Identification, Analysis and Gene Cloning of the SWEET Gene Family Provide Insights into Sugar Transport in Pomegranate (Punica granatum)

Members of the sugars will eventually be exported transporter (SWEET) family regulate the transport of different sugars through the cell membrane and control the distribution of sugars inside and outside the cell. The SWEET gene family also plays important roles in plant growth and development and physiological processes. So far, there are no reports on the SWEET family in pomegranate. Meanwhile, pomegranate is rich in sugar, and three published pomegranate genome sequences provide resources for the study of the SWEET gene family. 20 PgSWEETs from pomegranate and the known Arabidopsis and grape SWEETs were divided into four clades (Ⅰ, Ⅱ, Ⅲ and Ⅳ) according to the phylogenetic relationships. PgSWEETs of the same clade share similar gene structures, predicting their similar biological functions. RNA-Seq data suggested that PgSWEET genes have a tissue-specific expression pattern. Foliar application of tripotassium phosphate significantly increased the total soluble sugar content of pomegranate fruits and leaves and significantly affected the expression levels of PgSWEETs. The plant growth hormone regulator assay also significantly affected the PgSWEETs expression both in buds of bisexual and functional male flowers. Among them, we selected PgSWEET17a as a candidate gene that plays a role in fructose transport in leaves. The 798 bp CDS sequence of PgSWEET17a was cloned, which encodes 265 amino acids. The subcellular localization of PgSWEET17a showed that it was localized to the cell membrane, indicating its involvement in sugar transport. Transient expression results showed that tobacco fructose content was significantly increased with the up-regulation of PgSWEET17a, while both sucrose and glucose contents were significantly down-regulated. The integration of the PgSWEET phylogenetic tree, gene structure and RNA-Seq data provide a genome-wide trait and expression pattern. Our findings suggest that tripotassium phosphate and plant exogenous hormone treatments could alter PgSWEET expression patterns. These provide a reference for further functional verification and sugar metabolism pathway regulation of PgSWEETs.