PtrVINV2 is dispensable for cellulose synthesis but essential for salt tolerance in Populus trichocarpa Torr. and Gray

Invertase (EC.3.2.1.26), a key enzyme in sucrose breakdown, is crucial for cellulose synthesis. However, the function of the vacuolar invertase (VINV) in woody plants remains unclear. In this study, transgenic lines of Populus trichocarpa Torr. and Gray were generated to investigate the role of PtrVINV2 in wood formation and under high salinity stress. Compared to wild-type (WT), VINV activity in the developing xylem of knockout lines was reduced, resulting in a decrease in lignin content and an increase in hemicellulose content, while cellulose content remained unaffected. These changes in structural carbohydrate content were accompanied by reductions in xylem width and fibre cell wall thickness. The overexpression lines of the developing xylem exhibited opposite trends. Transcriptome analyses of developing xylem indicated that the expression level of PtrVINV2 affects the expression of genes involved in hemicellulose and lignin biosynthesis pathways, such as AXS, UAMs, HCT, COMT, CAD and peroxidases, while CesA expression remained unaffected. WGCNA analysis revealed that Potri.001G219100, Potri.009G106600 and Potri.002G081000 serve as 'hub' transcription factor genes within the structural/non-structural carbohydrate modules of PtrVINV2 transgenic lines, potentially involved in plant salt tolerance. Additionally, under 200 mmol/L NaCl treatment, the knockout lines exhibited increased salt sensitivity compared to WT. This increased sensitivity was accompanied by elevated activities of SOD, CAT and MDA, as well as higher sucrose content and reduced contents of glucose and fructose. The findings indicate that although PtrVINV2 is not essential for cellulose synthesis, it enhances salt tolerance in poplar and presents a promising candidate gene for breeding salt-tolerant poplar.