PtrWRKY75 overexpression reduces stomatal aperture and improves drought tolerance by salicylic acid-induced reactive oxygen species accumulation in poplar

Drought is a severe environmental stress that limits plant growth and productivity. Plant hormones play a key role in response to biological and abiotic stresses. And phytohormones can regulate stomatal opening and transpiration for improving water-use efficiency under water stress conditions. In this study, PtrWRKY75, a member of the WRKY transcription factor family, which is involved in many abiotic stress responses, was cloned from Populus trichocarpa (clone 'Nisqually-1'). PtrWRKY75 was mainly expressed in senescent leaves and could be induced by both dehydration and treatment with salicylic acid (SA). PtrWRKY75 was localized to the nucleus and bound specifically to the PHENYLALANINE AMMONIA LYASE 1 (PAL 1) promoter. To investigate the biological functions of PtrWRKY75, it was overexpressed in triploid white poplar (Populus tomentosa). Compared with wild-type plants, PtrWRKY75-overexpressing poplar lines (OE) maintained a higher photosynthetic rate and growth under drought stress. Under drought stress, PtrWRKY75 acted upstream of PAL1 and directly regulated PAL1 expression by binding to its promoter. Under drought stress, activated PAL1 increased reactive oxygen species (ROS) accumulation by promoting SA biosynthesis, ultimately leading to smaller stomatal aperture. Under drought conditions, the OE lines showed improved water-use efficiency by reducing their stomatal conductance and transpiration rate, and thus drought tolerance was enhanced. In conclusion, PtrWRKY75 is a promising gene target for improving the drought tolerance of Populus by SA.