Investigation of PtSGT1 and PtSGT4 Function in Cellulose Biosynthesis in Populus tomentosa Using CRISPR/Cas9 Technology

Cellulose synthesis is a complex process in plant cells that is important for wood processing, pulping, and papermaking. Cellulose synthesis begins with the glycosylation of sitosterol by sitosterol glycosyltransferase (SGT) to produce sitosterol-glucoside (SG), which acts as the guiding primer for cellulose production. However, the biological functions of SGTs in Populus tomentosa (P. tomentosa) remain largely unknown. Two full-length PtSGT genes (PtSGT1 and PtSGT4) were previously isolated from P. tomentosa and characterized. In the present study, CRISPR/Cas9 gene-editing technology was used to construct PtSGT1-sgRNA and PtSGT4-sgRNA expression vectors, which were genetically transformed into P. tomentosa using the Agrobacterium-mediated method to obtain transgenic lines. Nucleic acid and amino acid sequencing analysis revealed both base insertions and deletions, in addition to reading frame shifts and early termination of translation in the transgenic lines. Sugar metabolism analysis indicated that sucrose and fructose were significantly downregulated in stems and leaves of mutant PtSGT1-1 and PtSGT4-1. Glucose levels did not change significantly in roots and stems of PtSGT1-1 mutants; however, glucose was significantly upregulated in stems and downregulated in leaves of the PtSGT4-1 mutants. Dissection of the plants revealed disordered and loosely arranged xylem cells in the PtSGT4-1 mutant, which were larger and thinner than those of the wild-type. This work will enhance our understanding of cellulose synthesis in the cell walls of woody plants.