*2.8. OsbHLH079 Increases Grain Length by Altering TGW6 Expression*

In rice, several genes and QTLs, including *GS3*, *GS5*, *GW2*, *GW5*, *GW8*, *TGW6*, *GW6a*, *qGL3*, and *BG1*, affect grain size by regulating cell number, and *GS2*/*GL2*, *GL7*, and *PGL1* affect grain size by influencing cell size [33–43]. GW2 is a RING-type E3 ubiquitin ligase and acts as a negative regulator of cell division [33]. *GW2*-overexpressing transgenic rice showed a reduced grain-width phenotype. Increased expression of *GW2* could be one of the reasons for the slender-grain phenotype of the *osbhlh079-D* mutant (Figure S1). *TGW6* encodes an IAA-glucose hydrolase, and loss of function of *TGW6* results in enhanced grain weight, resulting in increased grain yield [38]. *RNAi-TGW6* rice plants showed an increased-grain-length phenotype, which could be one of the reasons for the long-grain phenotype of the *osbhlh079-D* mutant (Figure S1). It is possible that the regulatory function of OsbHLH079 is associated with not only the BR signaling pathway, but also the auxin-related pathway, which is closely involved in the control of grain size and grain yield [38].

### *2.9. OsbHLH079 is an Ortholog of Arabidopsis CRYPTOCHROME-INTERACTING bHLH 1*

In the genome-wide analysis, 167 *bHLH* genes in rice and 162 *bHLH* genes in *Arabidopsis* were identified and analyzed by amino acid sequence-based alignments. As a result, they were divided into 25 subfamilies [45]. *OsbHLH079* belongs to the C group with 24 genes in rice and 23 genes in *Arabidopsis*. The C group contains all the *Arabidopsis CRYPTOCHROME-INTERACTING bHLH* (*AtCIB*) genes (*AtCIB1, AtCIB2, AtCIB3, AtCIB4*, and *AtCIB5*), and *OsbHLH079* was annotated to be an *AtCIB1*-like gene [45]. In *Arabidopsis*, CIB1 interacts with cryptochrome 2 (CRY2), and this complex affects various developmental processes, such as hypocotyl elongation, flowering time, stomata opening, hypocotyl bending, programmed cell death, plastid development, and silique elongation [35,78–85]. In soybean, GmCIB1 binds to the E-box (CANNTG) motif and acts as a transcriptional activator to regulate leaf senescence-associated genes [86]. In rice, OsbHLH079 might be an ortholog of CIB1, and the bHLH domain was a highly conserved among amino acid sequences of AtCIB1, GmCIB1, and OsbHLH079 (Figure S2). Additionally, the *osbhlh079-D* mutant flowered earlier than WT under long-day and short-day conditions (Figure S3), which provides more insight into additional regulatory functions of OsbHLH079 in rice growth and development, similar to AtCIB1.
