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Scientific Reports
Brassinosteroids (BRs), plant steroid hormones, play important roles in plant cell elongation and differentiation. To investigate the mechanisms of BR signaling, we previously used the BR biosynthesis inhibitor Brz as a chemical biology tool and identified the Brz-insensitive-long hypocotyl4 mutant (bil4). Although the BIL4 gene encodes a seven-transmembrane-domain protein that is evolutionarily conserved in plants and animals, the molecular function of BIL4 in BR signaling has not been elucidated. Here, we demonstrate that BIL4 is expressed in early elongating cells and regulates cell elongation in Arabidopsis. BIL4 also activates BR signaling and interacts with the BR receptor brassinosteroid insensitive 1 (BRI1) in endosomes. BIL4 deficiency increases the localization of BRI1 in the vacuoles. Our results demonstrate that BIL4 regulates cell elongation and BR signaling via the regulation of BRI1 localization. Steroid hormones are bioactive substances that are widely conserved in eukaryotes 1 and serve as signaling molecules to control growth and development. The plant steroid hormones known as brassinosteroids (BRs) play important roles in plant development and in responses to environmental cues, such as cell elongation, cell division, xylem development, stresses and pathogen resistance 2-6. BRs are recognized by the BR receptor brassinosteroid insensitive 1 (BRI1), a Ser/Thr kinase, which has a leucine-rich repeat (LRR) domain and resides on the plasma membrane and endosomes. The dwarf phenotype of BRI1-deficient mutants (bri1) 7 suggests that BRI1 plays an important role in plant growth. The plasma membrane proteins BRI1-associated receptor kinase 1 (BAK1) 8, 9 , BR-signaling kinase 1 (BSK1) 10 , and BRI1 kinase inhibitor 1 (BKI1) 11 are involved in BR signaling by associating with BRI1. BRI1 is localized not only to plasma membranes but also to endosomes 12. The requirement that ligand-bound receptors, including innate immunity-related Toll-like receptors (TLRs), be localized to endosomes has been widely observed in animals 13. BRI1 endocytosis is regulated by the guanine nucleotide exchange factor for ADP-ribosylation factor (ARF-GEF) GNOM 14 , adaptor protein complex-2 (AP-2) 15 , and plant-specific TPLATE adaptor complex (TPC), which is associated with key components of clathrin-mediated endocytosis (CME) 16. BRI1 is considered to translocate to the vacuoles through processes that are regulated by ubiquitination 17 and
Differential expression of the brassinosteroid receptor-encoding BRI1 gene in Arabidopsis
Planta, 2014
the wild phenotype, expression from the photosynthesisassociated CAB3 and the vasculature-specific SUC2 and ATHB8 promoters resulted in plants with varying morphogenic defects. Our results reveal complex differential regulation of BRI1 expression, and suggest that by influencing the distribution and abundance of the receptor this regulation can enhance or attenuate BR signalling.
Oligomerization between BSU1 Family Members Potentiates Brassinosteroid Signaling in Arabidopsis
Molecular Plant, 2015
Brassinosteroid (BR) regulates diverse physiological and developmental processes in plants (Choudhary et al., 2012). BR binds to the receptor kinase BRI1 to trigger a phosphorylation/ dephosphorylation cascade, which includes phosphorylation of Brassinosteroid Signaling Kinase 1 (BSK1) and Constitutive Differential Growth 1 (CDG1) by BRI1, phosphorylation of BSU1 by CDG1, and dephosphorylation of BIN2 (a GSK3-like kinase) by BSU1 (Kim et al., 2009, 2011; Wang et al., 2014). Almost all the downstream components of BR signaling are encoded by multiple genes of a family. The Arabidopsis BSU1 family (BSUf; BSU1, BSL1, BSL2, and BSL3) is classified into a unique phosphatase family, the PPKL (Protein Phosphatases with Kelch-Like repeat domains), which is not found in animals (Kutuzov and Andreeva, 2002; Mora-Garcia et al., 2004). BSUf members contain an N-terminal Kelch-repeat domain and a C-terminal phosphatase domain that is closely related to protein phosphatase 1. Knockdown of BSL2 and BSL3 by artificial micro-RNA in the bsu1bsl1 mutant (amiBSL2,3/bsu1bsl1) causes pleiotropic defects, including severe dwarfism similar to strong BR-deficient or BR-insensitive mutants and abnormal stomatal development (Kim et al., 2009, 2012; Youn et al., 2013). A recent study suggested that BSUf members exhibit specific/non-overlapping functions in addition to BR signaling, although all four members are involved in BR signaling (Maselli et al., 2014). However, the biochemical characteristics underlying the functional redundancy and diversity of PPKL family members are largely unknown.
Plant physiology, 2015
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). BRI1is a BR receptor and database searches and additional genomic sequencing identified 5 maize homologs including duplicate copies of BRI1 itself. RNA interference (RNAi) using the extracellular coding region of a maize zmbri1 cDNA knocked down expression of all 5 homologs. Decreased response to exogenously applied brassinolide (BL), 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 con...
Developmental Cell, 2005
from most RTKs (Shiu and Bleecker, 2001b). Among the and Joanne Chory 1, * plant RLKs, 214 genes belong to a super class of LRR-1 Howard Hughes Medical Institute RLKs that feature an extracellular leucine-rich-repeat and Plant Biology Laboratory (LRR) domain. These LRR-RLKs have diverse biological 2 Molecular and Cell Biology Laboratory functions in disease resistance, growth, and develop-The Salk Institute for Biological Studies ment (see reviews of Shiu and Bleecker, 2001a; Becraft, La Jolla, California 92037 2002; Morris and Walker, 2003; Diévart and Clark, 2004). 3 Plant Functions Laboratory However, unlike many animal receptor kinases, the RIKEN knowledge of a ligand-induced activation mechanism The Institute of Physical and Chemical Research of the plant RLKs is lacking. Wako-shi, Saitama 351-0198 Brassinosteroid insensitive 1 (BRI1) is one of the few Japan plant LRR-RLKs with a known ligand and a well-defined downstream signaling pathway. BRI1 binds brassinolide (BL), the most active brassinosteroid (BR), at the Summary cell surface via its extracellular domain (Kinoshita et al., 2005) and regulates various aspects of plant develop-The leucine-rich-repeat receptor serine/threonine kiment, including stem elongation, pollen tube growth, nase, BRI1, is a cell-surface receptor for brassinosterfertility, senescence, and resistance to biotic and abioids (BRs), the steroid hormones of plants, yet its acotic stresses (Clouse et al., 1996; Mandava, 1988). So tivation mechanism is unknown. Here, we report a far, all known bri1 alleles are loss-of-function mutants unique autoregulatory mechanism of BRI1 activation. resulting in extremely dwarfed plants with delayed leaf Removal of BRI1's C terminus leads to a hypersensisenescence and with reduced male fertility or complete tive receptor, indicated by suppression of dwarfism sterility (Li and Chory, 1997; Noguchi et al., 1999). BRof BR-deficient and BR-perception mutants and by deficient mutants display similar phenotypes (Fujioka enhanced BR signaling as a result of elevated phosand Yokota, 2003), indicating that BRI1 plays a key role phorylation of BRI1. Several sites in the C-terminal in BR perception. region can be phosphorylated in vitro, and transgenic In addition to BRI1, another LRR-RLK, BAK1, can in-Arabidopsis expressing BRI1 mutated at these sites teract with BRI1 and may be involved in the early steps demonstrates an essential role of phosphorylation in of BR signaling (Nam and Li, 2002; Li et al., 2002). BRI1 activation. BRI1 is a ligand-independent homo-Downstream of BRI1, BIN2, a glycogen synthase kioligomer, as evidenced by the transphosphorylation nase-3 family member (Li and Nam, 2002) acts as a of BRI1 kinase in vitro, the dominant-negative effect negative regulator by phosphorylating two nearly idenof a kinase-inactive BRI1 in transgenic Arabidopsis, tical positive regulators, BES1 and BZR1, thereby and coimmunoprecipitation experiments. Our results targeting them for turnover by the proteasome (Wang support a BRI1-activation model that involves inhibiet al., 2002; Yin et al., 2002; He et al., 2002; Zhao et al., tion of kinase activity by its C-terminal domain, which 2002). In the presence of BRs, BIN2 is inactivated by is relieved upon ligand binding to the extracellular an unknown mechanism, allowing hypophosphorylated domain. BES1 and BZR1 to accumulate in the nucleus where they regulate BR-responsive gene expression (Yin et Introduction al., 2005; He et al., 2005). BSU1, a nuclear serine/threonine phosphatase, counteracts the action of BIN2 to Plasma-membrane-localized receptor kinases play regulate the dephosphorylation and accumulation of essential roles in perceiving and processing various BES (Mora-García et al., 2004). Although a significant stimuli in both plants and animals. The Arabidopsis gegap in the pathway is present between BRI1 and BIN2, nome contains more than 400 genes encoding putative the phosphorylation status and abundance of BES1/ receptor-like kinases (RLKs) with a variety of extracellu-BZR1 proteins appears to be a key downstream regulalar domains (Shiu and Bleecker, 2001b), which together tory point in the pathway and can be used as a biorepresent one of the largest gene families in Arabidopchemical measure of BRI1's signaling output. These sis and provide a great challenge to dissect their funcfindings make BRI1 an excellent system for the investions and regulatory mechanisms. Although these plant tigation of the regulatory mechanisms of the plant RLKs are structurally similar to their animal counter-LRR-RLKs. parts, there are some major distinctions between the Mutational analyses of BRI1 and other LRR-RLKs intwo. All plant RLKs are predicted to be serine/threonine dicate that the intracellular kinase domain is essential kinases, whereas most animal receptor kinases are tyfor their function. BRs induce BRI1 phosphorylation in rosine kinases (RTKs) (Schlessinger, 2000), apart from vivo (Wang et al., 2001), but the biological relevance of a notable exception, the transforming growth factor β BRI1 phosphorylation and molecular mechanisms involved in this process remain unknown. In addition, receptor oligomerization is a general principle for the li-
Plant Physiology
Brassinosteroids (BRs) are an important group of plant steroid hormones that regulate growth and development. Several members of the SMALL AUXIN UP RNA (SAUR) family have roles in BR-regulated hypocotyl elongation and root growth. However, the mechanisms are unclear. Here, we show in Arabidopsis (Arabidopsis thaliana) that SAUR15 interacts with cell surface receptor-like kinase BRASSINOSTEROID-INSENSITIVE 1 (BRI1) in BR-treated plants, resulting in enhanced BRI1 phosphorylation status and recruitment of the co-receptor BRI1-ASSOCIATED RECEPTOR KINASE 1. Genetic and phenotypic assays indicated that the SAUR15 effect on BRI1 can be uncoupled from BRASSINOSTEROID INSENSITIVE 2 activity. Instead, we show that SAUR15 promotes BRI1 direct activation of plasma membrane H+-ATPase (PM H+-ATPase) via phosphorylation. Consequently, SAUR15–BRI1–PM H+-ATPase acts as a direct, PM-based mode of BR signaling that drives cell expansion to promote the growth and development of various organs. These d...
An Essential Role for 14-3-3 Proteins in Brassinosteroid Signal Transduction in Arabidopsis
Developmental Cell, 2007
Brassinosteroids (BRs) are essential hormones for plant growth and development. BRs regulate gene expression by inducing dephosphorylation of two key transcription factors BZR1 and BZR2/BES1 through a signal transduction pathway, which involves cell surface receptors (BRI1 and BAK1) and a GSK3 kinase (BIN2). How BR-regulated phosphorylation controls the activities of BZR1/BZR2 is not fully understood. Here we show that BIN2-catalyzed phosphorylation of BZR1/ BZR2 not only inhibits DNA binding but also promotes binding to the 14-3-3 proteins. Mutations of a BIN2-phosphorylation site in BZR1 abolish 14-3-3 binding and lead to increased nuclear localization of BZR1 protein and enhanced BR-responses in transgenic plants. Further, BRdeficiency increases cytoplasmic localization and BR treatment induces rapid nuclear localization of BZR1/BZR2. Thus 14-3-3 binding is required for efficient inhibition of phosphorylated BR transcription factors largely through cytoplasmic retention. This study demonstrates that multiple mechanisms are required for BR regulation of gene expression and plant growth.
Plant and Cell Physiology, 2008
Loss-of-function alleles of the sole heterotrimeric G-protein a subunit in Arabidopsis, GPA1, display defects in cell proliferation throughout plant development. Previous studies indicated that GPA1 is involved in brassinosteroid (BR) response. Here we provide genetic evidence that lossof-function mutations in GPA1, gpa1-2 and gpa1-4, enhance the developmental defects of bri1-5, a weak allele of a BR receptor mutant, and det2-1, a BR-deficient mutant in Arabidopsis. gpa1-2 bri1-5 and gpa1-4 det2-1 double mutants had shorter hypocotyls, shorter roots and fewer lateral roots, and displayed more severe dwarfism than bri1-5 and det2-1 single mutants, respectively. By using the Arabidopsis hypocotyl as a model system where the parameters of cell division and cell elongation can be simultaneously measured, we found that gpa1 can specifically enhance the cell division defects of bri1-5 and det2-1 mutants. Similarly, gpa1 specifically enhances cell division defects in the primary roots of bri1-5 and det2-1 mutants. Furthermore, an additive effect on cell division between gpa1 and bri1-5 or det2-1 mutations was observed in the hypocotyls, whereas a synergistic effect was observed in the roots. Taken together, these results provided the first genetic evidence that G-protein-and BR-mediated pathways may be converged to modulate cell proliferation in a cell/tissue-specific manner.