The WUSCHEL-related homeobox (WOX) belongs to a plant-specific transcription factor gene family that plays crucial roles in plant growth and development. Barley ranks as the fourth global cereal crop and is recognized as a model crop for the study of cereal genetics. However, genome-wide characterization, functional validation, and stress-related studies of the WOX gene family in barley remain limited, hindering efforts to leverage their potential for improving salt tolerance and regeneration efficiency in breeding programs. In this study, we identified 12 HvWOX genes assigned from chromosome 1 to chromosome 5. Phylogenetic analysis revealed that these HvWOX genes can be classified into three clades (WUS, ancient, and intermediate). Gene structure analysis revealed that the exon numbers of HvWOX genes varied in the WUS and intermediate clades but were highly conserved in the ancient clade. Tissue-specific analysis revealed that the most common HvWOX genes were highly expressed in reproductive tissues such as anthers or ovaries. Cis-element analysis suggested that there were multiple stress- and hormone-responsive elements in the HvWOX gene promoters. In addition, overexpression of HvWOX8 in Arabidopsis significantly enhanced root elongation under salt stress (50–100 mM NaCl), suggesting its direct role in salt tolerance. Transcriptomic analysis further revealed that HvWOX8 modulates hormone signaling and electron transfer pathways during ATP synthesis under stress conditions. In conclusion, our results provided a comprehensive understanding of the gene characteristics, expression patterns, and potential roles of barley WOX genes.

Crop breeding for high nitrogen use efficiency (NUE) or tolerance to low nitrogen fertilization is thought to be an ideal solution to reduce the cost, carbon footprint, and other environmental problems caused by the excess use of nitrogen fertilizers. As a model plant for cereal crops, barley has many advantages, including good adaptability, a short growth period, and high natural stress resistance or tolerance. Therefore, research on improving NUE in barley is not only beneficial for nitrogen-efficient barley breeding but will also inform NUE improvement in other cereal crops. In this review, recent progress in understanding barley’s response to nitrogen nutrition, evaluation of NUE or low-nitrogen tolerance, quantitative trait loci (QTL) mapping and gene cloning associated with improving NUE, and breeding of nitrogen-efficient barley is summarized. Furthermore, several biotechnological tools that could be used for revealing the molecular mechanisms of NUE or breeding for improving NUE in barley are introduced, including GWAS, omics, and gene editing. The latest research ideas in unraveling the molecular mechanisms of improving NUE in other crops are also discussed. Thus, this review provides a better understanding of improving the NUE of barley and some directions for future research in this area.