Overexpression of sugar transporter gene PbSWEET4 of pear causes sugar reduce and early senescence in leaves.

As the main energy source for generating ATP during plant growth and development, sugars are synthesized in leaves, while sugar allocation depends on both intracellular transport between different organelles and source-to-sink transport. However, sugar transport related research is limited in pear. Here, a sugar transporter PbSWEET4 was identified that control sugar content and senescence in leaf. Phylogenetic analysis and multiple sequence alignment results indicated that PbSWEET4 was homologous to AtSWEET15, which contained two conserved domains and could promote senescence. The qRT-PCR and transcriptome database result showed that the expression of PbSWEET4 was positively correlated with leaf development, especially highly expressed in older leaves. Furthermore, the evaluation of promoter-GUS activity also indicated that PbSWEET4 exhibited the highest expression level in older leaves. The subcellular localization revealed that the PbSWEET4 localized in the plasma membrane. Finally, overexpression of the PbSWEET4 in strawberry plants could reduce leaf sugar content and chlorophyll content, while accelerate leaf senescence, which might be due to enhanced export of sugars from leaves. These results enrich the knowledge about the function of sugar exporter in regulating the fruit species development, and provide a novel genetic resource for future improvement in carbohydrate partitioning for pear and other fruit trees.