Populus trichocarpa EXPA6 Facilitates Radial and Longitudinal Transport of Na+ under Salt Stress

Expansins are cell wall (CW) proteins that mediate the CW loosening and regulate salt tolerance in a positive or negative way. However, the role of Populus trichocarpa expansin A6 (PtEXPA6) in salt tolerance and the relevance to cell wall loosening is still unclear in poplars. PtEXPA6 gene was transferred into the hybrid species, Populus alba × P. tremula var. glandulosa (84K) and Populus tremula × P. alba INRA ‘717-1B4’ (717-1B4). Under salt stress, the stem growth, gas exchange, chlorophyll fluorescence, activity and transcription of antioxidant enzymes, Na+ content, and Na+ flux of root xylem and petiole vascular bundle were investigated in wild-type and transgenic poplars. The correlation analysis and principal component analysis (PCA) were used to analyze the correlations among the characteristics and principal components. Our results show that the transcription of PtEXPA6 was downregulated upon a prolonged duration of salt stress (48 h) after a transient increase induced by NaCl (100 mM). The PtEXPA6-transgenic poplars of 84K and 717-1B4 showed a greater reduction (42–65%) in stem height and diameter growth after 15 days of NaCl treatment compared with wild-type (WT) poplars (11–41%). The Na+ accumulation in roots, stems, and leaves was 14–83% higher in the transgenic lines than in the WT. The Na+ buildup in the transgenic poplars affects photosynthesis; the activity of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT); and the transcription of PODa2, SOD Cu-Zn, and CAT1. Transient flux kinetics showed that the Na+ efflux of root xylem and leaf petiole vascular bundle were 1.9–3.5-fold greater in the PtEXPA6-transgenic poplars than in the WT poplars. PtEXPA6 overexpression increased root contractility and extensibility by 33% and 32%, indicating that PtEXPA6 increased the CW loosening in the transgenic poplars of 84K and 717-1B4. Noteworthily, the PtEXPA6-promoted CW loosening was shown to facilitate Na+ efflux of root xylem and petiole vascular bundle in the transgenic poplars. We conclude that the overexpression of PtEXPA6 leads to CW loosening that facilitates the radial translocation of Na+ into the root xylem and the subsequent Na+ translocation from roots to leaves, resulting in an excessive Na+ accumulation and consequently, reducing salt tolerance in transgenic poplars. Therefore, the downregulation of PtEXPA6 in NaCl-treated Populus trichocarpa favors the maintenance of ionic and reactive oxygen species (ROS) homeostasis under long-term salt stress.