PtrCWINV3 encoding a cell wall invertase regulates carbon flow to wood in Populus trichocarpa

Cell wall invertase (CWINV) catalyzes the hydrolysis of sucrose into glucose and fructose in the apoplastic unloading pathway, with carbon sources provided for sink tissues. However, its role in wood formation remains undetermined. Therefore, transgenic lines overexpressing PtrCWINV3 or with knocked-out PtrCWINV3 expression were generated in Populus trichocarpa. Compared with wild type, the PtrCWINV3-knockout lines showed decreased CWINV activity (by 7.4 %-10.8 %), which resulted in a 1.5 %-1.8 % decrease in cellulose content, a 0.82 %-0.98 % decrease in hemicellulose content, and an increase in lignin content (by 2.9 %-4.7 %). These changes in structural carbohydrate contents were accompanied with anomalies in the late stages of secondary xylem development, characterized by reduced width of the secondary xylem, fewer cell layers in secondary xylem, and thinner fiber cell walls. The lines overexpressing PtrCWINV3 under the control of the DX15 promoter in the developing xylem showed the opposite phenotype. Transcriptome data from the developing xylem indicated that PtrCWINV3 regulated the expression of genes involved in the biosynthesis of cellulose (CesA, EG, and CB), hemicellulose/pectin (UGD, AXS, GATL, UAM, PAE, and GAUT), and starch (GBSS), which suggested its involvement in multiple polysaccharide metabolic pathways. Ultimately, this facilitated the synthesis of structural carbohydrate components such as cellulose and hemicellulose, which promoted the later stages of secondary xylem development. These findings not only demonstrate the significant role of CWINV activity in wood formation, but also highlight an excellent candidate gene for breeding new poplar varieties with high cellulose and low lignin contents.