Abstract
Sugar allocation is based on the source-to-sink and intracellular transport between different organelles, and sugar transporters are usually involved in these processes. Tonoplast sugar transporters (TST) are responsible for transporting sugar into vacuoles; however, the role of TSTs in root growth and the response to abiotic stress is poorly studied. Here, RNA analysis and promoter-β-glucuronidase staining revealed that a melon TST1 gene (CmTST1) is highly expressed in the roots. The sugar feeding experiment results showed that the expression of CmTST1 in the roots was induced by a relatively high level of sucrose (6%), glucose (3%), and fructose (3%). The ectopic overexpression of CmTST1 in Arabidopsis improved the root and shoot growth of seedlings under high exogenous sugar stress. Furthermore, the ectopic expression of CmTST1 promoted the expression of plasma membrane-located sugar transporters. We proposed that CmTST1 plays a key role in importing sugar transport into the vacuoles of roots in response to metabolic demands to maintain cytosolic sugar homeostasis.
Highlights
Vacuoles are the largest organelles of plant cells, accounting for more than 90% of the volume of mature cells and containing the largest part of monosaccharides in mesophyll cells [1]
The results revealed that the expression of CmTST1 can be induced by higher sucrose (Suc), glucose (Glc), and fructose (Fru) levels
To examine the detailed expression profile of CmTST1 in melon plants, quantitative real-time PCR (qRT-PCR) was performed with RNA extracted from various tissues of 1-month-old plants
Summary
Vacuoles are the largest organelles of plant cells, accounting for more than 90% of the volume of mature cells and containing the largest part of monosaccharides in mesophyll cells [1]. The members of the early responsive to dehydration-6-like (ERD6L) subfamily and the inositol transporter (INT) subfamily of MSTs work as glucose and inositol exporters on the vacuolar membrane, respectively [7,8] Another type of proton-driven sugar symporters has been identified in the vacuolar membrane, namely, the sucrose exporter SUC4 [9]. TST and VGT act as vacuolar sugar/proton antiporters, while ERD6L, INT, and SUC4 act as proton/sugar symporters [10] In addition to these proton-coupled transporters, the tonoplast harbors facilitators that transport their substrates to their respective concentration gradients, such as SWEET [11] and SWEET [12]. The relationship between CmTST1 and the sugar signaling pathways will be discussed
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