**1. Introduction**

In cereal crops, leaf angle (defined as the angle between the leaf blade and the leaf sheath) is a key factor determining plant architecture, which also includes plant height, tiller number, and panicle morphology [1,2]. In cereal crops including rice (*Oryza sativa*), plant architecture has been an important agronomic trait for increasing crop yield. In particular, leaf angle is closely associated with photosynthetic capacity [3]. Plants with erect leaves capture more sunlight for photosynthesis and are amenable to much denser planting in populations with a high leaf area index for increasing total grain yield. The lamina joint, which connects the leaf blade and leaf sheath, is central in controlling leaf angle [4], as the degree of leaf inclination largely depends on cell proliferation or cell expansion as well as the cell wall composition at the lamina joint.

Brassinosteroid (BR) phytohormones affect lamina joint morphology and increase leaf angle in rice [5]. BRs are a group of steroid phytohormones that are widely distributed in plants; more than 69 types of BRs have been isolated from diverse plants [6]. BRs play pivotal roles in cell expansion, cell division, vascular bundle differentiation, male fertility, senescence, seed germination, grain filling, photomorphogenesis, flowering time, root growth, and abiotic/biotic stress responses [7–13]. In rice, BR functions in the regulation of grain size, leaf angle, and yield potential. For instance, several mutants with low BR contents or weak BR signaling, such as *dwarf2* (*d2*), *d11*, and *d61*, exhibit dwarfism and produce short grains and erect leaves [14–16]. Additionally, many genes have a role in controlling leaf angle, such as *TILLER ANGLE1* (*Ta1*), *EBISU DWARF* (*D2*), *INCREASED LAMINA INCLINATION1* (*ILI1*), *LEAF INCLINATION2* (*LC2*), *INCREASED LEAF ANGLE1* (*ILA1*), and *SLENDER GRAIN* (*SLG*) [1,17–21]. Moreover, loss-of-function mutants of BR-related genes, including *OsDWARF4* and *OsBRI1*, show improved grain yield due to their ability to be planted at a higher density and their enhanced photosynthetic rate [1,22]. Therefore, understanding the effects of BR on rice architecture has important implications for improving yield.

BR signal transduction has been intensively studied in *Arabidopsis thaliana* [23]. Under normal BR levels, BR interacts with BRASSINOSTEROID INSENSITIVE1 (BRI1) and BRASSINOSTEROID ASSOCIATED RECEPTOR KINASE1 (BAK1), forming a BRI–BR–BAK1 complex [24,25]. This complex inhibits the activity of BRASSINOSTEROID INSENSITIVE2 (BIN2) and activates PHOSPHATASE 2A (PP2A) for the activation of BRASSINAZOLE RESISTANT1 (BZR1). The activated BZR1 is translocated into the nucleus and regulates its downstream genes at the transcriptional level [23,26–28].

In rice, the BR signaling pathway remains largely unknown, since only a few components have been reported [23]. BR interacts with OsBRI1 and is involved in the formation of the OsBRI1–OsBAK1 complex [29,30], which inactivates OsBIN2 by an unknown pathway [23]. OsBIN2 phosphorylates OsBZR1, LEAF AND TILLER ANGLE INCREASED CONTROLLER (LIC), and DWARF AND LOW TILLERING (DLT) and inhibits their activities. OsBZR1 upregulates *ILI1* and downregulates *LIC* and *DLT*, thus transmitting the BR signal to their downstream genes, which affect plant growth and development [23].

BR mainly affects cell elongation and cell division; moreover, cell number and cell size largely determine organ size during organogenesis [31,32]. Grain size (GS), another key trait determining yield, is mainly determined by grain length (GL), grain width (GW), and grain thickness, all of which are closely related to cell elongation or cell division. Various genes and quantitative trait loci (QTLs) in rice, such as *GS3*, *GS5*, *GW2*, *GW5*, *GW8*, *GW6a*, *qGL3*, *THOUSAND-GRAIN WEIGHT6* (*TGW6*), and *BIG GRAIN1* (*BG1*), affect grain size by regulating cell number [33–41]. In addition, *GS2*/*GL2*, *GL7*, and *POSITIVE REGULATOR OF GRAIN LENGTH1* (*PGL1*) regulate grain size by influencing cell size in rice [40,42,43].

The basic helix–loop–helix (bHLH) domain transcription factors act in various biological processes in animals and plants [44]. In flowering plants, 162 bHLH proteins have been identified in *Arabidopsis thaliana* and 167 in rice [45]. These proteins are divided into two groups: typical bHLH proteins harboring both motifs (basic and HLH motif) bind to DNA through the basic region, whereas atypical, non-DNA-binding bHLH proteins lacking the basic region require other bHLH proteins to bind to DNA as protein dimers [46]. For example, rice ILI1 is an atypical bHLH protein that interacts with the typical bHLH protein OsIBH1 and represses OsIBH1 function [47]. This antagonistic regulation controls cell length in the lamina joint. Several bHLH transcription factors, such as BRASSINOSTEROID UPREGULATED1 (BU1), *O. sativa* BU1-LIKE1 (OsBUL1), and OsbHLH107, are involved in controlling leaf angle or grain size in rice [47–50].

In this study, we show that OsbHLH079 acts as a key regulator in determining leaf angle and grain length. *OsbHLH079-*overexpressing lines exhibited exaggerated leaf inclination, with longer cells on the adaxial surface of lamina joint. In addition, *OsbHLH079* is involved in modulating grain shape because the *OsbHLH079-*overexpressing mutant produced long grains. Several molecular genetic approaches showed that the function of OsbHLH079 is closely associated with the BR signaling pathway. This study provides new insight into the roles of OsbHLH079 in determining leaf angle and grain shape.
