The PagAFP2a-PagAREB1 Module Form a Negative Feedback Loop to Regulate Salt Tolerance in Populus

Salt stress is a major abiotic stress restrict plant growth and distribution. In our previous study, we found the ABI5-BINDING PROTEIN 2a (PagAFP2a) gene was rapidly and significantly induced by salt stress in hybrid poplar (Populus alba × Populus glandulosa), however, its function in salt stress responses was unclear. In this study, we further demonstrated that the PagAFP2a gene expression is significantly induced by salt and ABA treatments. Additionally, the ABA-responsive element (ABRE) binding proteins (PagAREB1s) directly bind to PagAFP2a promoter and activate its expression. Physiological analysis showed that PagAFP2a overexpression (PagAFP2aOE) or PagAREB1-3 knockout (PagAREB1-3KO) significantly reduced salt tolerance whereas PagAFP2a knockout (PagAFP2aKO) or PagAREB1-3 overexpression (PagAREB1-3OE) significantly enhanced salt tolerance in poplar. Correspondingly, salt stress responsive genes were significantly upregulated in PagAFP2aKO and PagAREB1-3OE plants while downregulated in PagAFP2aOE and PagAREB1-3KO plants. Furthermore, we demonstrated that PagAFP2a directly interacts with PagAREB1s and represses its transcriptional activity at the target genes. In summary, our results unveil the PagAFP2a-PagAREB1s module form a negative feedback loop in ABA signaling to fine-tune salt stress responses in Populus.