Plasma Membrane-Localized Transporter MdSWEET12 Is Involved in Sucrose Unloading in Apple Fruit

Sugar content is an important factor
determining the flavor in
apple fruit. Sugar unloading is a prerequisite step for sugar accumulation.
However, little is known about sugar unloading mechanisms in apple.
Transcriptomic sequencing of two apple varieties, “Envy”
and “Pacific Rose,” with significantly different sugar
content was performed. MdSWEET12a from the SWEET
transporter family was differentially expressed. Further study of
the MdSWEET12a showed that this plasma membrane-localized transporter
protein-encoding gene was mainly expressed in sieve element-companion
cells (SE-CC) in the fruit, which was positively correlated with the
sucrose accumulation during the development of “Envy”
apple. Consistently manipulating the gene expression through either
transient overexpression or silencing significantly increased or decreased
the sugar content in apple fruit, respectively. Complementary growth
experiments in mutant yeast cells indicated that MdSWEET12a transported
sucrose. Heterologous expression of MdSWEET12a in
tomato increased the expression of genes related to sugar metabolism
and transport, leading to increased sugar content. These findings
underpin the involvement of MdSWEET12a in sugar unloading in apple
fruit.