Integrative analysis reveals evolutionary patterns and potential functions of SWEET transporters in Euphorbiaceae

As a macromolecular substance, sucrose contributes to the plant growth and development. SWEET genes, a group of sugar transporters, are a recently found plant gene family and play important roles in sugar efflux, pollen nutrition, nectar secretion, phloem transport, and seed development. The SWEET genes have been identified and characterized in some plants, but the systematic study in tung tree (Vernicia fordii) was limited. Here, we identified 121 SWEETs in five Euphorbiaceae, and could be divided into four classes with 20 different motifs. Multiple sequence alignment revealed seven transmembrane helixes (TMHs) in the SWEET proteins which were created by an internal duplication of an ancestral three-TMHs unit, connected by TMH4. This study provides direct evidence for the first time for internal duplication in Euphorbiaceae. The large-scale duplication events represented the main driving force for SWEET family expansion in Euphorbiaceae. In addition, we determined the key VfSWEETs for sucrose transport from source to sink tissues in V. fordii and proposed a possible sucrose transport model, which would be helpful for understanding the mechanism of sucrose transport in V. fordii. This study provided a new insight into the evolution, expression and structural variations of SWEETs in V. fordii and four other Euphorbiaceae.