Genome-Wide Analysis of Barley bHLH Transcription Factors and the Functional Characterization of HvbHLH56 in Low Nitrogen Tolerance in Arabidopsis

Improvement of low nitrogen (LN) tolerance or nitrogen use efficiency (NUE) in crops is imperative for environment-friendly agriculture development. The basic helix-loop-helix (bHLH) transcription factors are involved in multiple abiotic stresses and are suitable as candidate genes for improving LN tolerance. Few studies were performed on the characterization of the HvbHLH gene family and their function in response to LN stress in barley. In this study, 103 HvbHLH genes were identified through genome-wide analysis. HvbHLH proteins were classified into 20 subfamilies based on phylogenetic analysis in barley, which was supported by conserved motifs and gene structure analysis. The stress-related cis-element analysis in the promoters showed that HvbHLHs are probably involved in multiple stress responses. By phylogenetic analysis of HvbHLHs and bHLHs in other plants, some HvbHLHs were predicted to play roles in response to nutrition deficiency stress. Furthermore, at least 16 HvbHLHs were differentially expressed in two barley genotypes differing in LN tolerance under LN stress. Finally, overexpression of HvbHLH56 enhanced LN stress tolerance in transgenic Arabidopsis, suggesting it is an important regulator in LN stress response. The differentially expressed HvbHLHs identified herein may be valuable for the breeding of barley cultivars with LN tolerance.