Monolignols are the building blocks for lignin polymerization in the apoplastic domain. Monolignol biosynthesis, transport, storage, glycosylation, and deglycosylation are the main biological processes partaking in their homeostasis. In Arabidopsis thaliana, members of the uridine diphosphate-dependent glucosyltransferases UGT72E and UGT72B subfamilies have been demonstrated to glycosylate monolignols. Here, the poplar UGT72 family, which is clustered into four groups, was characterized: Group 1 UGT72AZ1 and UGT72AZ2, homologs of Arabidopsis UGT72E1-3, as well as group 4 UGT72B37 and UGT72B39, homologs of Arabidopsis UGT72B1-3, glycosylate monolignols. In addition, promoter-GUS analyses indicated that poplar UGT72 members are expressed within vascular tissues. At the subcellular level, poplar UGT72s belonging to group 1 and group 4 were found to be associated with the nucleus and the endoplasmic reticulum. However, UGT72A2, belonging to group 2, was localized in bodies associated with chloroplasts, as well as possibly in chloroplasts. These results show a partial conservation of substrate recognition between Arabidopsis and poplar homologs, as well as divergent functions between different groups of the UGT72 family, for which the substrates remain unknown.
Plant UDP-glycosyltransferases (UGT) transfer sugars to small acceptor molecules and thereby play key roles in the biosynthesis of secondary metabolites, including phenylpropanoids. Some of those metabolites are involved in the xylem lignification of a broad range of terrestrial plants, particularly trees. Here, we focused on poplar UGT72B37, coding for an enzyme glycosylating monolignols by investigating CRISPR/Cas9 mutant lines. The cell wall characterization revealed a 10% lignin content increase in the xylem of three-month-old mutant lines compared to the wild type. No ectopic lignification was evidenced in the pith of the stems of the mutants, suggesting that the increased lignin deposition is restricted to lignified cell walls. The analysis of the expression level of lignin biosynthesis and polymerization genes did not show significant changes between the WT and the ugt72b37 mutants, except for CINNAMOYL-COA REDUCTASE 2 which was significantly upregulated by 1.2–1.5-fold. Noticeably, UGT72B38, the closest related gene to UGT72B37, is upregulated in mutant lines, suggesting a functional compensation between UGT72B37 and UGT72B38 possibly linked with lignin biosynthesis and accumulation in poplar. Overall, these results reinforce a plausible role of monolignol glycosylation in the cell wall lignification process.
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