Characterization of the SWEET Gene Family in Blueberry (Vaccinium corymbosum L.) and the Role of VcSWEET6 Related to Sugar Accumulation in Fruit Development

Abstract

Sugars will eventually be exported transporters (SWEETs) are essential transmembrane proteins involved in plant growth, stress responses, and plant-pathogen interactions. Despite their importance, systematic studies on SWEETs in blueberries (Vaccinium corymbosum L.) are limited. Blueberries are recognized for their rapid growth and the significant impact of sugar content on fruit flavor, yet the role of the SWEET gene family in sugar accumulation during fruit development remains unclear. In this study, 23 SWEET genes were identified in blueberry, and their phylogenetic relationships, duplication events, gene structures, cis-regulatory elements, and expression profiles were systematically analyzed. The VcSWEET gene family was classified into four clades. Structural and motif analysis revealed conserved exon-intron organization within each clade. RT-qPCR analysis showed widespread expression of VcSWEETs across various tissues and developmental stages, correlating with promoter cis-elements. VcSWEET6a, in particular, was specifically expressed in fruit and showed reduced expression during fruit maturation. Subcellular localization indicated that VcSWEET6a is located in the endoplasmic reticulum. Functional assays in yeast confirmed its role in glucose and fructose uptake, with transport activity inhibited at higher sugar concentrations. Overexpression of VcSWEET6a in blueberries resulted in reduced sugar accumulation. These findings offer valuable insights into the role of VcSWEETs in blueberry sugar metabolism.

Keywords: SWEET transporter; VcSWEET6; blueberry; expression pattern; sugar accumulation.

Figure 1

Polygenetic relationship and synteny analysis of VcSWEETs from blueberry: (a) Phylogenetic tree of the SWEET gene family in blueberry (Vaccinium corymbosum L.), rice (Oryza sativa), Arabidopsis (Arabidopsis thaliana L.), and grape (Vitis vinifera L.). (b) Synteny analysis of VcSWEETs in blueberry. (c,d) Synteny analysis of VcSWEET genes between blueberry and Arabidopsis (Arabidopsis thaliana L.), cranberry (Arabidopsis thaliana L.), corn (Zea mays L.), and rice(Oryza sativa). Gray lines in the background indicated the syntenic blocks within blueberry and other plant genomes. The highlighted color represented VcSWEETs with synteny in different genomes.

Figure 2

Phylogenetic relationships, conserved motif composition, and gene structure of 23 VcSWEETs: (a) Phylogenetic tree. (b) Motif pattern of VcSWEETs. The 8 colored boxes represent 8 different motifs, and their positions represent the positions on the protein. (c) Gene structure. CDS and introns are represented by red rectangles and black single lines, respectively.

Figure 3

Prediction of cis-acting elements in the promoters of VcSWEETs. The gradient color in the cell represents the number.

Figure 4

Expression pattern of VcSWEETs during different organizations and during fruit development periods.

Figure 5

Subcellular location analysis of VcSWEET6a was performed in N. benthamiana leaf. The free GFP was the control. Red fluorescence represents the control.

Figure 6

Yeast growth of VcSWEET6a expressed in yeast (Saccharomyces cerevisiae). Yeast growth assay of EBY.VW4000 transformed with pDR196 and pDR196-VcSWEET6a on 2% maltose, galactose, glucose, and fructose: (a) Yeast growth under different carbon sources; (b) The growth of yeast under different sugar concentrations.

Figure 7

Overexpression of VcSWEET6a enhances sucrose accumulation in transgenic blueberry fruits: (a) Expression of VcSWEET6a in different overexpressed lines. (b) Images of wild-type (WT), pGREENII-62-SK-overexpression, and VcSWEET6a-overexpression blueberry. (c) The content of sucrose. (d) The content of fructose. (e) The content of glucose. “*” indicates significant differences at the 0.05 level, and “**” indicates significant differences at the 0.01 level. “***” indicates significant differences at the 0.001 level.