Brassinosteroids (BR) are essential growth hormones found throughout the plant kingdom. BR bind to the receptor kinase BRI1 on the cell surface to activate a signal transduction pathway that regulates nuclear gene expression and plant growth. To understand the downstream BR signaling mechanism in rice, we studied the function of OsBZR1 using reverse genetic approaches and identified OsBZR1-interacting proteins. Suppressing OsBZR1 expression by RNAi resulted in dwarfism, erect leaves, reduced BR sensitivity, and altered BR-responsive gene expression in transgenic rice plants, demonstrating an essential role of OsBZR1 in BR responses in rice. Moreover, a yeast two-hybrid screen identified 14-3-3 proteins as OsBZR1-interacting proteins. Mutation of a putative 14-3-3-binding site of OsBZR1 abolished its interaction with the 14-3-3 proteins in yeast and in vivo. Such mutant OsBZR1 proteins suppressed the phenotypes of the Arabidopsis bri1-5 mutant and showed an increased nuclear distribution compared with the wild-type protein, suggesting that 14-3-3 proteins directly inhibit OsBZR1 function at least in part by reducing its nuclear localization. These results demonstrate a conserved function of OsBZR1 and an important role of 14-3-3 proteins in brassinosteroid signal transduction in rice.steroid ͉ nuclear localization ͉ GSK3 ͉ phosphorylation B rassinosteroids (BR) are polyhydroxylated steroid phytohormones with important roles in regulating a wide range of cellular and development processes, such as stem elongation, senescence, seed germination, reproductive development, and photomorphogenesis (1, 2). BR-deficient and BR-insensitive mutants have been identified in Arabidopsis thaliana (1), pea, tomato, barley, and rice (3). Although strong BR mutants show multiple growth defects, including dwarfism, male sterility, delayed senescence, and constitutive photomorphogenesis in the dark (1), weak BR mutants in rice and barley have increased grain yield because of erect leaves and reduced plant height (4-6). It is believed that manipulating BR level or sensitivity can improve plant productivity.Molecular genetic studies in A. thaliana have illustrated a BRsignaling pathway that includes two receptor kinases (BRI1 and BAK1) on the cell surface, a soluble glycogen synthase kinase 3-like kinase (BIN2), a phosphatase (BSU1), and two homologous transcription factors (BZR1 and BZR2/BES1) (1, 7). BR directly bind to the extracellular domain of BRI1 and activate its intracellular kinase activity (8). Activation of BRI1 leads to interaction with, and activation of, another receptor kinase, BAK1, which has been proposed to act as BRI1's coreceptor (9, 10). Downstream of the receptor kinases, the BIN2 kinase and BSU1 phosphatase regulate the phosphorylation status of BZR1 and BZR2/BES1 (11-14). BR activate BZR1 and BZR2/BES1 by inducing their dephosphorylation, possibly by inhibiting BIN2 or activating BSU1. Activated BZR1 and BES1 bind to promoters of BR target genes to regulate their expression (15, 16).BZR1 and BZR2/BES1 share 88...

XA21 is a receptor-like kinase protein in rice (Oryza sativa) that confers gene-for-gene resistance to specific races of the causal agent of bacterial blight disease, Xanthomonas oryzae pv oryzae. We identified XA21 binding protein 3 (XB3), an E3 ubiquitin ligase, as a substrate for the XA21 Ser and Thr kinase. The interaction between XB3 and the kinase domain of XA21 has been shown in yeast and in vitro, and the physical association between XB3 and XA21 in vivo has also been confirmed by coimmunoprecipitation assays. XB3 contains an ankyrin repeat domain and a RING finger motif that is sufficient for its interaction with the kinase domain of XA21 and for its E3 ubiquitin ligase activity, respectively. Transgenic plants with reduced expression of the Xb3 gene are compromised in resistance to the avirulent race of X. oryzae pv oryzae. Furthermore, reduced levels of Xb3 lead to decreased levels of the XA21 protein. These results indicate that Xb3 is necessary for full accumulation of the XA21 protein and for Xa21-mediated resistance.

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SummaryUbiquitin-mediated protein modification plays a key role in many cellular signal transduction pathways. The Arabidopsis gene XBAT32 encodes a protein containing an ankyrin repeat domain at the N-terminal half and a RING finger motif. The XBAT32 protein is capable of ubiquitinating itself. Mutation in XBAT32 causes a number of phenotypes including severe defects in lateral root production and in the expression of the cell division marker CYCB1;1::GUS. The XBAT32 gene is expressed abundantly in the vascular system of the primary root, but not in newly formed lateral root primordia. Treatment with auxin increases the expression of XBAT32 in the primary root and partially rescues the lateral root defect in xbat32-1 mutant plants. Thus, XBAT32 is a novel ubiquitin ligase required for lateral root initiation.

TitleXa21D encodes a receptor-like molecule with a leucine-rich repeat domain that determines race-specific recognition and is subject to adaptive evolution. Publication Date 1998-05-01Peer reviewed eScholarship.orgPowered by the California Digital Library University of CaliforniaThe Plant Cell, Vol. 10, 765-779, May 1998, www.plantcell.org © 1998 The rice Xa21 gene confers resistance to Xanthomonas oryzae pv oryzae in a race-specific manner. Analysis of the inheritance patterns and resistance spectra of transgenic plants carrying six Xa21 gene family members indicated that one member, designated Xa21D , displayed a resistance spectrum identical to that observed for Xa21 but conferred only partial resistance. Xa21D encodes a receptor-like protein carrying leucine-rich repeat (LRR) motifs in the presumed extracellular domain. The Xa21D transcript terminates shortly after the stop codon introduced by the retrotransposon Retrofit . Comparison of nucleotide substitutions in the LRR coding regions of Xa21 and Xa21D provided evidence of adaptive selection. Both functional and evolutionary evidence indicates that the Xa21D LRR domain controls race-specific pathogen recognition. INTRODUCTIONReceptor kinases (RKs) play a key role in important cellular processes in plants and animals (Fantl et al., 1993; Song et al., 1995;Becraft et al., 1996;Heldin and Ostman, 1996; Stein et al., 1996;Ten Dijke et al., 1996;Torii et al., 1996;Li and Chory, 1997). Three functional domains are commonly associated with RK proteins: an extracellular domain, a transmembrane domain, and an intracellular catalytic domain. Studies of animal RKs have revealed a common mechanism for RK-mediated cellular signaling (Hunter, 1995;Pawson, 1995;Heldin and Ostman, 1996). In this model, ligand binding to the extracellular receptor domain induces receptor dimerization and subsequent activation of the intracellular kinase domain. The specificity of the interaction with the ligand is controlled by amino acid residues in the extracellular domain (Heldin and Ostman, 1996). Plant RKs can be divided into six subclasses based on the protein motif in the presumed extracellular domains (Walker, 1994;Becraft et al., 1996). The largest subclass of plant RKs is the leucine-rich repeat (LRR) group, which encodes proteins with an extracellular domain containing 20 to 25 imperfect repeats of a 24-amino acid leucine-rich motif. The LRR subclass of plant RKs includes proteins that govern pollen development, plant elongation, regulation of meristem and flower development, disease resistance, and brassinosteroid signal transduction, as well as other functions that remain to be determined (Chang et al., 1992;Valon et al., 1993; Song et al., 1995;Torii et al., 1996;Clark et al., 1997;Li and Chory, 1997). Plant LRRs have also been found in secreted proteins (polygalacturonase inhibitor proteins or PGIPs) (De Lorenzo et al., 1994) and in membrane-bound resistance gene products (Dixon et al., 1996). LRR domains are present in a variety of proteins involved in peptide ligand ...