Identification of sugar transporter (SWEET) genes involved in pomegranate seed coat sugar accumulation

Sugar content of the outer seed coat and hardness of the inner seed coat are important traits of the pomegranate fruit. The translocation of sugars across biological membranes, mediated by SWEET transporters, is critical to seed development. In this study, we identified 16 PgrSWEET genes distributed on six chromosomes in the pomegranate genome. According to the phylogenetic analysis, PgrSWEET proteins were divided into four groups. Tandem and segmental duplications contributed to the expansion of the PgrSWEET family, while functional redundancy and diversification may have occurred among SWEET members according to analyses of evolution and gene expression. RNA-seq and qRT-PCR analyses revealed that PgrSWEET1a and PgrSWEET9 were highly expressed in the inner seed coat, and the expression levels gradually increased during seed development. Moreover, the relative expression levels of PgrSWEET1a and PgrSWEET9 in a hard-seeded cultivar were higher than those in a soft-seeded cultivar, indicating that PgrSWEET1a and PgrSWEET9 might function in the inner seed coat development by accumulating sugar metabolites. We also found that PgrSWEET2 was highly expressed in the outer seed coat during seed development, and the protein was localized to the tonoplast, indicating that PgrSWEET2 is likely a candidate regulating sugar accumulation or reutilization in the vacuoles of the outer seed coat. Genes encoding transcription factors probably regulating the candidate PgrSWEET genes were chosen by co-expression analysis. These results not only helped to characterize PgrSWEET genes but also provided an insight into their functions in relation to seed coat development.