| Summary: | <p>Abstract</p> <p>Background</p> <p>Since the identification of BRI1 (BRASSINOSTEROID-INSENSITIVE1), a brassinosteroids (BRs) receptor, most of the critical roles of BR in plant development have been assessed using various <it>bri1 </it>mutant alleles. The characterization of individual <it>bri1 </it>mutants has shown that both the extracellular and cytoplasmic domains of BRI1 are important to its proper functioning. Particularly, in the extracellular domain, regions near the 70-amino acid island are known to be critical to BR binding. In comparison, the exact function of the leucine rich-repeats (LRR) region located before the 70-amino acid island domain in the extracellular cellular portion of BRI1 has not yet been described, due to a lack of specific mutant alleles.</p> <p>Results</p> <p>Among the mutants showing altered growth patterns compared to wild type, we further characterized <it>cp3</it>, which displayed defective growth and reduced BR sensitivity. We sequenced the genomic DNA spanning <it>BRI1 </it>in the <it>cp3 </it>and found that <it>cp3 </it>has a point mutation in the region encoding the 13<sup>th </sup>LRR of BRI1, resulting in a change from serine to phenylalanine (S399F). We renamed it <it>bri1-120</it>. We also showed that overexpression of the wild type BRI1 protein rescued the phenotype of <it>bri1-120</it>. Using a GFP-tagged <it>bri1-120 </it>construct, we detected the <it>bri1-120 </it>protein in the plasma membrane, and showed that the phenotypic defects in the rosette leaves of <it>bri1-301</it>, a kinase-inactive weak allele of <it>BRI1</it>, can be restored by the overexpression of the <it>bri1-120 </it>proteins in <it>bri1-301</it>. We also produced <it>bri1-301 </it>mutants that were wild type in appearance by performing a genetic cross between <it>bri1-301 </it>and <it>bri1-120 </it>plants.</p> <p>Conclusions</p> <p>We identified a new <it>bri1 </it>allele, <it>bri1-120</it>, whose mutation site has not yet been found or characterized. Our results indicated that the extracellular LRR regions before the 70-amino acid island domain of BRI1 are important for the appropriate cellular functioning of BRI1. Also, we confirmed that a successful interallelic complementation occurs between the extracellular domain mutant allele and the cytoplasmic kinase-inactive mutant allele of <it>BRI1 in vivo.</it></p>
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