[Functional analysis on sucrose transporters in sweet potato]

Sweet potato is an important food crop that can also be used as an industrial raw material. Sucrose is the main form of long-distance carbohydrate transport in plants, and sucrose transporter (SUT) regulates the transmembrane transport and distribution of sucrose during plant growth and metabolism. Moreover, SUT plays a key role in phloem mediated source-to-sink sucrose transport and physiological activities, supplying sucrose for the sink tissues. In this study, the full-length cDNA sequences of IbSUT and IbSUT were obtained by rapid amplification of cDNA ends (RACE) cloning according to the transcripts of the two SUT coding genes which were differentially expressed in sweet potato storage roots with different starch properties. Phylogenetic analysis was performed to clarify the classification of IbSUT₆₂₇₈₈ and IbSUT₈₁₆₁₆. The subcellular localization of IbSUT₆₂₇₈₈ and IbSUT₈₁₆₁₆ was determined by transient expression in Nicotiana benthamiana. The function of IbSUT₆₂₇₈₈ and IbSUT₈₁₆₁₆ in sucrose and hexose absorption and transport was identified using yeast functional complementarity system. The expression pattern of IbSUT and IbSUT in sweet potato organs were analyzed by real-time fluorescence quantitative PCR (RT-qPCR). Arabidopsis plants heterologous expressing IbSUT and IbSUT genes were obtained using floral dip method. The differences in starch and sugar contents between transgenic and wild-type Arabidopsis were compared. The results showed IbSUT and IbSUT encoded SUT proteins with a length of 505 and 521 amino acids, respectively, and both proteins belonged to the SUT1 subfamily. IbSUT₆₂₇₈₈ and IbSUT₈₁₆₁₆ were located in the cell membrane and were able to transport sucrose, glucose and fructose in the yeast system. In addition, IbSUT₆₂₇₈₈ was also able to transport mannose. The expression of IbSUT was higher in leaves, lateral branches and main stems, and the expression of IbSUT was higher in lateral branches, stems and storage roots. After IbSUT and IbSUT were heterologously expressed in Arabidopsis, the plants grew normally, but the biomass increased. The heterologous expression of IbSUT increased the soluble sugar content, leaf size and 1 000-seed weight of Arabidopsis plants. Heterologous expression of IbSUT increased starch accumulation in leaves and root tips and 1 000-seed weight of seeds, but decreased soluble sugar content. The results obtained in this study showed that IbSUT and IbSUT might be important genes regulating sucrose and sugar content traits in sweet potato. They might carry out physiological functions on cell membrane, such as transmembrane transport of sucrose, sucrose into and out of sink tissue, as well as transport and unloading of sucrose into phloem. The changes in traits result from their heterologous expression in Arabidopsis indicates their potential in improving the yield of other plants or crops. The results obtained in this study provide important information for revealing the functions of IbSUT and IbSUT in starch and glucose metabolism and formation mechanism of important quality traits in sweet potato.