The function of OsbHLH068 is partially redundant with its homolog, AtbHLH112, in the regulation of the salt stress response but has opposite functions to control flowering in Arabidopsis

The transcription factor (TF) basic/Helix-Loop-Helix (bHLH) is important for plant growth, development, and stress responses. OsbHLH068, which is a homologous gene of AtbHLH112 that is up-regulated under drought and salt stresses, as indicated by previous microarray data analysis. However, the intrinsic function of OsbHLH068 remains unknown. In the present study, we characterized the function and compared the role of OsbHLH068 with that of its homolog, AtbHLH112. Histochemical GUS staining indicated that OsbHLH068 and AtbHLH112 share a similar expression pattern in transgenic Arabidopsis during the juvenile-to-adult phase transition. Heterologous overexpression of OsbHLH068 in Arabidopsis delays seed germination, decreases salt-induced H2O2 accumulation, and promotes root elongation, whereas AtbHLH112 knock-out mutant displays an opposite phenotype. Both OsbHLH068-overexpressing transgenic Arabidopsis seedlings and the Atbhlh112 mutant display a late-flowering phenotype. Moreover, the expression of OsbHLH068-GFP driven by an AtbHLH112 promoter can compensate for the germination deficiency in the Atbhlh112 mutant, but the delayed-flowering phenotype tends to be more severe. Further analysis by microarray and qPCR indicated that the expression of FT is down-regulated in both OsbHLH068-overexpressing Arabidopsis plants and Atbhlh112 mutant plants, whereas SOC1 but not FT is highly expressed in AtbHLH112-overexpressing Arabidopsis plants. A comparative transcriptomic analysis also showed that several stress-responsive genes, such as AtERF15 and AtPUB23, were affected in both OsbHLH068- and AtbHLH112-overexpressing transgenic Arabidopsis plants. Thus, we propose that OsbHLH068 and AtbHLH112 share partially redundant functions in the regulation of abiotic stress responses but have opposite functions to control flowering in Arabidopsis, presumably due to the evolutionary functional divergence of homolog-encoded proteins.