Isolation, phylogenetic relationship and expression profiling of sugar transporter genes in sweet orange (Citrus sinensis)

Abstract

Soluble sugars, including sucrose, glucose and fructose, are crucial components that determine the nutritional and commercial quality of sweet oranges (Citrus sinensis). Sugar transporters have been well demonstrated to mediate the long distance transportation and the subcelluar distribution of sugars in plants. Here, a genome-wide characterization of the sweet orange sugar transporter gene family was reported. We identified a set of putative orange sugar transporter genes containing 3 sucrose transporters (SUTs), 58 monosaccharide transporters (MSTs) that could be classified into 7 distinct subfamilies, and 16 SWEET transporters. Phylogenetic analysis among Arabidopsis thaliana, orange and Vitis vinifera allowed us to identify orthologous groups among these species. Three SUTs, CsSUT1, CsSUT2 and CsSUT4, were expressed in fruits, and exhibited increased transcripts levels as fruit sucrose accumulated, which suggested they participated in fruit sucrose accumulation. A large number of MSTs, CsSTPs, CsPMTs, CsVGTs, CspGlcTs, CsTMTs, CsERD6Ls and CsSWEETs showed fruit-expressed and up-regulated profiles, while glucose and fructose did not obviously accumulate as the fruit ripened. We then discussed the possibilities that fruit glucose and fructose had no evident accumulation, which was in contrast to sucrose. Additionally, many cis-elements such as ACGTATERD1, ARR1AT, MYCCONSENSUSAT, WRKY71OS, IBOXCORE, WBOXNTERF3, SUCROSE BOX 3 and WBOXHVISO1 were found in the promoter regions of orange sugar transporter genes, which suggested that they were transcriptionally regulated by sugars, phytohormones and stresses. This study might provide insights into the genomic organizations, evolutionary characteristics and expression profiling of the orange sugar transporter gene family.