Elucidating the role of SWEET13 in phloem loading of the C4 grass Setaria viridis.

Photosynthetic efficiency and sink demand are tightly correlated with rates of phloem loading where maintaining low cytosolic sugar concentrations is paramount to prevent downregulation of photosynthesis. Sugars Will Eventually be Exported Transporters (SWEETs) are thought to have a pivotal role in apoplastic phloem loading of C4 grasses. SWEETs have not been well-studied in C4 species, and their investigation is complicated by photosynthesis taking place across two cell types and therefore, photoassimilate export can occur out of either one. SWEET13 homologues in C4 grasses have been proposed to facilitate apoplastic phloem loading. Here we provide evidence for this hypothesis using the C4 grass Setaria viridis. Expression analyses on the leaf gradient of C4 species Setaria and sorghum show abundant transcript levels for SWEET13 homologues. Carbohydrate profiling along the Setaria leaf show total sugar content to be significantly higher in the mature leaf tip compared with the younger tissue at the base. We present the first known immunolocalization results for SvSWEET13a and SvSWEET13b using novel isoform-specific antisera. These results show localization to the bundle sheath and phloem parenchyma cells of both minor and major veins. We further present the first transport kinetics study of C4 monocot SWEETs by utilizing a Xenopus laevis oocyte heterologous expression system. We demonstrate that SvSWEET13a and SvSWEET13b are high-capacity transporters of glucose and sucrose with a higher apparent Vmax for sucrose than glucose, typical of clade III SWEETs. Collectively, these results provide evidence for an apoplastic phloem loading pathway in Setaria and possibly other C4 species.