Phylogenetic Relationships of Sucrose Transporters (SUTs) in Plants and Genome-wide Characterization of SUT Genes in Orchidaceae

Background: Sucrose is the primary form of photosynthetically produced carbohydrates transported long distance in many plant species, which significantly 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. Results: In this study, we investigated the relationship of SUTs in 24 representative plant species and performed a comprehensive analysis of SUT genes in three sequenced Orchidaceae species, Dendrobium officinale , Phalaenopsis equestris , and Apostasia shenzhenica . All the SUTs from 24 plants were classified into three groups and five subgroups: subgroups A, B1, B2.1, B2.2, and C, based on the evolutionary relationships. A total of 22 SUT genes were identified in Orchidaceae species, among which D. officinale had 8 genes ( DenSUT01-08 ), P. equestris had 8 genes ( PeqSUT01-08 ) and A. shenzhenica had 6 genes ( PeqSUT01-06 ). For the 22 Orchidaceae SUTs , each of the subgroups A, B2.2 and C contains three genes, whereas the SUT genes were significantly expanded in the monocot-specific subgroup B2.1 which contained 12 genes. To shed light into sucrose partitioning and functions of sucrose transporters in Orchidacea species, we analysed water-soluble sugar content and performed RNA sequencing of different tissues of D. officinale , including leaves, stems, flowers and roots. The results showed that although total content of water-soluble polysaccharides was highest in the stems of D. officinale , the sucrose content was highest in flowers. Moreover, gene expression analysis showed that most of the DenSUTs were expressed in flowers, among which DenSUT01 , DenSUT07 and DenSUT06 had significantly high expression levels. Conclusions: These results indicated that stems are used as main storage sinks for photosynthetically produced sugar in D. officinale , and that the DenSUTs mainly take functions in the cellular machinery and development of floral organs. Our findings provide valuable information on sucrose partitioning and the evolution and functions of SUT genes in Orchidaceae and other species.