SUT is a class of sucrose–proton symporter only present in plants, they are extremely important for the acquisition of sucrose from the intercellular apoplast outside of the cell into the cytosol, particularly when no plasmodesma exists between adjacent cells, and a membrane transporter or channel is the only pathway for material exchange. Moreover, in long-distance transport of sugar via the symplastic pathway, sucrose must be first uploaded into the phloem. The uploading process relies on SUT transporter(s) since very few plasmodesmata exist between the SE–CC complexes and their surrounding cells. Additionally, during symplastic transport within the phloem, a small portion of sucrose may leak into the apoplast outside of the SE–CC complexes; retrieving this portion of sucrose back to phloem also requires SUT transporter's participation.

Before being taken into the acceptor cell, sugar first needs to be released from the donor cell. As SUT sucrose transporters need a proton gradient to drive the transport process, it cannot accomplish sucrose export from the cytosol to apoplast because the proton concentration of the cytosol is usually lower than that of the intercellular apoplast. The presence of SWEET transporter solves the problem. SWEET is a class of unidirectional sugar transporters that can transport sugar in and out of the cell depending only on the concentration gradient across the membrane. They are passive sugar transporters that can only transport sugar from higher concentration to lower concentration across the membrane. As a result; they can be used to export sugar including sucrose and glucose from the cytosol of the donor cell to the interface between the donor cell and the acceptor cell. Subsequently, the plasma membrane-located SUT transporter at the acceptor cell imports the sugar by active transport. Sugar undergoes the apoplastic transport process from one cell to another in this way.

Physiological functions of SUT and SWEET transporters

SUTs sugar transporters are involved in a variety of physiological roles in plant growth and development. As sucrose active transporters, they are particularly important in the uptake of sucrose from the apoplast into the cytosol, e.g. uploading sucrose into phloem for sugar long-distance transport, retrieving leaked sucrose back into the cytosol, absorbing sucrose from the matrix for pollen tube growth, and transferring sucrose from the vacuole to the cytosol, and mobilizing sucrose from the endosperm for seed germination, etc. 

SWEET's role in plants includes nectar secretion，releasing sucrose or glucose from maternal cells to the apoplast between maternal-filial interface, efflux of sugar from the plant cell for symbiont, or pathogen' living, etc.

Unlike the plant-specific SUT transporter, SWEET transporter also present in the animal, their roles in the animal includes uptake of glucose into the cytosol in the intestine cell, or brain, etc.  Moreover, homologs of the SWEET gene, SemiSWEET, is identified in prokaryotes and virus.

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This entry is adapted from the peer-reviewed paper 10.3390/ijms222011198