In
A. thaliana, AtPLT5 (polyol transporter 5) was reported to be a plasma membrane-localized MST-like protein that mediates the transport of sorbitol, glucose, galactose, ribose, xylose, mannitol, glycerol, and inositol
[24]. Using transgenic
Xenopus oocytes as the model and glucose and glycerol as substrate examples, the transport activity of AtPLT5 was demonstrated to be pH-dependent
[24]. Maximal transport activity was demonstrated at pH 5.5
[24]. At pH 6.5, the transport activity was reduced; at pH 7.5, there was no transport activity
[24]. When the extracellular pH was brought back to 5.5, the transport activity resumed
[24]. AtPLT5 has a broad spectrum of substrates and is found to be expressed in various tissues
[24]. Therefore, it is proposed that AtPLT5 is possibly involved in the retrieval of sugars from the apoplast
[24]. The pH-dependent sugar/proton symporter activity of STPs was also reported in apple (
Malus domestica)
[25]. MdSTP13a was reported to be the transporter of both hexose and sucrose competitively to provide the sugars for pollen tube growth
[25]. Using transgenic yeast as the model, optimal glucose or sucrose uptake by MdSTP13a was established at pH 6
[25]. An increase or decrease in the pH resulted in declined transport activity
[25]. Different STPs have different optimal pH for their transport activities. For example, DgSTP1 from
Datisca glomerata has the optimal pH for transport activity at pH 4.5
[26]. In plants, the transport of sugars is a major strategy to distribute or store nutrients
[10]. Since different cellular compartments have different pH, understanding the pH dependence of the activities of sugar transporters is essential for the interpretation of the biological functions.
3.3. Paralogs of MSTs Have Differential Expression Patterns to Serve Different Functions
In
A. thaliana,
AtSTP1 was found to have a consistently high expression level in both the root and leaf among all 14 of the
STPs identified, under normal conditions
[27]. However, in the root, the expression of
AtSTP13 was highly inducible by salinity and ABA treatment
[27]. Although the expression of
ATSTP1 in the root was also induced by salinity, the fold change is much less than that of
AtSTP13 [27]. Both
stp1 and
stp13 mutants had reduced abilities to uptake glucose and fructose, while
stp1 also had reduced galactose uptake
[27]. After salt treatment, the leak of glucose from the
stp13 mutant was enhanced
[27]. Based on the expression data and the sugar flux data, it was suggested that AtSTP13 mediates the reabsorption of monosaccharides leaked from damaged cells under salt stress while AtSTP1 is the major contributor of monosaccharide uptake under normal conditions
[27]. Such differential functions of STPs in the same species are in line with another expression study on
STPs in
O. sativa.
TIn this study, the expressions of
STPs were found to be responsive to stresses including cold, high temperature, and submergence
[28]. However, the patterns of expression upon the same stress were diverse among various
STPs, which also had different expression patterns in different tissues
[28].