Top Bending Panicle1 is involved in brassinosteroid signaling and regulates the plant architecture in rice.

Brassinosteroids (BRs) regulate several aspects of plant growth and development. Although extensive studies have shown that BR signaling is conservative in higher plants, the molecular mechanism of regulating plant architecture in rice still remains to be explored. Here, we characterized a rice mutant named top bending panicle1 (tbp1). Compared to wild type, tbp1 mutant plants displayed semi-dwarf stature, erect leaves, small and round grains, as well as more tillers. Remarkably, the panicles of tbp1 plants were shorter and denser, and the tops of the panicles were curved by rolling of the base of flag leaves, which was later verified as due to reduced bulliform cell numbers. Map-based cloning, together with transgenic complementation and RNA-interference tests, revealed that TBP1 is a member of the somatic embryogenesis receptor kinases (SERKs) family involved in BR signaling. Furthermore, bimolecular fluorescence complementation and co-immunoprecipitation analysis demonstrated that a substitution at 61st amino acid (His 61Leu ) in the tbp1 mutant may result in a reduction of the interaction between TBP1 and OsBRI1 (BR receptor in rice). Taken together, our results demonstrate that TBP1 plays a significant role in regulating plant architecture via the brassinosteroid signaling pathway.