The brassinosteroid insensitive1-like3 signalosome complex regulates Arabidopsis root development

BRL3 and AtRGS1 cooperate to fine tune growth inhibition and ROS activation

Plasma membrane-localized leucine-rich repeat receptor-like kinases directly activates G protein complex via interaction with seven transmembrane domain Regulator of G-protein Signaling 1 (AtRGS1) protein. Brassinosteroid insensitive 1 (BRI1) LIKE3 (BRL3) phosphorylates AtRGS1 in vitro. FRET analysis showed that BRL3 and AtRGS1 interaction dynamics change in response to glucose and flg22. Both BRL3 and AtRGS1 function in glucose sensing and brl3 and rgs1-2 single mutants are hyposensitive to high glucose as well as the brl3/rgs1 double mutant. BRL3 and AtRGS1 function in the same pathway linked to high glucose sensing. Hypocotyl elongation, another sugar-mediated pathway, is also implicated to be partially mediated by BRL3 and AtRGS1 because rgs1-2, brl3-2 and brl3-2/ rgs1-2 mutants share the long hypocotyl phenotype. BRL3 and AtRGS1 modulate the flg22-induced ROS burst and block one or more components positively regulating ROS production because the brl3/rgs1 double mutant has ~60% more ROS production than wild type while rgs1-2 has a partial ROS burst impairment and brl3 has slightly more ROS production. Here, we proposed a simple model where both BRL3 and AtRGS1 are part of a fine-tuning mechanism sensing glucose and flg22 to prevent excess ROS burst and control growth inhibition.

Probing Activation and Deactivation of the BRASSINOSTEROID INSENSITIVE1 Receptor Kinase by Immunoprecipitation

Brassinosteroids (BRs) are sterol-derived hormones that control plant growth and development. The BR receptor complex is encoded by the BRASSINOSTEROID INSENSITIVE1 (BRI1) and members of the SOMATIC EMBRYOGENESIS RECEPTOR KINASE family. For activation and deactivation, the BR receptor complex uses different posttranslational modifications and recruits various partner proteins. Here, we describe optimized immunoprecipitation protocols and variants for biochemical analyses of posttranslational modifications of BRI1 and of protein-protein interactions.

The Primary Root of Sorghum bicolor (L. Moench) as a Model System to Study Brassinosteroid Signaling in Crops

Roots anchor plants to the soil and are essential for a successful plant growth and adaptation to the environment. Research on the primary root in the plant model system Arabidopsis thaliana has yielded important advances in the molecular and cellular understanding of root growth and development. Several studies have uncovered how the hormones brassinosteroids (BRs) control cell cycle and differentiation programs through different cell-specific signaling pathways that are key for root growth and development. Currently, an important challenge resides in the translation of the current knowledge on Arabidopsis roots into agronomically valuable species to improve the agricultural production and to meet the food security goals of the millennium. In this chapter, we characterize the primary root apex of the cereal Sorghum bicolor (L. Moench) (sorghum), analyze the physiological response of sorghum roots to BRs, and examine the phylogeny of the BRASSINOSTEROID INSENSITIVE1-like receptor family in Arabidopsis and its orthologous genes in sorghum. Overall, we support the use of sorghum as a suitable crop model species for the study of BR signaling in root growth and development. The methods presented enable any laboratory worldwide to use sorghum primary roots as a favorite organ for the study of growth and development in crops.

Identification of Brassinosteroid Signaling Complexes by Coimmunoprecipitation and Mass Spectrometry

A combination of coimmunoprecipitation (coIP) of tagged proteins followed by protein identification and quantitation using Liquid Chromatography Mass Spectrometry/Mass Spectrometry (LCMS/MS) has proven to be a reliable method to qualitatively characterize membrane-bound receptor complexes from plants. Success depends on a range of parameters, such as abundance and stability of the complex and functionality of the tagged receptors, efficiency of the protein complex isolation procedure, MS equipment, and analysis software in use. In this Chapter, we focus on the use of one of the green fluorescent protein-tagged receptors of the SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) family, of which SERK3, also known as BRASSINOSTEROID INSENSITIVE1 (BRI1) ASSOCIATED KINASE1 (BAK1), is a coreceptor of BRI1. Like BRI1 itself, SERK3 is a leucine-rich repeat receptor kinase (LRR RK) with a single-pass transmembrane domain. The latest updated laboratory protocol is presented as well as examples of data analysis and typical results obtained. Potential drawbacks of the procedure employed for plant membrane proteins will be pointed out.

BES1 regulates the localization of the brassinosteroid receptor BRL3 within the provascular tissue of the Arabidopsis primary root

Brassinosteroid (BR) hormones are important regulators of plant growth and development. Recent studies revealed the cell-specific role of BRs in vascular and stem cell development by the action of cell-specific BR receptor complexes and downstream signaling components in Arabidopsis thaliana Despite the importance of spatiotemporal regulation of hormone signaling in the control of plant vascular development, the mechanisms that confer cellular specificity to BR receptors within the vascular cells are not yet understood. The present work shows that BRI1-like receptor genes 1 and 3 (BRL1 and BRL3) are differently regulated by BRs. By using promoter deletion constructs of BRL1 and BRL3 fused to GFP/GUS (green fluorescent protein/β-glucuronidase) reporters in Arabidopsis, analysis of their cell-specific expression and regulation by BRs in the root apex has been carried out. We found that BRL3 expression is finely modulated by BRs in different root cell types, whereas the location of BRL1 appears to be independent of this hormone. Physiological and genetic analysis show a BR-dependent expression of BRL3 in the root meristem. In particular, BRL3 expression requires active BES1, a central transcriptional effector within the BRI1 pathway. ChIP analysis showed that BES1 directly binds to the BRRE present in the BRL3 promoter region, modulating its transcription in different subsets of cells of the root apex. Overall our study reveals the existence of a cell-specific negative feedback loop from BRI1-mediated BES1 transcription factor to BRL3 in phloem cells, while contributing to a general understanding of the spatial control of steroid signaling in plant development.

Diverse roles of SERK family genes in plant growth, development and defense response

Plant receptor-like protein kinases (RLKs) are transmembrane proteins with an extracellular domain and an intracellular kinase domain, which enable plant perceiving diverse extracellular stimuli to trigger the intracellular signal transduction. The somatic embryogenesis receptor kinases (SERKs) code the leucine-rich-repeat receptor-like kinase (LRR-RLK), and have been demonstrated to associate with multiple ligand-binding receptors to regulate plant growth, root development, male fertility, stomatal development and movement, and immune responses. Here, we focus on the progress made in recent years in understanding the versatile functions of Arabidopsis SERK proteins, and review SERK proteins as co-receptor to perceive different endogenous and environmental cues in different signaling pathway, and discuss how the kinase activity of SERKs is regulated by various modification.

Protein-protein interactions in plant mitogen-activated protein kinase cascades

Mitogen-activated protein kinases (MAPKs) form tightly controlled signaling cascades that play essential roles in plant growth, development, and defense. However, the molecular mechanisms underlying MAPK cascades are still elusive, due largely to our poor understanding of how they relay the signals. Extensive effort has been devoted to characterization of MAPK-substrate interactions to illustrate phosphorylation-based signaling. The diverse MAPK substrates identified also shed light on how spatiotemporal-specific protein-protein interactions function in distinct MAPK cascade-mediated biological processes. This review surveys various technologies used for characterizing MAPK-substrate interactions and presents case studies of MPK4 and MPK6, highlighting the multiple functions of MAPKs. Mass spectrometry-based approaches in identifying MAPK-interacting proteins are emphasized due to their increasing utility and effectiveness. The potential for using MAPKs and their substrates in enhancing plant stress tolerance is also discussed.

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.

Immunoprecipitation of Plasma Membrane Receptor-Like Kinases for Identification of Phosphorylation Sites and Associated Proteins

Membrane proteins are difficult to study for numerous reasons. The surface of membrane proteins is relatively hydrophobic and sometimes very unstable, additionally requiring detergents for their extraction from the membrane. This leads to challenges at all levels, including expression, solubilization, purification, identification of associated proteins, and the identification of post-translational modifications. However, recent advances in immunoprecipitation technology allow to isolate membrane proteins efficiently, facilitating the study of protein-protein interactions, the identification of novel associated proteins, and to identify post-translational modifications, such as phosphorylation. Here, we describe an optimized immunoprecipitation protocol for plant plasma membrane receptor-like kinases.

RNA Interference Knockdown of BRASSINOSTEROID INSENSITIVE1 in Maize Reveals Novel Functions for Brassinosteroid Signaling in Controlling Plant Architecture

Brassinosteroids (BRs) are plant hormones involved in various growth and developmental processes. The BR signaling system is well established in Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa) but poorly understood in maize (Zea mays). BRASSINOSTEROID INSENSITIVE1 (BRI1) is a BR receptor, and database searches and additional genomic sequencing identified five maize homologs including duplicate copies of BRI1 itself. RNA interference (RNAi) using the extracellular coding region of a maize zmbri1 complementary DNA knocked down the expression of all five homologs. Decreased response to exogenously applied brassinolide and altered BR marker gene expression demonstrate that zmbri1-RNAi transgenic lines have compromised BR signaling. zmbri1-RNAi plants showed dwarf stature due to shortened internodes, with upper internodes most strongly affected. Leaves of zmbri1-RNAi plants are dark green, upright, and twisted, with decreased auricle formation. Kinematic analysis showed that decreased cell division and cell elongation both contributed to the shortened leaves. A BRASSINOSTEROID INSENSITIVE1-ETHYL METHANESULFONATE-SUPPRESSOR1-yellow fluorescent protein (BES1-YFP) transgenic line was developed that showed BR-inducible BES1-YFP accumulation in the nucleus, which was decreased in zmbri1-RNAi. Expression of the BES1-YFP reporter was strong in the auricle region of developing leaves, suggesting that localized BR signaling is involved in promoting auricle development, consistent with the zmbri1-RNAi phenotype. The blade-sheath boundary disruption, shorter ligule, and disrupted auricle morphology of RNAi lines resemble KNOTTED1-LIKE HOMEOBOX (KNOX) mutants, consistent with a mechanistic connection between KNOX genes and BR signaling.

Brassinazole resistant 1 (BZR1)-dependent brassinosteroid signalling pathway leads to ectopic activation of quiescent cell division and suppresses columella stem cell differentiation

Previous publications have shown that BRI1 EMS suppressor 1 (BES1), a positive regulator of the brassinosteroid (BR) signalling pathway, enhances cell divisions in the quiescent centre (QC) and stimulates columella stem cell differentiation. Here, it is demonstrated that BZR1, a BES1 homologue, also promotes cell divisions in the QC, but it suppresses columella stem cell differentiation, opposite to the action of BES1. In addition, BR and its BZR1-mediated signalling pathway are shown to alter the expression/subcellular distribution of pin-formed (PINs), which may result in changes in auxin movement. BR promotes intense nuclear accumulation of BZR1 in the root tip area, and the binding of BZR1 to the promoters of several root development-regulating genes, modulating their expression in the root stem cell niche area. These BZR1-mediated signalling cascades may account for both the ectopic activation of QC cell divisions as well as the suppression of the columella stem cell differentiation. They could also inhibit auxin-dependent distal stem cell differentiation by antagonizing the auxin/WOX5-dependent pathway. In conclusion, BZR1-/BES1-mediated BR signalling pathways show differential effects on the maintenance of root apical meristem activities: they stimulate ectopic QC division while they show opposite effects on the differentiation of distal columella stem cells in a BR concentration- and BZR1-/BES1-dependent manner.

On the Origin of SERKs: Bioinformatics Analysis of the Somatic Embryogenesis Receptor Kinases

Somatic embryogenesis receptor-like kinases (SERKs) are leucine-rich repeat receptor-like kinases involved in several, seemingly unrelated, plant-signaling pathways. In Arabidopsis thaliana, functional and genetic analysis of four SERK proteins has indicated that they are only partly redundant; their functions overlap but each performs a specific subset of signaling roles. The molecular basis for the functional specificity within this highly homologous protein family is currently not known. Sequence analysis of SERK proteins from different plant species indicates that the SERKs are a highly conserved protein family present in monocots, dicots, and non-vascular plants. Residues in the extracellular domain that are important for interaction with other receptor kinases are highly conserved, even among SERK members without a function in the corresponding pathways. SERK2, for instance, does not function in the brassinosteroid pathway, does not interact with BRI1, but is conserved in its BRI1-interacting domain. Further sequence analysis indicates that SERK3/BAK1 and SERK4/BKK1 have diverged from the original SERK protein in both their extracellular and cytoplasmic domains. Functional analysis of chimeric SERK proteins shows that different domains provide the SERK proteins with different functional specificity. For instance, the SERK1 or SERK2 extracellular domains are essential for SERK function in male sporogenesis, while the SERK3 extracellular and cytoplasmic domains are essential for SERK3 activity in brassinosteroid and flagellin signaling. The emerging picture is that SERKs are ancient genes, whose products have been recruited as co-receptors in the newly evolved signaling pathways. The SERK ligand-binding and protein-protein interaction domains are highly conserved, allowing all SERKs to form complexes, albeit with different affinity. However, specific functional residues must have been altered, in both the extracellular and intracellular domains, to allow for the observed differences in functionality.

Calcium and ZmCCaMK are involved in brassinosteroid-induced antioxidant defense in maize leaves

Brassinosteroids (BRs) have been shown to enhance stress tolerance by inducing antioxidant defense systems. However, the mechanisms of BR-induced antioxidant defense in plants remain to be determined. In this study, the role of calcium (Ca(2+)) and maize calcium/calmodulin-dependent protein kinase (CCaMK), ZmCCaMK, in BR-induced antioxidant defense, and the relationship between ZmCCaMK and Ca(2+) in BR signaling were investigated. BR treatment led to a significant increase in cytosolic Ca(2+) concentration in protoplasts from maize mesophyll, and Ca(2+) was shown to be required for BR-induced antioxidant defense. Treatment with BR induced increases in gene expression and enzyme activity of ZmCCaMK in maize leaves. Transient overexpression and silencing of ZmCCaMK in maize protoplasts demonstrated that ZmCCaMK was required for BR-induced antioxidant defense. The requirement for CCaMK was further investigated using a loss-of-function mutant of OsCCaMK, the orthologous gene of ZmCCaMK in rice. Consistent with the findings in maize, BR treatment could not induce antioxidant defense in the rice OsCCAMK mutant. Furthermore, Ca(2+) was required for BR-induced gene expression and activation of ZmCCaMK, while ZmCCaMK was shown to enhance the BR-induced increase in cytosolic Ca(2+) concentration. Moreover, our results also showed that ZmCCaMK and H2O2 influenced each other. These results indicate that Ca(2+) works together with ZmCCaMK in BR-induced antioxidant defense, and there are two positive feedback loops between Ca(2+) or H2O2 and ZmCCaMK in BR signaling in maize.

The molecular circuitry of brassinosteroid signaling

Because they are tethered in space, plants have to make the most of their local growth environment. In order to grow in an ever-changing environment, plants constantly remodel their shapes. This adaptive attribute requires the orchestration of complex environmental signals at the cellular and organismal levels. A battery of small molecules, classically known as phytohormones, allows plants to change their body plan by using highly integrated signaling networks and transcriptional cascades. Amongst these hormones, brassinosteroids (BRs), the polyhydroxylated steroid of plants, influence plant responsiveness to the local environment and exquisitely promote, or interfere with, many aspects of plant development. The molecular circuits that wire steroid signals at the cell surface to the promoters of thousands of genes in the nucleus have been defined in the past decade. This review recapitulates how the transduction of BR signals impacts the temporally unfolding programs of plant growth. First, we summarize the paradigmatic BR signaling pathway acting primarily in cellular expansion. Secondly, we describe the current wiring diagram and the temporal dynamics of the BR signal transduction network. And finally we provide an overview of how key players in BR signaling act as molecular gates to transduce BR signals onto other signaling pathways.

Growth control: brassinosteroid activity gets context

Brassinosteroid activity controls plant growth and development, often in a seemingly opposing or complex manner. Differential impact of the hormone and its signalling components, acting both as promoters and inhibitors of organ growth, is exemplified by meristem differentiation and cell expansion in above- and below-ground organs. Complex brassinosteroid-based control of stomata count and lateral root development has also been demonstrated. Here, mechanisms underlying these phenotypic outputs are examined. Among these, studies uncovering core brassinosteroid signalling components, which integrate with distinct peptide, hormone, and environmental pathways, are reviewed. Finally, the differential spatiotemporal context of brassinosteroid activity within the organ, as an important determinant of controlled growth, is discussed.

Translatome analyses capture of opposing tissue-specific brassinosteroid signals orchestrating root meristem differentiation

The mechanisms ensuring balanced growth remain a critical question in developmental biology. In plants, this balance relies on spatiotemporal integration of hormonal signaling pathways, but the understanding of the precise contribution of each hormone is just beginning to take form. Brassinosteroid (BR) hormone is shown here to have opposing effects on root meristem size, depending on its site of action. BR is demonstrated to both delay and promote onset of stem cell daughter differentiation, when acting in the outer tissue of the root meristem, the epidermis, and the innermost tissue, the stele, respectively. To understand the molecular basis of this phenomenon, a comprehensive spatiotemporal translatome mapping of Arabidopsis roots was performed. Analyses of wild type and mutants featuring different distributions of BR revealed autonomous, tissue-specific gene responses to BR, implying its contrasting tissue-dependent impact on growth. BR-induced genes were primarily detected in epidermal cells of the basal meristem zone and were enriched by auxin-related genes. In contrast, repressed BR genes prevailed in the stele of the apical meristem zone. Furthermore, auxin was found to mediate the growth-promoting impact of BR signaling originating in the epidermis, whereas BR signaling in the stele buffered this effect. We propose that context-specific BR activity and responses are oppositely interpreted at the organ level, ensuring coherent growth.

Co-Immunoprecipitation of Membrane-Bound Receptors

The study of cell-surface receptor dynamics is critical for understanding how cells sense and respond to changing environments. Therefore, elucidating the mechanisms by which signals are perceived and communicated into the cell is necessary to understand immunity, development, and stress. Challenges in testing interactions of membrane-bound proteins include their dynamic nature, their abundance, and the complex dual environment (lipid/soluble) in which they reside. Co-Immunoprecipitation (Co-IP) of tagged membrane proteins is a widely used approach to test protein-protein interaction in vivo. In this protocol we present a method to perform Co-IP using enriched membrane proteins in isolated microsomal fractions. The different variations of this protocol are highlighted, including recommendations and troubleshooting guides in order to optimize its application. This Co-IP protocol has been developed to test the interaction of receptor-like kinases, their interacting partners, and peptide ligands in stable Arabidopsis thaliana lines, but can be modified to test interactions in transiently expressed proteins in tobacco, and potentially in other plant models, or scaled for large-scale protein-protein interactions at the membrane.

Brassinosteroid regulates cell elongation by modulating gibberellin metabolism in rice

Brassinosteroid (BR) and gibberellin (GA) are two predominant hormones regulating plant cell elongation. A defect in either of these leads to reduced plant growth and dwarfism. However, their relationship remains unknown in rice (Oryza sativa). Here, we demonstrated that BR regulates cell elongation by modulating GA metabolism in rice. Under physiological conditions, BR promotes GA accumulation by regulating the expression of GA metabolic genes to stimulate cell elongation. BR greatly induces the expression of D18/GA3ox-2, one of the GA biosynthetic genes, leading to increased GA1 levels, the bioactive GA in rice seedlings. Consequently, both d18 and loss-of-function GA-signaling mutants have decreased BR sensitivity. When excessive active BR is applied, the hormone mostly induces GA inactivation through upregulation of the GA inactivation gene GA2ox-3 and also represses BR biosynthesis, resulting in decreased hormone levels and growth inhibition. As a feedback mechanism, GA extensively inhibits BR biosynthesis and the BR response. GA treatment decreases the enlarged leaf angles in plants with enhanced BR biosynthesis or signaling. Our results revealed a previously unknown mechanism underlying BR and GA crosstalk depending on tissues and hormone levels, which greatly advances our understanding of hormone actions in crop plants and appears much different from that in Arabidopsis thaliana.

A receptor-like protein mediates the response to pectin modification by activating brassinosteroid signaling

The brassinosteroid (BR) signaling module is a central regulator of plant morphogenesis, as indicated by the large number of BR-responsive cell wall-related genes and the severe growth defects of BR mutants. Despite a detailed knowledge of the signaling components, the logic of this auto-/paracrine signaling module in growth control remains poorly understood. Recently, extensive cross-talk with other signaling pathways has been shown, suggesting that the outputs of BR signaling, such as gene-expression changes, are subject to complex control mechanisms. We previously provided evidence for a role of BR signaling in a feedback loop controlling the integrity of the cell wall. Here, we identify the first dedicated component of this feedback loop: a receptor-like protein (RLP44), which is essential for the compensatory triggering of BR signaling upon inhibition of pectin de-methylesterification in the cell wall. RLP44 is required for normal growth and stress responses and connects with the BR signaling pathway, presumably through a direct interaction with the regulatory receptor-like kinase BAK1. These findings corroborate a role for BR in controlling the sensitivity of a feedback signaling module involved in maintaining the physico-chemical homeostasis of the cell wall during cell expansion.

An update on receptor-like kinase involvement in the maintenance of plant cell wall integrity

BACKGROUND

Plant cell walls form the interface between the cells and their environment. They perform different functions, such as protecting cells from biotic and abiotic stress and providing structural support during development. Maintenance of the functional integrity of cell walls during these different processes is a prerequisite that enables the walls to perform their particular functions. The available evidence suggests that an integrity maintenance mechanism exists in plants that is capable of both detecting wall integrity impairment caused by cell wall damage and initiating compensatory responses to maintain functional integrity. The responses involve 1-aminocyclopropane-1-carboxylic acid (ACC), jasmonic acid, reactive oxygen species and calcium-based signal transduction cascades as well as the production of lignin and other cell wall components. Experimental evidence implicates clearly different signalling molecules, but knowledge regarding contributions of receptor-like kinases to this process is less clear. Different receptor-like kinase families have been considered as possible sensors for perception of cell wall damage; however, strong experimental evidence that provides insights into functioning exists for very few kinases.

SCOPE AND CONCLUSIONS

This review examines the involvement of cell wall integrity maintenance in different biological processes, defines what constitutes plant cell wall damage that impairs functional integrity, clarifies which stimulus perception and signal transduction mechanisms are required for integrity maintenance and assesses the available evidence regarding the functions of receptor-like kinases during cell wall integrity maintenance. The review concludes by discussing how the plant cell wall integrity maintenance mechanism could form an essential component of biotic stress responses and of plant development, functions that have not been fully recognized to date.

The growth-defense pivot: crisis management in plants mediated by LRR-RK surface receptors

Plants must adapt to their environment and require mechanisms for sensing their surroundings and responding appropriately. An expanded family of more than 200 leucine-rich repeat (LRR) receptor kinases (LRR-RKs) transduces fluctuating and often contradictory signals from the environment into changes in nuclear gene expression. Two LRR-RKs, BRASSINOSTEROID INSENSITIVE 1 (BRI1), a steroid receptor, and FLAGELLIN SENSITIVE 2 (FLS2), an innate immune receptor that recognizes bacterial flagellin, act cooperatively to partition necessary growth-defense trade-offs. BRI1 and FLS2 share common signaling components and slightly different activation mechanisms. BRI1 and FLS2 are paradigms for understanding the signaling mechanisms of LRR-containing receptors in plants.

Regulation of plant stem cell quiescence by a brassinosteroid signaling module

The quiescent center (QC) maintains the activity of the surrounding stem cells within the root stem cell niche, yet specific molecular players sustaining the low rate of QC cell division remain poorly understood. Here, we identified a R2R3-MYB transcription factor, BRAVO (BRASSINOSTEROIDS AT VASCULAR AND ORGANIZING CENTER), acting as a cell-specific repressor of QC divisions in the primary root of Arabidopsis. Ectopic BRAVO expression restricts overall root growth and ceases root regeneration upon damage of the stem cells, demonstrating the role of BRAVO in counteracting Brassinosteroid (BR)-mediated cell division in the QC cells. Interestingly, BR-regulated transcription factor BES1 (BRI1-EMS SUPRESSOR 1) directly represses and physically interacts with BRAVO in vivo, creating a switch that modulates QC divisions at the root stem cell niche. Together, our results define a mechanism for BR-mediated regulation of stem cell quiescence in plants.

Turning on the microscope turret: a new view for the study of brassinosteroid signaling in plant development

Brassinosteroid (BR) hormones are essential for plant growth and development. In Arabidopsis, the general understanding of BR signaling has been greatly attained by genetic and biochemical approaches that led to the identification of central BR signaling components, from the BRI1 receptor at the plasma membrane to downstream acting BR-regulated BRZ1 and BES1 transcription factors in the nuclei. Recently, an emerging trend is being established to further advance our understanding of the BR signaling pathway in plant development. Scientists have turned on the microscope lens turret to revisit the pleiotropic phenotypes of the BR mutants at a higher magnification, uncovering novel and specific cellular defects in the plant. In-depth phenotypic analysis in combination with the search for cell-specific signaling components that are responsible for those particular defects in the mutants are leading to: (1) definition of novel roles for BRs in vascular development, (2) unraveling BR function in cell division through quantitative analysis of Arabidopsis root growth, (3) establishment of a molecular connection between known patterning and BR-signaling components in organ boundary and stomata development and (4) development of novel strategies toward the identification of BR signaling components with spatiotemporal resolution. In this review, we highlight the importance of these emerging studies to investigate the spatiotemporal control of BR pathways in plant development.

Two for all: receptor-associated kinases SOBIR1 and BAK1

Leucine-rich repeat-receptor-like proteins (LRR-RLPs) are ubiquitous cell surface receptors lacking a cytoplasmic signalling domain. For most of these LRR-RLPs, it remained enigmatic how they activate cellular responses upon ligand perception. Recently, the LRR-receptor-like kinase (LRR-RLK) SUPPRESSOR OF BIR1-1 (SOBIR1) was shown to be essential for triggering defence responses by certain LRR-RLPs that act as immune receptors. In addition to SOBIR1, the regulatory LRR-RLK BRI1-ASSOCIATED KINASE-1 (BAK1) is also required for LRR-RLP function. Here, we compare the roles of SOBIR1 and BAK1 as regulatory LRR-RLKs in immunity and development. BAK1 has a general regulatory role in plasma membrane-associated receptor complexes comprising LRR-RLPs and/or LRR-RLKs. By contrast, SOBIR1 appears to be specifically required for the function of receptor complexes containing LRR-RLPs.