OsbHLH073 Negatively Regulates Internode Elongation and Plant Height by Modulating GA Homeostasis in Rice

Abstract

Internode elongation is one of the key agronomic traits determining a plant's height and biomass. However, our understanding of the molecular mechanisms controlling internode elongation is still limited in crop plant species. Here, we report the functional identification of an atypical basic helix-loop-helix transcription factor (OsbHLH073) through gain-of-function studies using overexpression (OsbHLH073-OX) and activation tagging (osbhlh073-D) lines of rice. The expression of OsbHLH073 was significantly increased in the osbhlh073-D line. The phenotype of osbhlh073-D showed semi-dwarfism due to deficient elongation of the first internode and poor panicle exsertion. Transgenic lines overexpressing OsbHLH073 confirmed the phenotype of the osbhlh073-D line. Exogenous gibberellic acid (GA 3 ) treatment recovered the semi-dwarf phenotype of osbhlh073-D plants at the seedling stage. In addition, quantitative expression analysis of genes involving in GA biosynthetic and signaling pathway revealed that the transcripts of rice ent-kaurene oxidases 1 and 2 (OsKO1 and OsKO2) encoding the GA biosynthetic enzyme were significantly downregulated in osbhlh073-D and OsbHLH073-OX lines. Yeast two-hybrid and localization assays showed that the OsbHLH073 protein is a nuclear localized-transcriptional activator. We report that OsbHLH073 participates in regulating plant height, internode elongation, and panicle exsertion by regulating GA biosynthesis associated with the OsKO1 and OsKO2 genes.

Meta-Analysis of Yield-Related and N-Responsive Genes Reveals Chromosomal Hotspots, Key Processes and Candidate Genes for Nitrogen-Use Efficiency in Rice

“…OsbHLH TFs are known for their roles in regulating grain length, internode elongation, plant height, and yield in rice (Heang and Sassa, 2012a , b , c ; Yang et al, 2018 ; Lee et al, 2020 ; Seo et al, 2020 ). In the present study, we proposed a model for the functional role of OsbHLH079 and its target genes in carbon–nitrogen metabolism ( Figure 4B ).…”

Modulation of Rice Leaf Angle and Grain Size by Expressing OsBCL1 and OsBCL2 under the Control of OsBUL1 Promoter

“…Interestingly, a novel mechanism accounting for the negative regulation of rice leaf inclination was reported in a recent study showing that OsbHLH98, a typical bHLH transcription factor, counteracts the BR-induced cell elongation through transcriptional repression of OsBUL1 encoding an atypical bHLH protein. Thus, based on the increased GA levels in the transgenic rice plants containing pOsBUL1:OsBCL1 and pOsBUL1:OsBCL2, it will be worth investigating whether OsBCL1/2 has a suppressive effect on the OsbHLH073, a negative regulator of GA biosynthesis containing an atypical bHLH domain, through GA-related pathways for cell elongation [34,41]. Protein-to-protein interaction patterns also support the distinct regulation of OsBCL1 and OsBCL2.…”

“…In particular, 167 bHLH proteins have been identified in rice. These proteins can be divided into two groups, the atypical non-DNA-binding and the typical DNA-binding bHLH family based on to their DNA-binding activity [29], and they can be positive or negative growth regulators by interacting in an antagonistic and redundant manner to regulate various biological processes involved in growth including cell elongation, biosynthesis, stress resistance and signal transduction pathways [2,5,[30][31][32][33][34].…”

“…It has also been reported that some bHLH proteins are involved in hormone biosynthesis and/or signaling in plants [2,[34][35][36][37]. Arabidopsis PIF3, PIF4, PIF5, and PIF3-LIKE 5 (PIL5) are involved in the GA biosynthesis and signaling pathway [35,37] and the function of rice OsbHLH073 is associated with GA biosynthesis [34].…”

“…It has also been reported that some bHLH proteins are involved in hormone biosynthesis and/or signaling in plants [2,[34][35][36][37]. Arabidopsis PIF3, PIF4, PIF5, and PIF3-LIKE 5 (PIL5) are involved in the GA biosynthesis and signaling pathway [35,37] and the function of rice OsbHLH073 is associated with GA biosynthesis [34]. Other bHLH proteins play important roles in controlling BR signaling: Arabidopsis BEE1, BEE2, and BEE3, as products of early response genes are required for full BR response [35] and AIF2 interacts with BIN2 to participate in the BR signaling pathway [38].…”