PtrLAC17 act as a key laccase in catalyzing lignin polymerization in both the stem xylem cell walls and the guard cell walls, coordinately regulating drought tolerance in Populus.

Plant laccase is a crucial enzyme that plays a role in the polymerization of lignin macromolecules. Previous research on laccases has primarily concentrated on the synthesis of lignin in the secondary walls of xylem. In this study, PtrLAC17 was localized in both the secondary cell walls of the xylem in the stem and the guard cell walls of Populus trichocarpa Torr, functioning as a key enzyme for lignin polymerization. Enzymatic analysis revealed that PtrLAC17-encoded enzyme can polymerizes lignin monomers in vitro, with a preference for sinapyl alcohol. In Populus overexpressing PtrLAC17 (OE-PtrLAC17s), lignin content significantly increased in both the guard cell ventral wall and the xylem cell wall, while it decreased in antisense lines (AS-PtrLAC17s) and mutants lines. The syringyl/guaiacyl (S/G) lignin ratio was also varied in the transgenic Populus lines. OE-PtrLAC17s exhibited reduced stomatal apertures, conferring enhanced drought resistance, whereas AS-PtrLAC17 and mutants showed increased stomatal apertures, resulting in decreased drought tolerance. These results demonstrate that elevated lignin deposition in of guard cells walls reduces stomatal aperture and enhances plant drought resistance. Additionally, lignin deposition in stem secondary cell wall increases the thickness of the fiber cell wall and improves hydraulic conductivity. These findings provide new insights into how plant laccase in Populus synergistically regulates drought stress response by catalyzing lignin polymerization in both the secondary cell wall of the xylem in the stem and stomatal cell wall.