Abstract: [Objective] In higher plants, sugars function as the main energy reserves, important signaling molecules, osmotic adjustment substances and the building blocks for other organic compounds, playing important roles during the entire life cycle. As sucrose transporters(SUTs) in plant, SUTs participate in sucrose loading from source tissues to phloem, transporting in phloem, and unloading from phloem to sink tissues. Therefore, SUTs are important in plant growth and development. There are big differences in sink tissues between woody plants and annual herbs and the carbon allocation mechanism in woody plants needs further investigation. In the present study, to further investigate the roles of PagSUT4 (GenBank No. KX545405) in sugar transportation, photosynthesis, and secondary growth, we generated PagSUT4 overexpression Populus lines and analyzed their phenotypes.[Method] The coding sequence of PagSUT4 was cloned using gene-specific primers from hybrid poplar clone 84K (Populus alba×P.glandulosa). The PagSUT4 expressions in roots, young leaves, mature leaves, primary stem, secondary stem, male flowers, female flowers, xylem and phloem were examined using qRT-PCR. To analyze the subcellular localization of PagSUT4, the gene was transiently expressed in tobacco leaf epidermal cells fused with YFP to its C-terminus. The coding sequence was cloned into PMDC32 to produce the 35S:PagSUT4 construct, and the latter was transformed into poplar 84K mediated by Agrobacterium tumefaciens and the transgenic lines with high expression of PagSUT4 were selected for further phenotypic analysis. The stem length, ground diameter, photosynthetic rate, intercellular CO₂ concentration, conductance to H₂O and transpiration rate of 2-month-old transgenic lines and non-transgenic controls in the greenhouse were measured. Moreover, the anatomy of stem sections of the 7^th internodes from them was performed.[Result] The results showed that PagSUT4 gene encoding a tonoplast-localized sucrose transporter. The PagSUT4 gene were ubiquitously expressed and showed high expression level in mature leaves, lignified stem, phloem and flowers. 13 transgenic lines with overexpression of PagSUT4 were obtained and the level of expression in leaves was significantly higher than that in the control. Stomatal conductance, transpiration rate and water use efficiency of the selected transgenic lines (S1 and S12) were higher than that of the non-transgenic controls, while the intercellular CO₂ concentration was lower in transgenic lines than that of the controls. The 2 transgenic lines exhibited 24% and 21% higher photosynthetic rate compared with the non-transgenic controls, respectively. The transgenic lines showed 22% and 17% increase in stem height, 9% and 7% increase in ground diameter, respectively. In addition, the two transgenic lines showed 32% and 21% increase in the xylem width of the seventh internodes, respectively.[Conclusion] Overall, PagSUT4 functions in mature leaves, secondary stem, phloem and flowers. Overexpression of PagSUT4 may promote the sucrose efflux from the leaves, the sucrose transport and unloading in the stem, have positive feedback effect on the photosynthesis through the enhancement of sucrose loading in source tissues and the unloading in sink tissues. The enhanced photosynthesis and increased sucrose unloading efficiency in the stem have promoted both height and diameter growth (development of secondary xylem) of the transgenic poplar trees.

Key words: Populus, overexpression, PagSUT4, sugar transport, photosynthesis, xylem development