Loss of Function of a Rice brassinosteroid insensitive1 Homolog Prevents Internode Elongation and Bending of the Lamina Joint

Abstract

Brassinosteroids (BRs) are plant growth-promoting natural products required for plant growth and development. Physiological studies have demonstrated that exogenous BR, alone or in combination with auxin, enhance bending of the lamina joint of rice. However, little is known about the function of endogenous BR in rice or other grass species. We report here the phenotypical and molecular characterization of a rice dwarf mutant, d61 , that is less sensitive to BR compared to the wild type. We cloned a rice gene, OsBRI1 , with extensive sequence similarity to that of the Arabidopsis BRI gene, which encodes a putative BR receptor kinase. Linkage analysis showed that the OsBRI1 gene is closely linked to the d61 locus . Single nucleotide substitutions found at different sites of the d61 alleles would give rise to amino acid changes in the corresponding polypeptides. Furthermore, introduction of the entire OsBRI1 coding region, including the 5 Ј and 3 Ј flanking sequences, into d61 plants complemented the mutation to display the wild-type phenotype. Transgenic plants carrying the antisense strand of the OsBRI1 transcript showed similar or even more severe phenotypes than those of the d61 mutants. Our results show that OsBRI1 functions in various growth and developmental processes in rice, including (1) internode elongation, by inducing the formation of the intercalary meristem and the longitudinal elongation of internode cells; (2) bending of the lamina joint; and (3) skotomorphogenesis.

Heterotrimeric G protein α subunit is involved in rice brassinosteroid response

“…Since d61-1 mutant is well-documented as a rice BRs insensitive mutant by Yamamuro et al [16], it was used as a positive control in root elongation inhibition assay. Our data clearly showed that d1 mutant was less sensitive to 24-EBL compared to wild-type plants.…”

“…In the presence of BR, the coleoptile of wild-type plants elongated more than that in the absence of BR; even the foliage leaves cannot break out of the coleoptile at high concentrations of BR in light [16]. These results suggested that BR is involved in the process of coleoptile elongation, and the extent of elongation can be considered reflect sensitivity to BR, since the length of elongation increased with increasing BR concentration.…”

“…japonica Nipponbare), d1 mutant [7] and d61-1 mutant [16] were used. The seeds were germinated at 30 °C in darkness for 2 days, and then transferred to agar plates containing half-strength Murashige and Skoog (MS) medium supplemented with different phytohormones.…”

“…High concentrations of brassinolide can inhibit root growth in rice [16]. To test the response of the d1 mutant to 24-epiBL, 2-day-old seedlings were treated with 0.1 nM to 10 µM 24-epiBL for 3 days to monitor the length of primary roots.…”

“…Both BRinsensitive and -deficient rice mutants display erect leaves [16,19,20]. BR-insensitive mutants d61-1 and d61-2 were found to be less sensitive to BL than the wild type on lamina inclination assay [16]. To further identify the sensitivity of the d1 mutant to 24-epiBL, lamina inclination assay was adopted.…”