Phylogenetic relationships of sucrose transporters (SUTs) in plants and genome-wide characterization of genes in Orchidaceae reveal roles in floral organ development.

Sucrose is the primary form of photosynthetically produced carbohydrates transported long distance in many plant species and substantially affects plant growth, development and physiology. Sucrose transporters (SUTs or SUCs) are a group of membrane proteins that play vital roles in mediating sucrose allocation within cells and at the whole-plant level. In this study, we investigated the relationships among SUTs in 24 representative plant species and performed an analysis of genes in three sequenced Orchidaceae species: , , and . All the SUTs from the 24 plant species were classified into three groups and five subgroups, subgroups A, B1, B2.1, B2.2, and C, based on their evolutionary relationships. A total of 22 genes were identified among Orchidaceae species, among which had 8 genes (), had eight genes () and had 6 genes (). For the 22 Orchidaceae, subgroups A, B2.2 and C contained three genes, whereas the genes were found to have significantly expanded in the monocot-specific subgroup B2.1, which contained 12 genes. To understand sucrose partitioning and the functions of sucrose transporters in Orchidaceae species, we analyzed the water-soluble sugar content and performed RNA sequencing of different tissues of , including leaves, stems, flowers and roots. The results showed that although the total content of water-soluble polysaccharides was highest in the stems of , the sucrose content was highest in the flowers. Moreover, gene expression analysis showed that most of the were expressed in the flowers, among which , and had significantly increased expression levels. These results indicated that stems are used as the main storage sinks for photosynthetically produced sugar in and that mainly function in the cellular machinery and development of floral organs. Our findings provide valuable information on sucrose partitioning and the evolution and functions of genes in Orchidaceae and other species.