Drought and Salinity Tolerance of the <i>PtWRKY33</i> Gene in <i>Populus</i>

In this study, the <i>PtWRKY33</i> gene sequence was cloned by PCR from cDNA, and systematic evolutionary analysis was performed. qRT–PCR was performed to measure the expression of the <i>PtWRKY33</i> gene in the roots, stems and leaves of <i>Populus tremuloides</i> under drought and saline stresses, and overexpression vectors were constructed and genetically transformed into tobacco. The results showed that the WRKY transcription factors of <i>Populus trichocarpa</i>, <i>Populus euphratica</i> and <i>Populus alba</i> were in the same branch and were closely related to each other. Treating poplar seedlings with 20% PEG6000 solution to simulate drought stress showed that expression of the <i>PtWRKY33</i> gene was induced and increased 1.89 times, 3.45 times and 11.6 times in leaves, stems and roots. The relative expression of the <i>PtWRKY33</i> gene was slower to respond to 150 mM NaCl treatment but was nonetheless induced. At 48 h, the increase was 1–3 times in leaves, stems and roots, respectively. NaHCO₃ (60 mM) was used to treat poplar seedlings with alkaline salt stress; the results indicated that the <i>PtWRKY33</i> gene was sensitive to NaHCO₃ stress treatment, and that its relative expression was significantly increased and increased 10.51 times (24 h), 6.56 times (6 h) and 5.16 times (24 h) in leaves, stems and roots, respectively. In this study, we found that NaCl and NaHCO₃ stress treatments were able to induce an increase in the expression of the <i>NtSOD1</i> and <i>NtAPX2</i> genes using qRT–PCR, and the significant increase in expression under the treatments compared with WT may be caused by overexpression of the <i>PtWRKY33</i> gene. Overexpression of the <i>PtWRKY33</i> gene in tobacco enhanced the antioxidant stress capacity and improved the salinity tolerance of transgenic tobacco. These results indicate that the <i>PtWRKY33</i> gene is a key gene for improved salinity-tolerant growth, which is important for future molecular breeding of tree resistance.