Abstract
Sugar from plant photosynthesis is a basic requirement for life activities. Sugar transporters are the proteins that mediate sugar allocation among or within source/sink organs. The transporters of the major facilitator superfamily (MFS) targeting carbohydrates represent the largest family of sugar transporters in many plants. Strawberry (Fragaria × ananassa Duchesne) is an important crop appreciated worldwide for its unique fruit flavor. The involvement of MFS sugar transporters (STs) in cultivated strawberry fruit sugar accumulation is largely unknown. In this work, we characterized the genetic variation associated with fruit soluble sugars in a collection including 154 varieties. Then, a total of 67 ST genes were identified in the v4.0 genome integrated with the v4.0.a2 protein database of F. vesca, the dominant subgenome provider for modern cultivated strawberry. Phylogenetic analysis updated the nomenclature of strawberry ST homoeologs. Both the chromosomal distribution and structural characteristics of the ST family were improved. Semi-RT-PCR analysis in nine tissues from cv. Benihoppe screened 34 highly expressed ST genes in fruits. In three varieties with dramatically differing fruit sugar levels, qPCR integrated with correlation analysis between ST transcript abundance and sugar content identified 13 sugar-correlated genes. The correlations were re-evaluated across 19 varieties, including major commercial cultivars grown in China. Finally, a model of the contribution of the sugar transporter system to subcellular sugar allocation in strawberry fruits was proposed. Our work highlights the involvement of STs in controlling strawberry fruit soluble sugars and provides candidates for the future functional study of STs in strawberry development and responses and a new approach for strawberry genetic engineering and molecular breeding.
Highlights
Sugars are among the basic requirements of all living organisms, serving as energy sources, signaling molecules, and carbon skeletons and playing roles inIn plants, sugars are largely produced in green functional leaves via photosynthesis
The major facilitator superfamily (MFS), characterized by 12 transmembrane domains (TMDs), is an ancient, conserved family of secondary transporters, and MFS-type transporters targeting carbohydrates often represent the largest family of sugar transporters in plants[12]
Sugar transporter proteins (STPs) belong to a group of transporters known as monosaccharide transporters (MSTs) or hexose transporters (HTs), which function as proton/sugar symporters for a wide range of monosaccharides, including glucose, fructose, pentose, xylose, ribose, galactose, and mannose[1,18]
Summary
Sugars are among the basic requirements of all living organisms, serving as energy sources, signaling molecules, and carbon skeletons and playing roles inIn plants, sugars are largely produced in green functional leaves via photosynthesis. High plasticity of sugar accumulation in strawberry fruits could be observed from the analysis in 11 varieties with five or more samples collected from different conditions (Fig. 3, Table S2). Heterogeneity in the spatiotemporal expression of selected ST genes in strawberry varieties that accumulate soluble sugars at different levels
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