Leveraging Transcriptome Insights and GsHZ4 Gene Expression to Improve Alkaline Tolerance in Lupinus angustifolius

Soil alkalinity severely restricts the cultivation of <i>Lupinus angustifolius</i>, a valuable legume. Wild soybean (<i>Glycine soja</i>) is a leguminous plant with extremely strong alkaline resistance (pH 8.5). Transferring the alkali-tolerant genes from wild soybeans into lupinus can effectively enhance the alkali tolerance. In this study, we combined transcriptome profiling and genetic transformation to elucidate the molecular basis of alkaline stress response in lupinus. RNA-seq analysis of root tips under acid (HCl, pH 4.0) and alkali (NaHCO₃, pH 8.5) stress revealed 104,353 annotated unigenes, with differential expression patterns highlighting enrichment in cellular component, binding, and catalytic activity categories. KEGG pathway analysis indicated that early responses involved ribosome-related pathways, while later stages activated plant hormone signaling and MAPK pathways. Notably, no homeodomain-leucine zipper (HD-Zip) family genes were identified in the lupinus genome. Therefore, we transferred GsHZ4, an alkali-resistant HD-Zip transcription factor from wild soybean into lupinus hairy roots via <i>Agrobacterium rhizogenes</i>-mediated transformation. Overexpression of <i>GsHZ4</i> significantly enhanced antioxidant enzyme activities (CAT, POD, and SOD) and reduced malondialdehyde content under NaHCO₃ stress. Furthermore, the promoter of <i>GsHZ4</i> expression was strongly induced by indole-3-acetic acid (IAA). Key alkali-responsive genes (<i>LaKIN</i>, <i>LaMYB34</i>, <i>LaDnaJ1</i>, <i>LaDnaJ20</i>, <i>LaNAC22</i>, and <i>LaNAC35</i>) were upregulated in transgenic lines, suggesting that <i>GsHZ4</i> integrates into the endogenous stress-regulation network. Our findings demonstrate that heterologous expression of <i>GsHZ4</i> can enhance alkaline tolerance of lupinus, providing a novel strategy for breeding stress-resistant varieties and expanding lupinus cultivation in saline-alkali soils.