PagSWEET17a Mediates Sugar Transport in Root and Affects Drought Tolerance in Populus alba x P. glandulosa

Sugars are the principal carbon and energy sources and serve as osmotic regulators and radical scavengers, thus playing an important role in plant responses to drought. Sugar transporters steering the distribution of sugar are vital players involved in tolerance to drought. Sugars Will Eventually be Exported Transporters (SWEETs) facilitate both the influx and efflux of mono- and/or disaccharides and control both inter and intracellular distribution of sugars. PagSWEET17a in Populus alba x P. glandulosa is one of four orthologous genes of AtSWEET17 in Arabidopsis. Unlike AtSWEET17, which is a vacuolar localized transporter, PagSWEET17a is localized to the endoplasmic reticulum (ER). Here, the role of PagSWEET17a in poplar responses to drought stress was investigated. PagSWEET17a was specifically expressed in cambium cells in younger root but mainly located in phloem fibers, and xylem vessels and fiber cells in the root undergoing secondary growth. Loss of PagSWEET17a inhibited the growth of roots in poplar seedlings and led to a decreased tolerance to drought. Analysis of sugar profiles revealed that accumulations of predominant sugars were significantly suppressed in both knockout (KO) mutant lines under drought. PagSWEET17a might contribute to poplar tolerance to drought by promoting drought-induced root expansion and diminishing oxidative damages caused by the stress.