The Malus domestica sugar transporter gene family: identifications based on genome and expression profiling related to the accumulation of fruit sugars

Abstract

In plants, sugar transporters are involved not only in long-distance transport, but also in sugar accumulations in sink cells. To identify members of sugar transporter gene families and to analyze their function in fruit sugar accumulation, we conducted a phylogenetic analysis of the Malus domestica genome. Expression profiling was performed with shoot tips, mature leaves, and developed fruit of “Gala” apple. Genes for sugar alcohol [including 17 sorbitol transporters (SOTs)], sucrose, and monosaccharide transporters, plus SWEET genes, were selected as candidates in 31, 9, 50, and 27 loci, respectively, of the genome. The monosaccharide transporter family appears to include five subfamilies (30 MdHTs, 8 MdEDR6s, 5 MdTMTs, 3 MdvGTs, and 4 MdpGLTs). Phylogenetic analysis of the protein sequences indicated that orthologs exist among Malus, Vitis, and Arabidopsis. Investigations of transcripts revealed that 68 candidate transporters are expressed in apple, albeit to different extents. Here, we discuss their possible roles based on the relationship between their levels of expression and sugar concentrations. The high accumulation of fructose in apple fruit is possibly linked to the coordination and cooperation between MdTMT1/2 and MdEDR6. By contrast, these fruits show low MdSWEET4.1 expression and a high flux of fructose produced from sorbitol. Our study provides an exhaustive survey of sugar transporter genes and demonstrates that sugar transporter gene families in M. domestica are comparable to those in other species. Expression profiling of these transporters will likely contribute to improving our understanding of their physiological functions in fruit formation and the development of sweetness properties.

A kinetic model of sugar metabolism in peach fruit reveals a functional hypothesis of a markedly low fructose‐to‐glucose ratio phenotype

“…Similarly, Wei et al . () found that the expression of the MdSWEET4.1 transporter (phylogenetically similar to AtSWEET17) was higher in the leaves than in the fruit of apple trees, and that the fructose concentration was lower in the leaves. The AtSWEET17 transporter is a bidirectional passive transporter (Guo et al ., ), and its activity corresponds with the TpassifFru parameter of the model.…”

Proteomic fingerprinting of apple fruit, juice, and cider via combinatorial peptide ligand libraries and MS analysis

“…Sixty to eighty percent of Sor is produced in apple leaves and almost all Sor and half of Suc are converted to Fru, estimating that 80% of total carbon flux is due to Fru production in fruit . Many MS identifications (listed in Supporting Information Table 1 using Uniprot_ Malus database) are proteins involved in sugar cycle and transport: Sorbitol‐6‐phosphate dehydrogenase (A0A0S1TQH9_MALDO) converts Sor after being taken up into the cytosol of parenchyma cells, into Fru imported in vacuole for accumulation . The balance between sugar and organic acids, due to both glycolysis and TCA cycle, is largely responsible for apple flavor: the most predominant organic acids are malic acid and citric acid, which decrease their abundance during maturation process .…”

Functional uncoupling of the tonoplast proton pump and its effect on the flesh gelling physiological disorder in papaya fruit

“…During papaya fruit development, this organ represents a strong sink of the sugars and water, which allows the fruit to grow. The fruit receive the main sugars by long-distance transport by phloem and sugar accumulations requiring the operation of transporters due to their size and polarity (Wei et al, 2012). The accumulation of sugars in the fruit is directly related to the active transport of hexoses and the synthesis and metabolism of sugars (Zhou and Paull, 2001;Paull et al, 2008).…”

“…The transport of sugar and other solutes into the cell may occur by plasmodesmatas or sugar transporters by the H + /sugar symporter or antiporter, respectively, in the plasma membrane and tonoplast (Lalonde et al, 1999;Ruan, 2014). According to Wei et al (2012), three families of transporters-SUTs, MSTs (or HT), and SWEETs-are mainly implicated in the distribution of sugars within most plant cells. The plant sugar transporters are proton coupled energy-dependent, with stoichiometry of H + /sucrose cotransport determined as 1:1 (Lalonde et al, 1999).…”

“…During ripening the cell wall invertase cleaves the sucrose arriving via the phloem into hexoses, thus maintaining the sucrose source to sink gradient, which maintains the hydrostatic pressure, driving phloem sap movement toward fruit tissue (Ruan, 2014;Zhou and Paull, 2001). The hexoses are then taken up into the fruit parenchyma cells and vacuoles via hexose transporters (Lalonde et al, 1999;Wei et al, 2012). This may mean that the lower plasma membrane energization by decline of ATP hydrolysis activity of the P-ATPase in gelled papaya reduced the load capacity of sugars to the cell in attached fruit, accumulating sugar in the apoplast and causing the flesh gelling disorder.…”