BrassinosteroidsThe brassinosteroid BR class of steroid hormones regulates plant development and physiology. The BR signal is transduced by a brassinosteroid review kinase-mediated signal transduction pathway, which is distinct from animal steroid signalling systems. Recent studies have fully connected the BR signal transduction chain and have identified thousands of BR target genes, linking Herpes simplex oculare corticosteroidi signalling to numerous cellular processes. Molecular links between BR and several other signalling pathways have also been identified. Here, we provide an overview of the highly integrated BR revifw network brassinosteroid review explain how this steroid hormone functions as a master brassinosteroid review of plant growth, development and metabolism.
Brassinosteroid signalling | Development
The brassinosteroid BR class of steroid hormones regulates plant development and physiology. The BR signal is transduced by a receptor kinase-mediated signal transduction pathway, which is distinct from animal steroid signalling systems. Recent studies have fully connected the BR signal transduction chain and have identified thousands of BR target genes, linking BR signalling to numerous cellular processes.
Molecular links between BR and several other signalling pathways have also been identified. Here, we provide an overview of the highly integrated BR signalling network and explain how this steroid hormone functions as a master regulator of plant growth, development and metabolism.
Steroid hormones regulate gene expression and development in both plants and animals. Brassinosteroid BR was discovered in pollen extract based on its ability to promote cell elongation, but was later found in all growing tissues of higher plants, with the highest levels found in pollen, seeds and fruit. Subsequent physiological studies demonstrated that BR has a wide range of effects on growth and in responses to biotic and abiotic stresses.
Studies of mutants with defects in BR biosynthesis or signalling demonstrated that BR plays essential roles in nearly all phases of plant development, as these mutants show multiple developmental defects, such as reduced seed germination, extreme dwarfism, photomorphogenesis in the dark, altered distribution of stomata, delayed flowering and male sterility.
The molecular mechanisms of BR signal transduction and BR-mediated regulation of gene expression and plant development have been dissected in detail by studies using molecular genetics, biochemistry, proteomics and genomics, making the BR signalling pathway one of the best understood signal transduction pathways in plants. Phosphorylated BKI1 also interacts with the family of phosphopeptide-binding proteins to promote BR signalling Wang et al.
BRI1 and BAK1 are mainly localised at the plasma membrane but also undergo endocytosis and cycle between the endosome and plasma membrane Geldner et al. Unphosphorylated BZR1 and BZR2 can then move into the nucleus and bind to the promoters of their target genes, leading to gene activation or repression He et al. In particular, cell wall modification and cellular transport are major cellular functions targeted by BR, consistent with its effects on cell elongation and growth Sun et al.
Interestingly, a recent study showed that compromised cell wall integrity activates BR signalling, suggesting a feedback mechanism for the BR-mediated balance between cell extension and integrity of the cell wall Wolf et al.
Also highly represented in the BZR1 targets are transcription factors and components of many other signalling pathways, such as the light, gibberellin GA and auxin pathways Sun et al. In addition to BIM1 Yin et al.
Genetic and transgenic experiments indicated that these BZR2-interacting proteins have only minor effects on BR-regulated growth responses such as hypocotyl elongation; whether they interact with BZR1 remains unknown. Nearly all BR-regulated processes are also regulated by other hormonal or environmental signals Depuydt and Hardtke, In particular, suppression of photomorphogenesis in the dark requires BR and two other hormones: GA and auxin Li et al.
A large body of evidence shows that BR antagonises light signals, has similar physiological effects to GA, and interacts synergistically with auxin during cell elongation and gene expression Depuydt and Hardtke, ; Lau and Deng, Recent studies have uncovered the molecular mechanisms underlying the interactions between BR and these other signalling pathways Bai et al.
Light switches seedling development from etiolation skotomorphogenesis to de-etiolation photomorphogenesis by inhibiting cell elongation and promoting chloroplast development and leaf expansion. BR is required not only for cell elongation but also for the suppression of light-induced genes in the dark Chory et al. Although it is generally believed that environmental signals affect plant growth by changing plant hormones, light has no obvious effect on BR levels or upstream BR signalling Luo et al.
First, BZR1 represses the expression levels of many positive regulators and activates negative regulators of the light signalling network Fan et al.
PIFs are negative regulators of photomorphogenesis and are degraded upon interaction with light-activated phytochromes, but are increased by dark and shade, as well as heat Leivar and Quail, PIFs and BZR1 directly interact with each other and interdependently regulate a large number of direct target genes, many of which encode transcription factors and proteins that function in the cell wall and chloroplast Oh et al.
Some of the transcription factors downstream of BZR1 play important roles in regulating cell elongation and chloroplast development. BR and GA are both growth-promoting hormones, having similar effects on various developmental processes throughout the life cycle of plants Depuydt and Hardtke, Until recently, the effects of BR and GA have been considered additive Depuydt and Hardtke, , and their signalling pathways have been proposed to act on largely non-overlapping transcriptional responses Nemhauser et al.
However, recent studies demonstrated an interdependent relationship and a direct interaction between the BR and GA signalling pathways Bai et al. GA is unable to increase hypocotyl elongation in BR-deficient and BR-insensitive mutants, whereas BR and the dominant gain-of-function bzrD mutation can increase cell elongation in GA-deficient mutants. Several observations suggest potential molecular mechanisms of mediating BR-auxin interactions.
First, BZR1 regulates many genes involved in auxin synthesis, transport and signalling Sun et al. Fifth, BR and auxin responses are integrated through the actin cytoskeleton, which is not only regulated by both BR and auxin but also feedback regulates auxin transport and BR signalling Lanza et al. The contributions of these mechanisms to the physiology of BR-auxin interaction remain to be evaluated further. There are over receptor kinases in Arabidopsis Shiu et al.
Sharing and cross-regulation of downstream components are thus likely to be common among receptor kinase pathways, particularly those that diverged recently in evolution and those that regulate related processes and thus would benefit from signalling crosstalk.
Indeed, recent studies have shown that direct crosstalk exists between BRI1 and two other well-characterised receptor kinases that control stomata development and innate immunity. Photosynthesis requires both chloroplasts and stomata, which are the epidermal valves that allow gas exchange between plant leaves and the atmosphere.
The density and distribution of stomata are tightly controlled to optimise the uptake of CO 2 and minimise water loss Dong and Bergmann, BR has been shown to inhibit and promote stomatal development in leaves and hypocotyls, respectively, due to BIN2-mediated phosphorylation of two components of the stomatal pathway. It has been proposed that such co-receptor sharing could create an antagonistic relationship between BR and flagellin due to competition for the co-receptor Belkhadir et al.
If the activated BAK1 can reassociate with different partners, activation of BAK1 by one signal might enhance the signalling of another pathway. However, there is evidence that BR can also inhibit FLS2-mediated signalling through an unknown mechanism downstream of the receptor kinases Albrecht et al. The contributions of these mechanisms to the trade-off between growth and immunity remain to be analysed.
In addition to direct crosstalk with other signalling pathways, BR also impinges on developmental pathways, often through BZR1-mediated regulation of specific developmental regulators. Recent studies have shed light on BR regulation of reproductive development and root growth. The transition from vegetative to reproductive growth is regulated by numerous interacting endogenous and environmental cues, such as GA, BR, photoperiod and temperature Li et al.
However, whether this interaction mediates BR repression of FLC expression and promotion of flowering remains unanswered Clouse, , as REF6 activates gene expression by removing repressive H3K27me3 histone marks Lu et al. BR may also indirectly affect flowering time by influencing the circadian clock and the photoperiod flowering pathway, as BR application shortens circadian rhythms Hanano et al. Defects in BR biosynthesis or signalling pathways also reduce male fertility due to shortening of the stamen and defects in pollen development Ye et al.
These developmental defects correlate with the reduced expression of several key genes involved in anther and pollen development, many of which are transcriptional targets of BZR2 Ye et al. Finally, recent studies have shown that, in maize, BR controls sex determination by promoting stamen and repressing pistil development in tassels Hartwig et al. BR also promotes quiescent centre division and columella stem cell differentiation Gonzalez-Garcia et al. Interestingly, expressing BRI1 using an epidermal-specific promoter rescued the root growth of the bri1 mutant, demonstrating that BR perception in the epidermis is sufficient to control root growth and meristem size, possibly through a mobile factor other than BZR1 or BZR2 Hacham et al.
BR also plays an important role in directing epidermal cell fate in roots, where epidermal cells differentiate into hair or non-hair cells depending on their position. Thus, regulating the expression levels of cell type-specific components seems to be a general mechanism by which BR exerts specific effects on diverse developmental responses. Over the past decade, our understanding of the BR signalling pathway has progressed rapidly thanks to a combination of genetic, proteomic and genomic approaches.
The BR pathway represents the first, and still the only, fully elucidated receptor kinase signalling pathway in plants. Its extensive molecular connections with other signalling pathways demonstrate a high degree of integration in the regulatory networks in plants.
In particular, the essential roles played by BR in plant responses to light and GA support the notion that BR is a master regulator at the centre of the plant growth regulation network. A framework has thus been established for building a detailed molecular map of the growth regulation network in plants. Many key questions remain to be answered, however. For example, what controls the wide range of BR levels found in different tissues and organs? How do plants use BR in the context of normal development and under environmental stresses?
Does BR serve as a positional cue for cell differentiation and morphogenesis? How does BR induce distinct responses in different tissues and cell types? How is the BR pathway integrated with additional signalling pathways? Finally, how does BR signalling integrate with both by regulating and being regulated by cellular processes such as vesicle trafficking, cytoskeleton organisation, and cell wall expansion and integrity?
Answers to these questions will advance our understanding of plant growth regulation, which is important for food and bioenergy production and for environmental conservation Vriet et al.
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Rap1, Canoe and Mbt cooperate with Bazooka to promote zonula adherens assembly in the fly photoreceptor Rhian F. J Cell Sci Skip to main content. Summary The brassinosteroid BR class of steroid hormones regulates plant development and physiology. Introduction Steroid hormones regulate gene expression and development in both plants and animals. BR-mediated regulation of photomorphogenesis through crosstalk with other pathways Nearly all BR-regulated processes are also regulated by other hormonal or environmental signals Depuydt and Hardtke, Regulation of the light response Light switches seedling development from etiolation skotomorphogenesis to de-etiolation photomorphogenesis by inhibiting cell elongation and promoting chloroplast development and leaf expansion.
The relationship between BR and GA BR and GA are both growth-promoting hormones, having similar effects on various developmental processes throughout the life cycle of plants Depuydt and Hardtke, Crosstalk between BRI1 and other receptor kinase pathways There are over receptor kinases in Arabidopsis Shiu et al. Crosstalk between the BRI1 and ERECTA pathways regulates stomata development Photosynthesis requires both chloroplasts and stomata, which are the epidermal valves that allow gas exchange between plant leaves and the atmosphere.
BR-mediated regulation of other aspects of plant development In addition to direct crosstalk with other signalling pathways, BR also impinges on developmental pathways, often through BZR1-mediated regulation of specific developmental regulators.
Regulation of reproductive development The transition from vegetative to reproductive growth is regulated by numerous interacting endogenous and environmental cues, such as GA, BR, photoperiod and temperature Li et al.
Perspectives Over the past decade, our understanding of the BR signalling pathway has progressed rapidly thanks to a combination of genetic, proteomic and genomic approaches. Acknowledgments We apologise to authors whose primary work we could not cite because of space constraints.
Competing interests statement The authors declare no competing financial interests.