Fig. 5

Schematic models of gene expression and roles of SWEET proteins in phloem loading and unloading. a Representative genes highly expressed in each tissue during the sucrose accumulation stage; those likely involved in phloem loading of sucrose in the leaf and unloading and accumulation in the stem are shown. b Sucrose efflux associated with SWEET proteins in the leaf. Sucrose is synthesized in leaf mesophyll cells and diffuses through the plasmodesmata. SWEET proteins facilitate sucrose efflux into the apoplast. Subsequently, sucrose is taken up and concentrated in the sieve element–companion cell complex by SUT sucrose symporters. Sucrose is transported through the sieve elements out of the leaves to the stem, roots, and seeds. SbSWEET8-1 (Sobic.008G094000) may play a role in the efflux of photosynthesized sucrose to the leaf apoplast. This model was constructed on the basis of an analogy to that in Arabidopsis. c SWEET-dependent sucrose accumulation in the stem. Synthesized sucrose is transported from the leaf through the sieve element, and SWEET proteins might facilitate sucrose efflux into the stem apoplast. SbSWEET4-3 (Sobic.004G136600) is a sugar transporter that might contribute to phloem unloading