and/or exogenous BRs. This study examines the physiological response of two different leaves (younger and older) of maize plants exposed to drought and treated with 24-epibrassinolide (epiBL), together with the contents of several C₂₇, C₂₈-and C₂₉-BRs. Two timepoints of epiBL application (prior to and during drought) were utilized to ascertain how this could affect plant drought response and the contents of endogenous BRs. Marked differences in the contents of individual BRs between younger and older maize leaves were found: the younger leaves diverted their BR biosynthesis from C₂₈-BRs to C₂₉-BRs, probably at the very early biosynthetic steps, as the levels of C₂₈-BR precursors were very low in these leaves, whereas C₂₉-BR levels vere extremely high. Drought also apparently negatively affected contents of C₂₈-BRs (particularly in the older leaves) and C₂₉-BRs (particularly in the younger leaves) but not C₂₇-BRs. The response of these two types of leaves to the combination of drought exposure and the application of exogenous epiBL differed in some aspects. The older leaves showed accelerated senescence under such conditions reflected in their reduced chlorophyll content and diminished efficiency of the primary photosynthetic processes. In contrast, the younger leaves of well-watered plants showed at first a reduction of proline levels in response to epiBL treatment, whereas in drought-stressed, epiBL pre-treated plants they were subsequently characterized by elevated amounts of proline. The contents of C₂₉- and C₂₇-BRs in plants treated with exogenous epiBL depended on the length of time between this treatment and the BR analysis regardless of plant water supply; they were more pronounced in plants subjected to the later epiBL treatment. The application of epiBL before or during drought did not result in any differences of plant response to this stressor.

Transcriptomic analysis of Chinese yam (Dioscorea polystachya Turcz.) variants indicates brassinosteroid involvement in tuber development

Dioscorea is an important but underutilized genus of flowering plants that grows predominantly in tropical and subtropical regions. Several species, known as yam, develop large underground tubers and aerial bulbils that are used as food. The Chinese yam (D. polystachya Turcz.) is one of the few Dioscorea species that grows well in temperate regions and has been proposed as a climate-resilient crop to enhance food security in Europe. However, the fragile, club-like tubers are unsuitable for mechanical harvesting, which is facilitated by shorter and thicker storage organs. Brassinosteroids (BRs) play a key role in plant cell division, cell elongation and proliferation, as well as in the gravitropic response. We collected RNA-Seq data from the head, middle and tip of two tuber shape variants: F60 (long, thin) and F2000 (short, thick). Comparative transcriptome analysis of F60 vs. F2000 revealed 30,229 differentially expressed genes (DEGs), 1,393 of which were differentially expressed in the growing tip. Several DEGs are involved in steroid/BR biosynthesis or signaling, or may be regulated by BRs. The quantification of endogenous BRs revealed higher levels of castasterone (CS), 28-norCS, 28-homoCS and brassinolide in F2000 compared to F60 tubers. The highest BR levels were detected in the growing tip, and CS was the most abundant (439.6 ± 196.41 pmol/g in F2000 and 365.6 ± 112.78 pmol/g in F60). Exogenous 24-epi-brassinolide (epi-BL) treatment (20 nM) in an aeroponic system significantly increased the width-to-length ratio (0.045 ± 0.002) compared to the mock-treated plants (0.03 ± 0.002) after 7 weeks, indicating that exogenous epi-BL produces shorter and thicker tubers. In this study we demonstrate the role of BRs in D. polystachya tuber shape, providing insight into the role of plant hormones in yam storage organ development. We found that BRs can influence tuber shape in Chinese yam by regulating the expression of genes involved cell expansion. Our data can help to improve the efficiency of Chinese yam cultivation, which could provide an alternative food source and thus contribute to future food security in Europe.

Advances in the Biosynthesis and Molecular Evolution of Steroidal Saponins in Plants

Steroidal saponins are an important type of plant-specific metabolite that are essential for plants' responses to biotic and abiotic stresses. Because of their extensive pharmacological activities, steroidal saponins are also important industrial raw materials for the production of steroidal drugs. In recent years, more and more studies have explored the biosynthesis of steroidal saponins in plants, but most of them only focused on the biosynthesis of their molecular skeleton, diosgenin, and their subsequent glycosylation modification mechanism needs to be further studied. In addition, the biosynthetic regulation mechanism of steroidal saponins, their distribution pattern, and their molecular evolution in plants remain unclear. In this review, we summarized and discussed recent studies on the biosynthesis, molecular regulation, and function of steroidal saponins. Finally, we also reviewed the distribution and molecular evolution of steroidal saponins in plants. The elucidation of the biosynthesis, regulation, and molecular evolutionary mechanisms of steroidal saponins is crucial to provide new insights and references for studying their distribution, diversity, and evolutionary history in plants. Furthermore, a deeper understanding of steroidal saponin biosynthesis will contribute to their industrial production and pharmacological applications.

OsCPD1 and OsCPD2 are functional brassinosteroid biosynthesis genes in rice

CONSTITUTIVE PHOTOMORPHOGENIC DWARF (CPD), member of the CYP90A family of cytochrome P450 (CYP450) monooxygenase, is an essential component of brassinosteroids (BRs) biosynthesis pathway. Compared with a single CPD/CYP90A1 in Arabidopsis thaliana, two highly homologous CPD genes, OsCPD1/CYP90A3 and OsCPD2/CYP90A4, are present in rice genome. There is still no genetic evidence so far about the requirement of OsCPD1 and OsCPD2 in rice BR biosynthesis. In this study, we reported the functional characterization of OsCPD genes using CRISPR/Cas9 gene editing technology. The overall growth and development of oscpd1 and oscpd2 single knock-out mutants was indistinguishable from the wild-type, whereas, the oscpd1 oscpd2 double mutant displayed multiple and obvious BR-related defects. Cytological analyses further indicated the defective cell elongation in oscpd1 oscpd2 double mutant. The oscpd double mutants had a lower endogenous BR level and could be restored by the application of the brassinolide (BL). Moreover, overexpression of OsCPD1 and OsCPD2 led to a typical BR enhanced phenotype, with enlarged leaf angle and increased grain size. Taken together, our results provide direct genetic evidence that OsCPD1 and OsCPD2 play essential and redundant roles in maintenance of plant architecture by modulating BR biosynthesis in rice.

Rapid and Efficient Regeneration of Populus ussuriensis Kom. from Root Explants through Direct De Novo Shoot Organogenesis

Populus ussuriensis is an important tree species with high economic and ecologic values. However, traditional sexual propagation is time-consuming and inefficient, challenging afforestation and wood production using P. ussuriensis, and requires a rapid and efficient regeneration system. The present study established a rapid, efficient, and stable shoot regeneration method from root explants in P. ussuriensis using several plant growth regulators. Most shoot buds (15.2 per explant) were induced at high efficiency under WPM medium supplemented with 221.98 μM 6-BA, 147.61 μM IBA, and 4.54 μM TDZ within two weeks. The shoot buds were further multiplicated and elongated under WPM medium supplemented with 221.98 μM 6-BA, 147.61 μM IBA, and 57.74 μM GA3 for four weeks. The average number and efficiency of elongation of multiplication and elongation for induced shoot buds were 75.2 and 78%, respectively. All the shoots were rooted within a week and none of them showed abnormality in rooting. The time spent for the entire regeneration of this direct shoot organogenesis was seven weeks, much shorter than conventional indirect organogenesis with the callus induction phase, and no abnormal growth was observed. This novel regeneration system will not only promote the massive propagation, but also accelerate the genetic engineering studies for trait improvement of P. ussuriensis species.

Phytohormones: Key players in the modulation of heavy metal stress tolerance in plants

Heavy metal (HM) stress in plants has received considerable global attention as it threatens sustainable growth in agriculture worldwide. Hence, desperate efforts have been undertaken for combating the effects of this stress in plants. Interestingly, the use of phytohormones in reducing the impact of HM toxicity has gained much momentum in the recent past. Phytohormones act as chemical messengers that improve the HM stress resistance in plants, thus allowing them to retain their growth and developmental plasticity. Their exogenous application as well as manipulation of endogenous levels through precise targeting of their biosynthesis/signaling components is a promising approach for providing a protective shield against HM stress in plants. However, for the successful use of phytohormones for field plants exposed to HM toxicity, in-depth knowledge of the key pathways regulated by them is of prime importance. Hence, the present review mainly summarizes the key conceptual developments on the involvement of phytohormones in the mitigation of HM stress in plants. The role of various genes, proteins, and signaling components involved in phytohormones associated HM stress tolerance and their modulation has also been discussed. Thus, this update will pave the way for improving HM stress tolerance in plants with the advent of phytohormones for sustainable agriculture growth in the future.

The transcriptional dynamics during de novo shoot organogenesis of Ma bamboo (Dendrocalamus latiflorus Munro): implication of the contributions of the abiotic stress response in this process

De novo shoot organogenesis is an important biotechnological tool for fundamental studies in plant. However, it is difficult in most bamboo species, and the genetic control of this highly dynamic and complicated regeneration process remains unclear. In this study, based on an in-depth analysis at the cellular level, the shoot organogenesis from calli of Ma bamboo (Dendrocalamus latiflorus Munro) was divided into five stages. Subsequently, single-molecule long-read isoform sequencing of tissue samples pooled from all five stages was performed to generate a full-length transcript landscape. A total of 83 971 transcripts, including 73 209 high-quality full-length transcripts, were captured, which served as an annotation reference for the subsequent RNA sequencing analysis. Time-course transcriptome analysis of samples at the abovementioned five stages was conducted to investigate the global gene expression atlas showing genome-wide expression of transcripts during the course of bamboo shoot organogenesis. K-means clustering analysis and stage-specific transcript identification revealed important dynamically expressed transcription regulators that function in bamboo shoot organogenesis. The majority of abiotic stress-responsive genes altered their expression levels during this process, and further experiments demonstrated that exogenous application of moderate but not severe abiotic stress increased the shoot regeneration efficiency. In summary, our study provides an overview of the genetic flow dynamics during bamboo shoot organogenesis. Full-length cDNA sequences generated in this study can serve as a valuable resource for fundamental and applied research in bamboo in the future.

Brassinosteroids inhibit miRNA-mediated translational repression by decreasing AGO1 on the endoplasmic reticulum

Translational repression is a conserved mechanism in microRNA (miRNA)-guided gene silencing. In Arabidopsis, ARGONAUTE1 (AGO1), the major miRNA effector, localizes in the cytoplasm for mRNA cleavage and at the endoplasmic reticulum (ER) for translational repression of target genes. However, the mechanism underlying miRNA-mediated translational repression is poorly understood. In particular, how the subcellular partitioning of AGO1 is regulated is largely unexplored. Here, we show that the plant hormone brassinosteroids (BRs) inhibit miRNA-mediated translational repression by negatively regulating the distribution of AGO1 at the ER in Arabidopsis thaliana. We show that the protein levels rather than the transcript levels of miRNA target genes were reduced in BR-deficient mutants but increased under BR treatments. The localization of AGO1 at the ER was significantly decreased under BR treatments while it was increased in the BR-deficient mutants. Moreover, ROTUNDIFOLIA3 (ROT3), an enzyme involved in BR biosynthesis, co-localizes with AGO1 at the ER and interacts with AGO1 in a GW motif-dependent manner. Complementation analysis showed that the AGO1-ROT3 interaction is necessary for the function of ROT3. Our findings provide new clues to understand how miRNA-mediated gene silencing is regulated by plant endogenous hormones.

The PEAPOD Pathway and Its Potential To Improve Crop Yield

A key strategy to increase plant productivity is to improve intrinsic organ growth. Some of the regulatory networks underlying organ growth and development, as well as the interconnections between these networks, are highly conserved. An example of such a growth-regulatory module with a highly conserved role in final organ size and shape determination in eudicot species is the PEAPOD (PPD)/KINASE-INDUCIBLE DOMAIN INTERACTING (KIX)/STERILE APETALA (SAP) module. We review the proteins constituting the PPD pathway and their roles in different plant developmental processes, and explore options for future research. We also speculate on strategies to exploit knowledge about the PPD pathway for targeted yield improvement to engineer crop traits of agronomic interest, such as leaf, fruit, and seed size.

Transcriptomic analysis of cytochrome P450 genes and pathways involved in chromium toxicity in Oryza sativa

In plants, cytochrome P450 monooxygenase (CYP) plays an important role in detoxifying xenobiotic chemicals and coordinating abiotic stresses. Agilent 44 K rice microarray has been used to focus on the transcriptional profile of osCYP genes in rice seedling exposed to Cr solution containing K₂CrO₄ or Cr(NO₃)₃. Our study showed that expression profiles of 264 osCYP genes identified were tissue, dose and stimulus specific in rice seedlings. Comparative genomics analysis revealed that more differentially expressed osCYP genes were discovered in roots than in shoots under both Cr exposures. Results from Venn diagram analysis of differentially expressed osCYP genes demonstrated that there were common osCYP genes and unique osCYP genes present in different rice tissue as well as in different Cr treatments, which may control and/or regulate involvement of different CYP isoenzymes under Cr exposure individually or combinedly. KEGG analysis indicated that significant up- and down-regulated osCYP genes in rice tissues were chiefly related to "biosynthesis of secondary metabolites". However, involvements of osCYP genes mapped in the "biosynthesis of secondary metabolites" were tissue and dose specific, implying their distinctly responsive and adaptive mechanisms during Cr exposure. Overall, our findings are evident to describe and clarify their individual roles of specific osCYP genes in regulating involvement of CYP isoforms in Cr detoxification by rice seedlings.

Putrescine metabolism modulates the biphasic effects of brassinosteroids on canola and Arabidopsis salt tolerance

Brassinosteroids (BRs) are known to improve salt tolerance of plants, but not in all situations. Here, we show that a certain concentration of 24-epibrassinolide (EBL), an active BR, can promote the tolerance of canola under high-salt stress, but the same concentration is disadvantageous under low-salt stress. We define this phenomenon as hormonal stress-level-dependent biphasic (SLDB) effects. The SLDB effects of EBL on salt tolerance in canola are closely related to H₂ O₂ accumulation, which is regulated by polyamine metabolism, especially putrescine (Put) oxidation. The inhibition of EBL on canola under low-salt stress can be ameliorated by repressing Put biosynthesis or diamine oxidase activity to reduce H₂ O₂ production. Genetic and phenotypic results of bri1-9, bak1, bes1-D, and bzr1-1D mutants and overexpression lines of BRI1 and BAK1 in Arabidopsis indicate that a proper enhancement of BR signaling benefits plants in countering salt stress, whereas excessive enhancement is just as harmful as a deficiency. These results highlight the involvement of crosstalk between BR signaling and Put metabolism in H₂ O₂ accumulation, which underlies the dual role of BR in plant salt tolerance.

Strigolactones and Brassinosteroids Antagonistically Regulate the Stability of the D53-OsBZR1 Complex to Determine FC1 Expression in Rice Tillering

Rice tillering, a key architecture trait determining grain yield, is highly regulated by a class of newly identified phytohormones, strigolactones (SLs). However, the whole SL signaling pathway from the receptor to downstream transcription factors to finally inhibit tillering remains unrevealed. In this study, we first found that brassinosteroids (BRs) strongly enhance tillering by promoting bud outgrowth in rice, which is largely different from the function of BRs in Arabidopsis. Genetic and biochemical analyses indicated that both the SL and BR signaling pathways control rice tillering by regulating the stability of D53 and/or the OsBZR1-RLA1-DLT module, a transcriptional complex in the rice BR signaling pathway. We further found that D53 interacts with OsBZR1 to inhibit the expression of FC1, a local inhibitor of tillering, and that this inhibition depends on direct DNA binding by OsBZR1, which recruits D53 to the FC1 promoter in rice buds. Taken together, these findings uncover a mechanism illustrating how SLs and BRs coordinately regulate rice tillering via the early responsive gene FC1.

Local and Systemic Effects of Brassinosteroid Perception in Developing Phloem

The plant vasculature is an essential adaptation to terrestrial growth. Its phloem component permits efficient transfer of photosynthates between source and sink organs but also transports signals that systemically coordinate physiology and development. Here, we provide evidence that developing phloem orchestrates cellular behavior of adjacent tissues in the growth apices of plants, the meristems. Arabidopsis thaliana plants that lack the three receptor kinases BRASSINOSTEROID INSENSITIVE 1 (BRI1), BRI1-LIKE 1 (BRL1), and BRL3 ("bri³" mutants) can no longer sense brassinosteroid phytohormones and display severe dwarfism as well as patterning and differentiation defects, including disturbed phloem development. We found that, despite the ubiquitous expression of brassinosteroid receptors in growing plant tissues, exclusive expression of the BRI1 receptor in developing phloem is sufficient to systemically correct cellular growth and patterning defects that underlie the bri³ phenotype. Although this effect is brassinosteroid-dependent, it cannot be reproduced with dominant versions of known downstream effectors of BRI1 signaling and therefore possibly involves a non-canonical signaling output. Interestingly, the rescue of bri³ by phloem-specific BRI1 expression is associated with antagonism toward phloem-specific CLAVATA3/EMBRYO SURROUNDING REGION-RELATED 45 (CLE45) peptide signaling in roots. Hyperactive CLE45 signaling causes phloem sieve element differentiation defects, and consistently, knockout of CLE45 perception in bri³ background restores proper phloem development. However, bri³ dwarfism is retained in such lines. Our results thus reveal local and systemic effects of brassinosteroid perception in the phloem: whereas it locally antagonizes CLE45 signaling to permit phloem differentiation, it systemically instructs plant organ formation via a phloem-derived, non-cell-autonomous signal.

A basic helix-loop-helix protein (GhFP1) promotes fibre elongation of cotton (Gossypium hirsutum) by modulating brassinosteroid biosynthesis and signalling

Basic helix-loop-helix (bHLH) proteins are involved in transcriptional networks controlling a number of biological processes in plants. However, little information is known on the roles of bHLH proteins in cotton fibre development so far. Here, we show that a cotton bHLH protein (GhFP1) positively regulates fibre elongation. GhFP1 transgenic cotton and Arabidopsis plants were generated to study how GhFP1 regulates fibre cell elongation. Fibre length of the transgenic cotton overexpressing GhFP1 was significantly longer than that of wild-type, whereas suppression of GhFP1 expression hindered fibre elongation. Furthermore, overexpression of GhFP1 in Arabidopsis promoted trichome development. Expression of the brassinosteroid (BR)-related genes was markedly upregulated in fibres of GhFP1 overexpression cotton, but downregulated in GhFP1-silenced fibres. BR content in the transgenic fibres was significantly altered, relative to that in wild-type. Moreover, GhFP1 protein could directly bind to the promoters of GhDWF4 and GhCPD to activate expression of these BR-related genes. Therefore, our data suggest that GhFP1 as a positive regulator participates in controlling fibre elongation by activating BR biosynthesis and signalling. Additionally, homodimerisation of GhFP1 may be essential for its function, and interaction between GhFP1 and other cotton bHLH proteins may interfere with its DNA-binding activity.

Brassinosteroids: Multidimensional Regulators of Plant Growth, Development, and Stress Responses

Brassinosteroids (BRs) are a group of polyhydroxylated plant steroid hormones that are crucial for many aspects of a plant's life. BRs were originally characterized for their function in cell elongation, but it is becoming clear that they play major roles in plant growth, development, and responses to several stresses such as extreme temperatures and drought. A BR signaling pathway from cell surface receptors to central transcription factors has been well characterized. Here, we summarize recent progress toward understanding the BR pathway, including BR perception and the molecular mechanisms of BR signaling. Next, we discuss the roles of BRs in development and stress responses. Finally, we show how knowledge of the BR pathway is being applied to manipulate the growth and stress responses of crops. These studies highlight the complex regulation of BR signaling, multiple points of crosstalk between BRs and other hormones or stress responses, and the finely tuned spatiotemporal regulation of BR signaling.

CIRCADIAN CLOCK ASSOCIATED 1 and ATAF2 differentially suppress cytochrome P450-mediated brassinosteroid inactivation

Brassinosteroids (BRs) are a group of steroid hormones regulating plant growth and development. Since BRs do not undergo transport among plant tissues, their metabolism is tightly regulated by transcription factors (TFs) and feedback loops. BAS1 (CYP734A1, formerly CYP72B1) and SOB7 (CYP72C1) are two BR-inactivating cytochrome P450s identified in Arabidopsis thaliana. We previously found that a TF ATAF2 (ANAC081) suppresses BAS1 and SOB7 expression by binding to the Evening Element (EE) and CIRCADIAN CLOCK ASSOCIATED 1 (CCA1)-binding site (CBS) on their promoters. Both the EE and CBS are known binding targets of the circadian regulatory protein CCA1. Here, we confirm that CCA1 binds the EE and CBS motifs on BAS1 and SOB7 promoters, respectively. Elevated accumulations of BAS1 and SOB7 transcripts in the CCA1 null mutant cca1-1 indicate that CCA1 is a repressor of their expression. When compared with either cca1-1 or the ATAF2 null mutant ataf2-2, the cca1-1 ataf2-2 double mutant shows higher SOB7 transcript accumulations and a stronger BR-insensitive phenotype of hypocotyl elongation in white light. CCA1 interacts with ATAF2 at both DNA-protein and protein-protein levels. ATAF2, BAS1, and SOB7 are all circadian regulated with distinct expression patterns. These results demonstrate that CCA1 and ATAF2 differentially suppress BAS1- and SOB7-mediated BR inactivation.

Integrating the dynamics of yield traits in rice in response to environmental changes

Reductions in crop yields as a consequence of global climate change threaten worldwide food security. It is therefore imperative to develop high-yielding crop plants that show sustainable production under stress conditions. In order to achieve this aim through breeding or genetic engineering, it is crucial to have a complete and comprehensive understanding of the molecular basis of plant architecture and the regulation of its sub-components that contribute to yield under stress. Rice is one of the most widely consumed crops and is adversely affected by abiotic stresses such as drought and salinity. Using it as a model system, in this review we present a summary of our current knowledge of the physiological and molecular mechanisms that determine yield traits in rice under optimal growth conditions and under conditions of environmental stress. Based on physiological functioning, we also consider the best possible combination of genes that may improve grain yield under optimal as well as environmentally stressed conditions. The principles that we present here for rice will also be useful for similar studies in other grain crops.

Designed Manipulation of the Brassinosteroid Signal to Enhance Crop Yield

Brassinosteroid (BR), a plant steroid hormone, plays crucial role in modulating plant growth and development, which affect crop architecture and yield. However, BR application cannot highly benefit to agricultural production as expectation, because it regulates multiple processes in different tissues and leads to side effect. In addition, accurately modifying BR signal at transcriptional level is difficult. Effective manipulation of the BR signal and avoidance of side effects are required to enhance yield in different crops. Application of BR by spraying at specific developmental stages can enhance crop yield, but this method is impractical for use on a large scale. The accurate molecular design of crops would be much more helpful to manipulate the BR signal in specific organs and/or at particular developmental stages to enhance crop yield. This minireview summarizes the BR regulation of yield in different crops, especially horticultural crops, and the strategies used to regulate the BR signal to enhance crop yield. One popular strategy is to directly modulate the BR signal through modifying the functions of important components in the BR signal transduction pathway. Another strategy is to identify and modulate regulators downstream of, or in crosstalk with, the BR signal to manipulate its role in specific processes and increase crop yield. Efforts to accurately design a BR manipulation strategy will ultimately lead to effective control of the BR signal to avoid side effects and enhance crop yield.

De novo shoot organogenesis during plant regeneration

Plants exhibit remarkable regeneration capacity, ensuring developmental plasticity. In vitro tissue culture techniques are based on plant regeneration ability and facilitate production of new organs and even the whole plant from explants. Plant somatic cells can be reprogrammed to form a pluripotent cell mass called the callus. A portion of pluripotent callus cells gives rise to a fertile shoot via de novo shoot organogenesis (DNSO). Here, we reconstitute the shoot regeneration process with four phases, namely pluripotency acquisition, shoot promeristem formation, establishment of the confined shoot progenitor, and shoot outgrowth. Additionally, other biological processes, including cell cycle progression and reactive oxygen species metabolism, which further contribute to successful completion of DNSO, are also summarized. Overall, this study highlights recent advances in the molecular and cellular events involved in DNSO, as well as the regulatory mechanisms behind key steps of DNSO.

Regulation of Brassinosteroid Homeostasis in Higher Plants

Brassinosteroids (BRs) are known as one of the major classes of phytohormones essential for various processes during normal plant growth, development, and adaptations to biotic and abiotic stresses. Significant progress has been achieved on revealing mechanisms regulating BR biosynthesis, catabolism, and signaling in many crops and in model plant Arabidopsis. It is known that BRs control plant growth and development in a dosage-dependent manner. Maintenance of BR homeostasis is therefore critical for optimal functions of BRs. In this review, updated discoveries on mechanisms controlling BR homeostasis in higher plants in response to internal and external cues are discussed.

Multiple Roles of Jasmonates in Shaping Rhizotaxis: Emerging Integrators

The root system and its architecture enormously contribute to plant survival and adaptation to the environment. Depending on the intrinsic genetic information and the surrounding rhizosphere, plants develop a highly plastic root system, which is a critical determinant for survival. Plant root system, which includes primary root (PR), lateral roots (LR) and adventitious roots (AR), is shaped by tightly controlled developmental programs. Phytohormones are the main signaling components that orchestrate and coordinate the genetic information and the external stimuli to shape the root system patterning or rhizotaxis. Besides their role in plant stress responses and defense against herbivory and pathogen attacks, jasmonic acid and its derivatives, including the receptor-active conjugate jasmonoyl-L-isoleucine (JA-Ile), emerge as potential regulators of rhizotaxis. In this chapter, we summarize and discuss the recent progress achieved during the recent years to understand the JA-mediated genetic and molecular networks guiding PR, LR, and AR initiation. We highlight the role of JAs as critical integrators in shaping rhizotaxis.

DWARF4 accumulation in root tips is enhanced via blue light perception by cryptochromes

Brassinosteroid (BR) signalling is known to be coordinated with light signalling in above ground tissue. Many studies focusing on the shade avoidance response in above ground tissue or hypocotyl elongation in darkness have revealed the contribution of the BR signalling pathway to these processes. We previously analysed the expression of DWARF 4 (DWF4), a key BR biosynthesis enzyme, and revealed that light perception in above ground tissues triggered DWF4 accumulation in root tips. To determine the required wavelength of light and photoreceptors responsible for this regulation, we studied DWF4-GUS marker plants grown in several monochromatic light conditions. We revealed that monochromatic blue LED light could induce DWF4 accumulation in primary root tips and root growth as much as white light, whereas monochromatic red LED could not. Consistent with this, a cryptochrome1/2 double mutant showed retarded root growth under white light whereas a phytochromeA/B double mutant did not. Taken together, our data strongly indicated that blue light signalling was important for DWF4 accumulation in root tips and root growth. Furthermore, DWF4 accumulation patterns in primary root tips were not altered by auxin or sugar treatment. Therefore, we hypothesize that blue light signalling from the shoot tissue is different from auxin and sugar signalling.

Genome-wide identification and expression analysis of brassinosteroid action-related genes during the shoot growth of moso bamboo

Brassinosteroids (BRs) are a group of plant steroid hormones that play crucial roles in a range of plant growth and development processes. BR action includes active BR formation by a complex biosynthesis process and driving BR biological function through signal transduction. Although the characterization of several BR action-related genes has been conducted in a few model plants, systematic information about these genes in bamboo is still lacking. We identified 64 genes related to BR action from the genome of moso bamboo (Phyllostachys edulis), including twenty that participated in BR biosynthesis and forty-four involved in BR signal transduction. The characteristics of all these candidate genes were identified by bioinformatics methods, including the gene structures, basic physical and chemical properties of proteins, conserved domains and evolutionary relationships. Based on the transcriptome data, the candidate genes demonstrated different expression patterns, which were further validated by qRT-PCR using templates from bamboo shoots with different heights. Thirty-four positive and three negative co-expression modules were identified by 44 candidate genes in the newly emerging bamboo shoot. The gene expression patterns and co-expression modules of BR action-related genes in bamboo shoots indicated that they might function to promote bamboo growth through BR biosynthesis and signal transduction processes. This study provides the first step towards the cloning and functional dissection of the role of BR action-related genes in moso bamboo, which also presents an excellent opportunity for genetic engineering using the candidate genes to improve bamboo quantity and quality.

Identification, Characterization, and Expression Patterns of TCP Genes and microRNA319 in Cotton

The TEOSINTE BRANCHED 1, CYCLOIDEA, and PROLIFERATING CELL FACTORS (TCP) gene family is a group of plant-specific transcription factors that have versatile functions in developmental processes and stress responses. In this study, a total of 73 TCP genes in upland cotton were identified and characterizated. Phylogenetic analysis classified them into three subgroups: 50 belonged to PCF, 16 to CIN, and 7 to CYC/TB1. GhTCP genes are randomly distributed in 22 of the 26 chromosomes in cotton. Expression patterns of GhTCPs were analyzed in 10 tissues, including different developmental stages of ovule and fiber, as well as under heat, salt, and drought stresses. Transcriptome analysis showed that 44 GhTCP genes exhibited varied transcript accumulation patterns in the tested tissues and 41 GhTCP genes were differentially expressed in response to heat, salt, and drought stresses. Furthermore, three GhTCP genes of the CIN clade were found to contain miR319-binding sites. An anti-correlation expression of GhTCP21 and GhTCP54 was analyzed with miR319 under salt and drought stress. Our results lay the foundation for understanding the complex mechanisms of GhTCP-mediated developmental processes and abiotic stress-signaling transduction pathways in cotton.

PIF4-induced BR synthesis is critical to diurnal and thermomorphogenic growth

The Arabidopsis PIF4 and BES1/BZR1 transcription factors antagonize light signaling by facilitating co-activated expression of a large number of cell wall-loosening and auxin-related genes. While PIF4 directly activates expression of these targets, BES1 and BZR1 activity switch from a repressive to an activator function, depending on interaction with TOPLESS and other families of regulators including PIFs. However, the complexity of this regulation and its role in diurnal control of plant growth and brassinosteroid (BR) levels is little understood. We show by using a protein array that BES1, PIF4, and the BES1-PIF4 complex recognize different DNA elements, thus revealing a distinctive cis-regulatory code beneath BES1-repressive and PIF4 co-activation function. BES1 homodimers bind to conserved BRRE- and G-box elements in the BR biosynthetic promoters and inhibit their expression during the day, while elevated PIF4 competes for BES1 homodimer formation, resulting in de-repressed BR biosynthesis at dawn and in response to warmth. Our findings demonstrate a central role of PIF4 in BR synthesis activation, increased BR levels being essential to thermomorphogenic hypocotyl growth.

Arabidopsis BRASSINOSTEROID INACTIVATOR2 is a typical BAHD acyltransferase involved in brassinosteroid homeostasis

Brassinosteroids (BRs) are plant-specific steroidal hormones; BR homeostasis is crucial for various aspects of plant growth and development. However, to date, the BR inactivation process has not been thoroughly elucidated. In this study, we identified and characterized a novel BAHD family acyltransferase gene, BRASSINOSTEROID INACTIVATOR2 (BIA2), involved in BR inactivation. BIA2-overexpressing (OE-BIA2) plants displayed typical BR-deficient phenotypes, which were rescued by exogenous BR treatment. Real-time qRT-PCR and transcriptome analyses showed that expression levels of virtually all of the BR biosynthetic genes were increased, whereas the expression of many BR inactivation genes was reduced in OE-BIA2 plants. Root inhibition assays showed that the root growth of OE-BIA2 plants was inhibited. We obtained plants with an intermediate phenotype by crossing the OE-BIA2 plants with BRASSINOSTEROID-INSENSITIVE1 (BRI1)-overexpressing plants. The null BIA2 mutants had longer hypocotyls in the dark. BIA2 was predominantly expressed in roots, and its expression was induced by 24-epibrassinolide or dark treatment, but it exhibited a differential expression pattern compared with its homologue, BIA1. Furthermore, genetic transformation with point-mutant and deleted-BIA2 constructs confirmed that the HXXXD motif is essential for the function of BIA2. Taken together, these findings indicate that BIA2 is a typical BAHD acyltransferase that is involved in BR homeostasis and may inactivate bioactive BRs by esterification, particularly in roots and hypocotyls under dark conditions.

Overexpression of Populus trichocarpa CYP85A3 promotes growth and biomass production in transgenic trees

Brassinosteroids (BRs) are essential hormones that play crucial roles in plant growth, reproduction and response to abiotic and biotic stress. In Arabidopsis, AtCYP85A2 works as a bifunctional cytochrome P450 monooxygenase to catalyse the conversion of castasterone to brassinolide, a final rate-limiting step in the BR-biosynthetic pathway. Here, we report the functional characterizations of PtCYP85A3, one of the three AtCYP85A2 homologous genes from Populus trichocarpa. PtCYP85A3 shares the highest similarity with AtCYP85A2 and can rescue the retarded-growth phenotype of the Arabidopsis cyp85a2-2 and tomato d^x mutants. Constitutive expression of PtCYP85A3, driven by the cauliflower mosaic virus 35S promoter, increased the endogenous BR levels and significantly promoted the growth and biomass production in both transgenic tomato and poplar. Compared to the wild type, plant height, shoot fresh weight and fruit yield increased 50%, 56% and 43%, respectively, in transgenic tomato plants. Similarly, plant height and stem diameter increased 15% and 25%, respectively, in transgenic poplar plants. Further study revealed that overexpression of PtCYP85A3 enhanced xylem formation without affecting the composition of cellulose and lignin, as well as the cell wall thickness in transgenic poplar. Our finding suggests that PtCYP85A3 could be used as a potential candidate gene for engineering fast-growing trees with improved wood production.

Genotype-specific physiological and transcriptomic responses to drought stress in Setaria italica (an emerging model for Panicoideae grasses)

Understanding drought-tolerance mechanisms and identifying genetic dominance are important for crop improvement. Setaria italica, which is extremely drought-tolerant, has been regarded as a model plant for studying stress biology. Moreover, different genotypes of S. italica have evolved various drought-tolerance/avoidance mechanisms that should be elucidated. Physiological and transcriptomic comparisons between drought-tolerant S. italica cultivar 'Yugu1' and drought-sensitive 'An04' were conducted. 'An04' had higher yields and more efficient photosystem activities than 'Yugu1' under well-watered conditions, and this was accompanied by positive brassinosteroid regulatory actions. However, 'An04's growth advantage was severely repressed by drought, while 'Yugu1' maintained normal growth under a water deficiency. High-throughput sequencing suggested that the S. italica transcriptome was severely remodelled by genotype × environment interactions. Expression profiles of genes related to phytohormone metabolism and signalling, transcription factors, detoxification, and other stress-related proteins were characterised, revealing genotype-dependent and -independent drought responses in different S. italica genotypes. Combining our data with drought-tolerance-related QTLs, we identified 20 candidate genes that contributed to germination and early seedling' drought tolerance in S. italica. Our analysis provides a comprehensive picture of how different S. italica genotypes respond to drought, and may be used for the genetic improvement of drought tolerance in Poaceae crops.

Light perception in aerial tissues enhances DWF4 accumulation in root tips and induces root growth

Many attempts have been made to characterize the activities of brassinosteroids (BRs), which are important plant hormones. The crosstalk between light perception and the BR signalling pathway has been extensively studied regarding its effects on photomorphogenesis, especially in elongating etiolated hypocotyls. In contrast, how and where the light induces BR biosynthesis remain uncharacterized. DWF4 is one of the main enzymes involved in the BR biosynthesis pathway in Arabidopsis thaliana. We established DWF4-GUS A. thaliana lines in a homozygous dwf4-102 genetic background, but functionally complemented with a genomic DWF4 sequence fused in-frame with a β-glucuronidase (GUS) marker gene. The DWF4-GUS plants enabled the visualization of the accumulation of DWF4 under different conditions. We investigated the effects of aboveground light on root and hypocotyl growth. We observed that root length increased when shoots were maintained under light irrespective of whether roots were exposed to light. We also determined that light perception in aerial tissues enhanced DWF4 accumulation in the root tips. Overall, our data indicate that BR biosynthesis is promoted in the root tip regions by an unknown mechanism in distantly located shoot tissues exposed to light, leading to increased root growth.

Functional analyses of Populus euphratica brassinosteroid biosynthesis enzyme genes DWF4 (PeDWF4) and CPD (PeCPD) in the regulation of growth and development of Arabidopsis thaliana

DWF4 and CPD are key brassinosteroids (BRs) biosynthesis enzyme genes. To explore the function of Populus euphratica DWF4 (PeDWF4) and CPD (PeCPD), Arabidopsis thaliana transgenic lines (TLs) expressing PeDWF4, PeCPD or PeDWF4 plus PeCPD, namely PeDWF4-TL, PeCPD-TL and PeCP/DW-TL, were characterized. Compared with wild type (WT), the changes of both PeDWF4-TL and PeCPD-TL in plant heights, silique and hypocotyls lengths and seed yields were similar, but in bolting time and stem diameters, they were opposite. PeCP/DW-TL was more in plant heights and the lengths of primary root, silique, and fruit stalk, but less in silique numbers and seed yields than either PeDWF4-TL or PeCPD-TL. PeDWF4 and PeCPD specially expressed in PeDWF4-TL or PeCPDTL, and the transcription level of PeDWF4 was higher than that of PeCPD. In PeCP/DW-TL, their expressions were all relatively reduced. Additionally, the expression of PeDWF4 and PeCPD differentially made the expression levels of AtDWF4, AtCPD, AtBR6OX2, AtFLC, AtTCP1 and AtGA5 change in the TLs. The total BRs contents were PeDWF4-TL greater than PeCP/DW-TL greater than WT greater than PeCPD-TL. These results imply that PeDWF4 is functionally not exactly the same as PeCPD and there may be a synergistic and antagonistic effects in physiology between both of them in the regulation of plant growth and development.

Identification of brassinosteroid genes in Brachypodium distachyon

BACKGROUND

Brassinosteroids (BRs) are steroidal phytohormones that are involved in diverse physiological processes and affect many important traits, such as plant stature, stress tolerance, leaf angle, fertility, and grain filling. BR signaling and biosynthetic pathways have been studied in various plants, such as the model dicot Arabidopsis thaliana; however, relatively little is known about these pathways in monocots.

RESULTS

To characterize BR-related processes in the model grass Brachypodium distachyon, we studied the response of these plants to the specific BR biosynthesis inhibitor, propiconazole (Pcz). We found that treatments with Pcz produced a dwarf phenotype in B. distachyon seedlings, similar to that observed in Pcz-treated Arabidopsis plants and in characterized BR-deficient mutants. Through bioinformatics analysis, we identified a list of putative homologs of genes known to be involved in BR biosynthesis and signaling in Arabidopsis, such as DWF4, BR6OX2, CPD, BRI1, and BIN2. Evaluating the response of these genes to Pcz treatments revealed that candidates for BdDWF4, BR6OX2 and, CPD were under feedback regulation. In addition, Arabidopsis plants heterologously expressing BdDWF4 displayed tall statures and elongated petioles, as would be expected in plants with elevated levels of BRs. Moreover, heterologous expression of BdBIN2 in Arabidopsis resulted in dwarfism, suggesting that BdBIN2 functions as a negative regulator of BR signaling. However, the dwarf phenotypes of Arabidopsis bri1-5, a weak BRI1 mutant allele, were not complemented by overexpression of BdBRI1, indicating that BdBRI1 and BRI1 are not functionally equivalent.

CONCLUSION

We identified components of the BR biosynthetic and signaling pathways in Brachypodium, and provided examples of both similarities and differences in the BR biology of these two plants. Our results suggest a framework for understanding BR biology in monocot crop plants such as Zea mays (maize) and Oryza sativa (rice).

Arabidopsis NAC Transcription Factor JUNGBRUNNEN1 Exerts Conserved Control Over Gibberellin and Brassinosteroid Metabolism and Signaling Genes in Tomato

The Arabidopsis thaliana NAC transcription factor JUNGBRUNNEN1 (AtJUB1) regulates growth by directly repressing GA3ox1 and DWF4, two key genes involved in gibberellin (GA) and brassinosteroid (BR) biosynthesis, respectively, leading to GA and BR deficiency phenotypes. AtJUB1 also reduces the expression of PIF4, a bHLH transcription factor that positively controls cell elongation, while it stimulates the expression of DELLA genes, which are important repressors of growth. Here, we extend our previous findings by demonstrating that AtJUB1 induces similar GA and BR deficiency phenotypes and changes in gene expression when overexpressed in tomato (Solanum lycopersicum). Importantly, and in accordance with the growth phenotypes observed, AtJUB1 inhibits the expression of growth-supporting genes, namely the tomato orthologs of GA3ox1, DWF4 and PIF4, but activates the expression of DELLA orthologs, by directly binding to their promoters. Overexpression of AtJUB1 in tomato delays fruit ripening, which is accompanied by reduced expression of several ripening-related genes, and leads to an increase in the levels of various amino acids (mostly proline, β-alanine, and phenylalanine), γ-aminobutyric acid (GABA), and major organic acids including glutamic acid and aspartic acid. The fact that AtJUB1 exerts an inhibitory effect on the GA/BR biosynthesis and PIF4 genes but acts as a direct activator of DELLA genes in both, Arabidopsis and tomato, strongly supports the model that the molecular constituents of the JUNGBRUNNEN1 growth control module are considerably conserved across species.

Brassinosteroids Regulate Root Growth, Development, and Symbiosis

Brassinosteroids (BRs) are natural plant hormones critical for growth and development. BR deficient or signaling mutants show significantly shortened root phenotypes. However, for a long time, it was thought that these phenotypes were solely caused by reduced cell elongation in the mutant roots. Functions of BRs in regulating root development have been largely neglected. Nonetheless, recent detailed analyses, revealed that BRs are not only involved in root cell elongation but are also involved in many aspects of root development, such as maintenance of meristem size, root hair formation, lateral root initiation, gravitropic response, mycorrhiza formation, and nodulation in legume species. In this review, current findings on the functions of BRs in mediating root growth, development, and symbiosis are discussed.

Transcriptome, microRNA, and degradome analyses of the gene expression of Paulownia with phytoplamsa

BACKGROUND

Paulownia witches' broom (PaWB) is a fatal disease of Paulownia caused by a phytoplasma. In previous studies, we found that plants with PaWB symptoms would revert to a healthy morphology after methyl methane sulfonate (MMS) treatment. To completely understand the gene expression profiles of the Paulownia-phytoplasma interaction, three high-throughput sequencing technologies were used to investigate changes of gene expression and microRNAs (miRNAs) in healthy Paulownia tomentosa plantlets, PaWB-infected plantlets, and PaWB-infected plantlets treated with 60 mg · L(-1) MMS.

METHODS

Transcriptome, miRNAs and degradome sequencing were performed to explore the global gene expression profiles in the process of Paulownia tomentosa with phytoplasma infection.

RESULTS

A total of 98,714 all-unigenes, 62 conserved miRNAs, and 35 novel miRNAs were obtained, among which 902 differentially expressed genes (DEGs) and 24 miRNAs were found to be associated with PaWB disease. Subsequently, the target genes of these miRNAs were predicted by degradome sequencing. Interestingly, we found that 19 target genes of these differentially expressed miRNAs were among the 902 DEGs. The targets of pau-miR156g, pau-miR403, and pau-miR166c were significantly up-regulated in the P. tomentosa plantlets infected with phytoplasma. Interaction of miRNA -target genes mediated gene expression related to PaWB were identified.

CONCLUSIONS

The results elucidated the possible roles of the regulation of genes and miRNAs in the Paulownia-phytoplasma interaction, which will enrich our understanding of the mechanisms of PaWB disease in this plant.

Characterization of Rolled and Erect Leaf 1 in regulating leave morphology in rice

Leaf morphology, particularly in crop, is one of the most important agronomic traits because it influences the yield through the manipulation of photosynthetic capacity and transpiration. To understand the regulatory mechanism of leaf morphogenesis, an Oryza sativa dominant mutant, rolled and erect leaf 1 (rel1) has been characterized. This mutant has a predominant rolled leaf, increased leaf angle, and reduced plant height phenotype that results in a reduction in grain yield. Electron microscope observations indicated that the leaf incurvations of rel1 dominant mutants result from the alteration of the size and number of bulliform cells. Molecular cloning revealed that the rel1 dominant mutant phenotype is caused by the activation of the REL1 gene, which encodes a novel unknown protein, despite its high degree of conservation among monocot plants. Moreover, the downregulation of the REL1 gene in the rel1 dominant mutant restored the phenotype of this dominant mutant. Alternatively, overexpression of REL1 in wild-type plants induced a phenotype similar to that of the dominant rel1 mutant, indicating that REL1 plays a positive role in leaf rolling and bending. Consistent with the observed rel1 phenotype, the REL1 gene was predominantly expressed in the meristem of various tissues during plant growth and development. Nevertheless, the responsiveness of both rel1 dominant mutants and REL1-overexpressing plants to exogenous brassinosteroid (BR) was reduced. Moreover, transcript levels of BR response genes in the rel1 dominant mutants and REL1-overexpressing lines were significantly altered. Additionally, seven REL1-interacting proteins were also identified from a yeast two-hybrid screen. Taken together, these findings suggest that REL1 regulates leaf morphology, particularly in leaf rolling and bending, through the coordination of BR signalling transduction.

Functional analysis of GmCPDs and investigation of their roles in flowering

The onset of floral development is a pivotal switch in the life of soybean. Brassinosteroids (BRs), a group of steroidal phytohormones with essential roles in plant growth and development, are associated with flowering induction. Genes involved in BR biosynthesis have been studied to a great extent in Arabidopsis, but the study of these genes has been limited in soybean. In this study, four CPD homologs (GmCPDs) catalyzing BR synthesis were isolated from soybean. Transcripts were mainly confined to cotyledons and leaves and were down-regulated in response to exogenous BR. Bioinformatic analysis showed strong sequence and structure similarity between GmCPDs and AtCPD as well as CPDs of other species. Overexpression of GmCPDs in an Arabidopsis BR-deficient mutant rescued the phenotype by restoring the biosynthesis pathway, revealing the functional roles of each GmCPDs in. Except for the rescue of root development, leaf expansion and plant type architecture, GmCPDs in expression also complemented the late flowering phenotype of Arabidopsis mutants deficient in CPD. Further evidence in soybean plants is that the expression levels of GmCPDs in are under photoperiod control in Zigongdongdou, a photoperiod-sensitive variety, and show a sudden peak upon floral meristem initiation. Together with increased GmCPDs in expression in the leaves and cotyledons of photoperiod-insensitive early-maturity soybean, it is clear that GmCPDs in contribute to flowering development and are essential in the early stages of flowering regulation.

The Arabidopsis thaliana TCP transcription factors: A broadening horizon beyond development

The TCP family of transcription factors is named after the first 4 characterized members, namely TEOSINTE BRANCHED1 (TB1) from maize (Zea mays), CYCLOIDEA (CYC) from snapdragon (Antirrhinum majus), as well as PROLIFERATING CELL NUCLEAR ANTIGEN FACTOR1 (PCF1) and PCF2 from rice (Oryza sativa). Phylogenic analysis of this plant-specific protein family unveils a conserved bHLH-containing DNA-binding motif known as the TCP domain. In accordance with the structure of this shared domain, TCP proteins are grouped into class I (TCP-P) and class II (TCP-C), which are suggested to antagonistically modulate plant growth and development via competitively binding similar cis-regulatory modules called site II elements. Over the last decades, TCPs across the plant kingdom have been demonstrated to control a plethora of plant processes. Notably, TCPs also regulate plant development and defense responses via stimulating the biosynthetic pathways of bioactive metabolites, such as brassinosteroid (BR), jasmonic acid (JA) and flavonoids. Besides, mutagenesis analysis coupled with biochemical experiments identifies several crucial amino acids located within the TCP domain, which confer the redox sensitivity of class I TCPs and determine the distinct DNA-binding properties of TCPs. In this review, developmental functions of TCPs in various biological pathways are briefly described with an emphasis on their involvement in the synthesis of bioactive substances. Furthermore, novel biochemical aspects of TCPs with respect to redox regulation and DNA-binding preferences are elaborated. In addition, the unexpected participation of TCPs in effector-triggered immunity (ETI) and defense against insects indicates that the widely recognized developmental regulators are capable of fine-tuning defense signaling and thereby enable plants to evade deleterious developmental phenotypes. Altogether, these recent impressive breakthroughs remarkably advance our understanding as to how TCPs integrate internal developmental cues with external environmental stimuli to orchestrate plant development.

Antagonistic regulation of Arabidopsis growth by brassinosteroids and abiotic stresses

To withstand ever-changing environmental stresses, plants are equipped with phytohormone-mediated stress resistance mechanisms. Salt stress triggers abscisic acid (ABA) signaling, which enhances stress tolerance at the expense of growth. ABA is thought to inhibit the action of growth-promoting hormones, including brassinosteroids (BRs). However, the regulatory mechanisms that coordinate ABA and BR activity remain to be discovered. We noticed that ABA-treated seedlings exhibited small, round leaves and short roots, a phenotype that is characteristic of the BR signaling mutant, brassinosteroid insensitive1-9 (bri1-9). To identify genes that are antagonistically regulated by ABA and BRs, we examined published Arabidopsis microarray data sets. Of the list of genes identified, those upregulated by ABA but downregulated by BRs were enriched with a BRRE motif in their promoter sequences. After validating the microarray data using quantitative RT-PCR, we focused on RD26, which is induced by salt stress. Histochemical analysis of transgenic Arabidopsis plants expressing RD26pro:GUS revealed that the induction of GUS expression after NaCl treatment was suppressed by co-treatment with BRs, but enhanced by co-treatment with propiconazole, a BR biosynthetic inhibitor. Similarly, treatment with bikinin, an inhibitor of BIN2 kinase, not only inhibited RD26 expression, but also reduced the survival rate of the plant following exposure to salt stress. Our results suggest that ABA and BRs act antagonistically on their target genes at or after the BIN2 step in BR signaling pathways, and suggest a mechanism by which plants fine-tune their growth, particularly when stress responses and growth compete for resources.

Brassinosteroid nuclear signaling recruits HSP90 activity

Heat shock protein 90 (HSP90) controls a number of developmental circuits, and serves a sophisticated and highly regulatory function in signaling pathways. Brassinosteroids (BRs) control many aspects of plant development. Genetic, physiological, cytological, gene expression, live cell imaging, and pharmacological approaches provide conclusive evidence for HSP90 involvement in Arabidopsis thalianaBR signaling. Nuclear-localized HSP90s translocate to cytoplasm when their activity is blocked by the HSP90 inhibitor geldanamycin (GDA). GDA treatment promoted the export of BIN2, a regulator of BR signaling, from the nucleus into the cytoplasm, indicating that active HSP90 is required to sustain BIN2 in the nucleus. HSP90 nuclear localization was inhibited by brassinolide (BL). HSP90s interact with BIN2 in the nucleus of untreated cells and in the cytoplasm of BL-treated cells, showing that the site-specific action of HSP90 on BIN2 is controlled by BRs. GDA and BL treatments change the expression of a common set of previously identified BR-responsive genes. This highlights the effect of active HSP90s on the regulation of BR-responsive genes. Our observations reveal that HSP90s have a central role in sustaining BIN2 nuclear function. We propose that BR signaling is mediated by HSP90 activity and via trafficking of BIN2-HSP90 complexes into the cytoplasm.

PAG1, a cotton brassinosteroid catabolism gene, modulates fiber elongation

Cotton (Gossypium hirsutum) is the major source of natural textile fibers. Brassinosteroids (BRs) play crucial roles in regulating fiber development. The molecular mechanisms of BRs in regulating fiber elongation, however, are poorly understood. pagoda1 (pag1) was identified via an activation tagging genetic screen and characterized by genome walking and brassinolide (BL) supplementation. RNA-Seq analysis was employed to elucidate the mechanisms of PAG1 in regulating fiber development. pag1 exhibited dwarfism and reduced fiber length due to significant inhibition of cell elongation and expansion. BL treatment rescued its growth and fiber elongation. PAG1 encodes a homolog of Arabidopsis CYP734A1 that inactivates BRs via C-26 hydroxylation. RNA-Seq analyses showed that the constitutive expression of PAG1 downregulated the expression of genes involved in very-long-chain fatty acids (VLCFA) biosynthesis, ethylene-mediated signaling, response to cadmium, cell wall development, cytoskeleton organization and cell growth. Our results demonstrate that PAG1 plays crucial roles in regulating fiber development via controlling the level of endogenous bioactive BRs, which may affect ethylene signaling cascade by mediating VLCFA. Therefore, BR may be a critical regulator of fiber elongation, a role which may in turn be linked to effects on VLCFA biosynthesis, ethylene and cadmium signaling, cell wall- and cytoskeleton-related gene expression.