VvTMT2 encodes a putative tonoplast monosaccharide transporter expressed during grape berry (Vitis vinifera cv. Sultanine) ripening

Sugars produced by photosynthesis are transported to the berries as sucrose via the phloem and imported into the fleshy cells through the plasma membrane and they accumulate in the vacuoles as glucose and fructose during ripening. Therefore, this process requires sugar transporter activity of plasma membrane as well as tonoplast. To examine sugar transport processes in Vitis vinifera, a fulllength cDNA clone encoding a monosaccharide transporter designated as VvTMT2 (Vitis vinifera Tonoplast Monosaccharide Transporter 2) was isolated by RT-PCR from the ripening stage of V. vinifera ‘Sultanine’ berries (GeneBank accession number: JX233818). VvTMT2 cDNA contains a 2220-bp ORF encoding a predicted protein of 739 amino acids with a molecular mass of 79.2 kDa. According to the hydropathy pattern, VvTMT2 possesses 11 transmembrane-spanning domains with an extended cytoplasmic loop between transmembrane helices 6 and 7. This loop spans 371 amino acids, which is approximately 4–5 times longer than the corresponding structures in all other known monosaccharide transporters from prokaryotes and eukaryotes. VvTMT2 exhibited high similarities with other tonoplast monosaccharide transporters from different plant species. A search for cis-regulatory elements in the VvTMT2 promoter region showed that this gene is probably regulated by phytohormones, sugars and abiotic stresses. A semiquantitative RT-PCR was conducted to determine the VvTMT2 expression pattern using mRNAs isolated from various organs such as berries, young and mature leaves, tendrils, roots and pollens. VvTMT2 was highly expressed during the initiation of ripening and over-ripening of berries. However, transcript levels of VvTMT2 were undetectable in some vegetative organs, suggesting that expression of VvTMT2 gene is tissue and organ specific. Thus, VvTMT2 gene may be involved in hexose transport from the cytoplasm to the vacuoles during berry ripening and over-ripening.