Comprehensive Identification and Expression Analysis of the SWEET Gene Family in Actinidia eriantha Reveals That Two AeSWEET11 Genes Function in Sucrose and Hexose Transport

Sugars are key metabolites influencing the flavor and quality of kiwifruit, with their accumulation in fruit relying on sugar transporters. Recently identified sugar transporters known as SWEETs play significant roles in modulating plant growth, development, and fruit ripening. However, the characteristics of SWEET genes in Actinidia eriantha remain poorly understood. In this study, a total of 26 AeSWEET genes were identified across 17 chromosomes. These genes encoded proteins ranging from 198 to 305 amino acids in length and contained 5 to 7 transmembrane helices. Both interspecific and intraspecific phylogenetic trees categorized AeSWEET proteins into four distinct clades. The motif and domain structures were conserved within each clade, although variations were observed in exon-intron organizations. One tandem and fourteen segmental duplication events were identified as primary drivers of the AeSWEET family expansion. Comparative syntenic mapping showed a closer homology of the AeSWEET family with that of dicotyledons compared to monocotyledons. Promoter cis-element analysis indicated the potential responses of AeSWEET genes to five phytohormones and seven environmental stressors. Quantitative real-time PCR analysis revealed tissue-specific expression profiles of AeSWEET genes, with two AeSWEET11 genes (AeSWEET11a and AeSWEET11b) showing significantly higher expression levels in fruit tissues. Their expressions were positively correlated with sucrose, fructose, and glucose contents throughout fruit development and ripening. Transient transformation tests in tobacco leaves verified the predominant localization of AeSWEET11a and AeSWEET11b to the plasma membrane. Functional assays in yeast mutants revealed that AeSWEET11a and AeSWEET11b both possessed sucrose and hexose transport activities. These findings highlight the potential of targeting AeSWEET11a and AeSWEET11b to enhance sugar accumulation in the fruit of A. eriantha, thereby providing a foundation for improving the flavor profile of commercial cultivars.