*Article* **The Rice** *Basic Helix–Loop–Helix 79* **(***OsbHLH079***) Determines Leaf Angle and Grain Shape**

**Hyoseob Seo 1,**†**, Suk-Hwan Kim 1,**†**, Byoung-Doo Lee 1,**†**, Jung-Hyun Lim 1, Sang-Ji Lee 1, Gynheung An <sup>2</sup> and Nam-Chon Paek 1,\***


Received: 26 February 2020; Accepted: 17 March 2020; Published: 18 March 2020

**Abstract:** Changes in plant architecture, such as leaf size, leaf shape, leaf angle, plant height, and floral organs, have been major factors in improving the yield of cereal crops. Moreover, changes in grain size and weight can also increase yield. Therefore, screens for additional factors affecting plant architecture and grain morphology may enable additional improvements in yield. Among the basic Helix-Loop-Helix (bHLH) transcription factors in rice (*Oryza sativa*), we found an enhancer-trap T-DNA insertion mutant of *OsbHLH079* (termed *osbhlh079-D*). The *osbhlh079-D* mutant showed a wide leaf angle phenotype and produced long grains, similar to the phenotypes of mutants with increased brassinosteroid (BR) levels or enhanced BR signaling. Reverse transcription-quantitative PCR analysis showed that BR signaling-associated genes are largely upregulated in *osbhlh079-D*, but BR biosynthesis-associated genes are not upregulated, compared with its parental *japonica* cultivar 'Dongjin'. Consistent with this, *osbhlh079-D* was hypersensitive to BR treatment. Scanning electron microscopy revealed that the expansion of cell size in the adaxial side of the lamina joint was responsible for the increase in leaf angle in *osbhlh079-D*. The expression of cell-elongation-associated genes encoding expansins and xyloglucan endotransglycosylases/hydrolases increased in the lamina joints of leaves in *osbhlh079-D*. The regulatory function of OsbHLH079 was further confirmed by analyzing *35S::OsbHLH079* overexpression and *35S::RNAi-OsbHLH079* gene silencing lines. The *35S::OsbHLH079* plants showed similar phenotypes to *osbhlh079-D*, and the *35S::RNAi-OsbHLH079* plants displayed opposite phenotypes to *osbhlh079-D*. Taking these observations together, we propose that *OsbHLH079* functions as a positive regulator of BR signaling in rice.

**Keywords:** bHLH transcription factor; lamina joint; leaf angle; long grain; brassinosteroid signaling
