Molecular Identification and Physiological Characterization of a Novel Monosaccharide Transporter from Arabidopsis Involved in Vacuolar Sugar Transport

Abstract

The tonoplast monosaccharide transporter (TMT) family comprises three isoforms in Arabidopsis thaliana, and TMT-green fluorescent protein fusion proteins are targeted to the vacuolar membrane. TMT promoter-b-glucuronidase plants revealed that the TONOPLAST MONOSACCHARIDE TRANSPORTER1 (TMT1) and TMT2 genes exhibit a tissue-and cell type-specific expression pattern, whereas TMT3 is only weakly expressed. TMT1 and TMT2 expression is induced by drought, salt, and cold treatments and by sugar. During cold adaptation, tmt knockout lines accumulated less glucose and fructose compared with wild-type plants, whereas no differences were observed for sucrose. Cold adaptation of wild-type plants substantially promoted glucose uptake into isolated leaf mesophyll vacuoles. Glucose uptake into isolated vacuoles was inhibited by NH 4 þ , fructose, and phlorizin, indicating that transport is energy-dependent and that both glucose and fructose were taken up by the same carrier. Glucose import into vacuoles from two cold-induced tmt1 knockout lines or from triple knockout plants was substantially lower than into corresponding wild-type vacuoles. Monosaccharide feeding into leaf discs revealed the strongest response to sugar in tmt1 knockout lines compared with wild-type plants, suggesting that TMT1 is required for cytosolic glucose homeostasis. Our results indicate that TMT1 is involved in vacuolar monosaccharide transport and plays a major role during stress responses.

A Sugar-Inducible Protein Kinase, VvSK1, Regulates Hexose Transport and Sugar Accumulation in Grapevine Cells

“…VvHT6 transcript abundance correlates well with hexose accumulation in the berries and was recently proposed to play a significant role in hexose accumulation during berry ripening (Deluc et al, 2007). If VvHT6, which is highly homologous to the tonoplast Glc transporter AtTMT2 (Wormit et al, 2006), acts as a tonoplast sugar transporter, it would only favor indirectly Glc uptake across the plasma membrane by decreasing the cytosolic Glc concentration. VvHT3 transcript accumulates in young berries but also later during the phase of sugar storage (Hayes et al, 2007).…”

Arabidopsis INOSITOL TRANSPORTER2 Mediates H+ Symport of Different Inositol Epimers and Derivatives across the Plasma Membrane

“…This includes most members of the STP subfamily (Bü ttner, 2007), one member of the PLT subfamily (AtPLT5; Klepek et al, 2005), one member of the TMT subfamily of tonoplastlocalized monosaccharide transporters (AtTMT1; Wormit et al, 2006), one member of the VGT subfamily of vacuolar Glc transporters (AtVGT1; Aluri and Bü ttner, 2007), and one member of the AtINT subfamily (AtINT4; Schneider et al, 2006). Individual members of other subfamilies were studied (AtERD6 [At1g08930], Kiyosue et al, 1998;pGlcT [At5g16150], Weber et al, 2000;AtSFP1 [At5g27350], Quirino et al, 2001), but not functionally characterized.…”

Tonoplast Sugar Transporters (SbTSTs) putatively control sucrose accumulation in sweet sorghum stems

“…Testing of this hypothesis will potentially require characterizing loss-of-function mutations in both genes, since they were found to be partially functionally redundant in Arabidopsis thaliana. 29 These efforts are currently underway. Furthermore, these data suggest TSTs have been the target of selection for sugar accumulation in both the sweet sorghum stem and the sugar beet taproot.…”

“…A different class of sugar transport proteins, Tonoplast Sugar Transporters (TSTs, also known as Tonoplast Monosaccharide Transporters), is located on the tonoplast and function as H C /sucrose antiporters to transport sucrose into the vacuole. [29][30][31][32] Recently, a TST was shown to be responsible for sucrose accumulation within the sugar beet taproot. 12 Whether TST or SWEET genes have also been selected during the domestication of other major sucrose storage crops, such as sweet sorghum or sugarcane stem tissues, is not known.…”