Isolation and Comparison of Eight SWEET17 Genes from Six Loquat Cultivars

Abstract

The plant SWEET (Sugars Will Eventually be Exported Transporters) proteins, as newly discovered sugar transporters, play various important roles during plant depending on the transmembrane gradient of sugar concentration. SWEET17 protein is one of the SWEET family members transporting fructose in particular. In this study, we identified eight SWEET17s from six loquats (Eriobotrya japonica (Thunb.) Lindl.) cultivars and named them as Ej_NHB_SWEET17, Ej_BL_SWEET17a, Ej_ZH_SWEET17, Ej_MM-_SWEET17a, Ej_MM_SWEET17b, Ej_BL_SWEET17b, Ej_GY_SWEET17 and Ep_LY_SWEET17. All of the eight loquat SWEET17s have five to six transmembrane structures. Phylogenetic analysis indicated that Ej_NHB_SWEET17 had the farthest genetic relationship with other seven loquat SWEET17s, indicating that ‘Ninghaibai’ cultivar was far away from the other five loquat varieties possibly. The similarities of nucleotide and amino acid sequences between Ej_NHB_SWEET17 and the other loquat SWEET17s were only about 61 and 46%, respectively, while that among other seven SWEET17s were greater than 90%. The expression levels of loquat SWEET17s in leaves were positively correlated with the content of fructose, sorbitol and total sugar in various fruit from the six cultivars, in which the sugar contents in fruits of different cultivars varied significantly, that was probably related to expression levels of loquat SWEET17. Our results indicated that the expression level of loquat SWEET17s was probably related to the regulation of sugar contents of mature fruit, experimental confirmation needs to be performed. Together, these results indicated that EjSWEET17s possibly played important roles in the sugar accumulation of fruit, which could serve as the basis for further functional identification of such genes.