The gene PbTMT4 from pear (Pyrus bretschneideri) mediates vacuolar sugar transport and strongly affects sugar accumulation in fruit

Abstract

Tonoplast monosaccharide transporters (TMTs) play important roles in vacuolar sugar accumulation in plants. In this study, six TMT genes (PbTMT1-6) were identified in the Pyrus bretschneideri genome database, and their expression profiles were correlated with soluble sugar contents during the pear (P. bretschneideri cv. Ya Li) fruit development process. Subsequently, PbTMT4 was identified as a strong contributor to fructose, glucose and sucrose accumulation in fructescence of pears. Heterologous expression of PbTMT4, in the hexose transporter-deficient yeast strain EBY.VW4000, facilitated growth in media containing low levels of glucose, fructose, sucrose or sorbitol. In addition, PbTMT4-transformed tomato plants flowered and bore fruit significantly earlier than wild-type (WT) plants, and glucose and fructose levels in mature tomatoes were increased by about 32 and 21% compared with those in WT plants. However, no obvious alterations in sucrose content, plant height and weight per fruit were observed. Finally, subcellular localization experiments in transformed Arabidopsis plants showed that PbTMT4 is localized to tonoplast vesicles of protoplasts. These preliminary results suggest that PbTMT4 participates in vacuolar accumulation of sugars, and thus affects plant growth and development.

Molecular cloning and functional analysis of a sugar transporter gene (CsTST2) from cucumber (Cucumis sativus L.)

“…Similar results have also been found for TST members in other plant species, such as CmTST2 [31], PbTMT4 and PbTMT5 [25]. Overexpression of PbTMT4 in tomato resulted in early flowering and ripening phenotypes compared with wild-type plants [32]. Furthermore, CsTST2 also displayed moderate expression levels in leaves and roots ( Figure 5), implying that it may also play a role in these organs.…”

“…However, heterologous expression of CsTST2 was unable to restore the growth on the medium containing glucose ( Figure 7(A)). The hexose transport capacity of CsTST2 is different from that of other identified TSTs, such as VvTMT1 [48], PbTMT4 [32], ClTST2 [27], AtTMT1 and AtTMT2 [17,18], implying that there might be other sugar transporters to import glucose into the vacuoles of cucumber. In addition, the growth of the SUSY7/ura3 transformants was restored to some extent on the medium containing sucrose, suggesting that CsTST2 also enables sucrose uptake (Figure 7(B)).…”

“…Sugars are always accumulated in the vacuoles as glucose and fructose during ripening, and both of the homologs of CsTST2 (CmTST2 and ClTST2) are located in vacuoles and their expression is positively related to sugar accumulation in fruits [27,31]. Some pear TST genes (PbTMT2, PbTMT3 and PbTMT4) have also been found to be correlated with the levels of fructose and glucose during pear fruit development [32,33]. Moreover, CsTST2 showed relatively high expression in male and female flowers ( Figure 5), suggesting that it may also be involved in the transport of sugars for flower development in cucumber.…”

“…TSTs belong to the major facilitator superfamily (MFS) and usually possess 12 transmembrane (TM) helices separated by a central cytosolic loop between helices 6 and 7 [25][26][27]. In recent years, many TST family genes have been identified and characterized at genome-wide level in various plant species, such as tomato [28], sweet orange [29], sweet sorghum [30], melon [31], watermelon [27], and pear [25,32,33]. In addition, several TST genes have been functionally characterized for their roles in sugar transport across the tonoplast.…”

“…For example, BvTST2.1 acts as a H þ /sucrose antiporter and plays an essential role in vacuolar sucrose uptake in sugar beet [24]. The expression of PbTMT4 is correlated with glucose and fructose contents during pear fruit development, and PbTMT4 could mediate the transport of glucose and fructose when expressed in a yeast mutant strain EBY.VW4000, suggesting that PbTMT4 is involved in the control of sugar accumulation in pear fruit [32]. These findings indicate that TST proteins can act as important sugar transporters and are responsible for sugar accumulation during plant growth and development.…”