A ray localized vacuolar sucrose transport is required for wood formation in Populus tomentosa

Wood development in perennial trees is a carbon demanding process essential for secondary cell wall formation and xylem development. Ray tissues are critical for organizing the radial transport system, but the regulatory mechanisms behind this transport and its influence on wood development remain largely unexplored. SUGAR WILL EVENTUALLY BE EXPORTED TRANSPORTERs (SWEETs) are key players in facilitating carbohydrate transport into the sink, yet their role in radial sugar transport from phloem to developing wood is unclear. In this study, we show that PtoSWEET17b is preferentially expressed in ray cells of Populus tomentosa secondary xylem, localizing to the vacuolar membrane. Functional analysis in yeast demonstrated that PtoSWEET17b functions as a high-capacity sucrose transporter. We demonstrated that the knockout of PtoSWEET17b resulted in reduced ¹³C incorporation into xylem and lower soluble sugar content in wood; this is accompanied by a decrease in xylem formation and a decrease in cellulose and hemicellulose levels. Conversely, overexpression of PtoSWEET17b increased xylem production and soluble sugar content compared with wild-type plants. Collectively, these results establish PtoSWEET17b's role in regulating radial transport from ray tissues to xylem, providing insights into post-phloem sugar transport in developing Populus stems.