Physiological and Transcriptional Regulation of Salt Tolerance in Thinopyrum ponticum and Screening of Salt-Tolerant Candidate Genes

The tall wheatgrass <i>Thinopyrum ponticum</i> has excellent saline-alkali tolerance and great potential for restoring saline-alkali land to enhance productivity. This study used the <i>Thinopyrum ponticum</i> cv. "Orbit" variety, which is widely planted in saline-alkali pastures, as the material and artificially simulated salt stress using 150 mM NaCl and 150 mM Na₂SO₄, respectively. The growth and physiological indexes of the leaves and roots of seedlings were measured after various treatment durations, and the transcriptomes of untreated and Na₂SO₄-treated leaves and roots were also analyzed after 24 h of treatment. The results showed that salt stress resulted in significant reductions in leaf relative water content in seedlings and inhibited root elongation growth, with Na₂SO₄ treatment producing a greater impact on plant growth than NaCl treatment. Salt stress significantly alters ion transport and distribution in <i>Thinopyrum ponticum</i>, characterized by pronounced Na⁺ accumulation and a concomitant decline in K⁺ uptake. Additionally, to adapt to salt stress, roots enhance their ability to absorb and transport essential cations, such as Ca²⁺, Mg²⁺, Fe³⁺, and Cu²⁺. RNA-Seq analysis identified 1682 and 2816 differentially expressed genes (DEGs) in leaves and roots under Na₂SO₄ stress, respectively, with 210 common DEGs. Enrichment analyses revealed that DEGs were primarily associated with redox homeostasis, ion balance, and signal transduction. Furthermore, transcription regulation analysis indicated the <i>Thinopyrum ponticum</i> can coordinate the activation of NAC/MYB/WRKY transcription factors, SA/ETH hormone signaling, and Ca²⁺ pathways in response to salt stress. In summary, this study systematically reveals for the first time the molecular mechanisms by which <i>Thinopyrum ponticum</i> responds to Na₂SO₄ stress through coordinated regulation of ion transport, transcription factor networks, and hormone-Ca²⁺ signaling pathways. Based on transcriptomic and protein-protein interaction analyses, nine key candidate genes for saline-alkali tolerance were identified, including <i>UGT7472</i>, <i>JMT</i>, <i>T4E14.7</i>, <i>CAX5</i>, <i>CP1</i>, <i>PXG2 NAMT1</i>, <i>BON3</i>, and <i>APX7</i>. These findings provide novel genetic resources and a theoretical foundation for breeding salt-alkali-tolerant crops.