杨树糖转运蛋白PtSWEET17a参与木质部碳积累的作用机理

The transport and accumulation of carbon in tree determine the quality and quantity of wood, and the unloading from phloem and transport in the xylem of sugar is the key to carbon import into the xylem. SWEETs are not only involved in the transport of sucrose across the membrane, but also involved in the transport of monosaccharides. Therefore, SWEETs play an important role in plant growth and development. In previous study, we identified the SWEETs family in Populus. Expression analysis in various tissues showed that PtSWEET17a exhibited a high expression level in the phloem and xylem suggesting that it may be involved in carbon import into the xylem. However, the actual role of PtSWEET17a is still unclear. In this project, we will carry out immunohistochemical analysis and 13C- sugar assimilating experiments on PtSWEET17a gene knockout mutants, transgenic plants with overexpression of PtSWEET17a, and pPtSWEET17a::PtSWEET17a-GFP transgenic plants. We aim to explore the mechanism of PtSWEET17a mediated carbon accumulation in poplar xylem by studying the dynamic relationship between the expression of PtSWEET17a and accumulation 13C- sugar, exploring the role of PtSWEET17a in the radial phloem-to-wood pathway, and the influence of PtSWEET17a on the secondary cell wall in xylem. The results obtained in the project will provide theoretical guidance and technical approach for molecular breeding of wood.

碳的运输及积累决定了木材形成的质和量，而糖在木质部中的卸载及转运是决定碳向木质部积累的关键环节。糖转运蛋白SWEET不仅参与蔗糖跨膜运输，还参与单糖跨膜运输，在植物生长发育过程中发挥重要作用。通过对杨树SWEET基因家族成员的鉴定及组织表达分析，我们发现PtSWEET17a在韧皮部和木质部中高丰度表达，可能参与了糖在杨树木质部碳积累过程。但是，其具体作用过程和机理尚不清楚。本项目拟通过创制PtSWEET17a基因敲除突变体、过表达及GFP融合表达转基因材料，结合免疫组化技术及13C-糖吸收实验，分析木质部发育过程中PtSWEET17a的组织定位、表达及与糖积累的动态关系，明确PtSWEET17a在木质部光合产物径向运输不同环节中的作用，及其对木质部次生细胞壁结构和组分的影响，揭示PtSWEET17a糖转运蛋白参与木质部碳积累的作用机理。为木材产量、质量的分子育种提供理论指导及技术途径。

木材是地球上重要的可再生资源。在木材形成过程中，需要不断地由光合组织获得碳源，以满足木质部的发育需求。糖转运蛋白在光合作用同化产物（蔗糖）从光合组织运输到木质部过程中发挥重要作用。SWEETs（Sugars Will Eventually Be Exported Transporters）蛋白是近年来发现的一类重要的糖转运蛋白。本项目通过创制超表达转基因植株和突变体材料，研究了杨树PtSWEET17a糖转运蛋白参与木质部碳积累的作用机理。研究表明，PtSWEET17a主要在韧皮部和木质部表达，超表达 PtSWEET17a显著提高了木质部葡萄糖和蔗糖的含量，推测其可能与葡萄糖和蔗糖的转运有关。通过在杨树植株叶片外施13C-Sucrose，并测定杨树茎树皮和木质部13C信号的积累，发现PtSWEET17a突变后显著降低了外施叶片中13C信号的外运，以及韧皮部和木质部中13C信号的积累，进一步证实了PtSWEET17a的糖转运活性。与未转基因对照相比，突变体植株的株高变矮和地径减小。进一步切片显微观察发现，突变体植株的木质部区域明显变窄。透射电镜分析表明，突变体中纤维细胞和导管细胞的次生细胞壁明显变薄。对木质部化学组分的分析表明，PtSWEET17a突变后可以导致纤维素含量显著下降。由此推测，PtSWEET17a可能参与了糖向发育中的纤维或导管细胞的转运。上述研究结果，为今后木材产量和质量的分子改良提供了新的思路和途径。