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

Xiaoyu Wei,Fengli Liu,Cheng Chen,Mingjun Li,Fengwang Ma

doi:10.3389/fpls.2014.00569

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.

Highlights

In plants, soluble sugars [i.e., sucrose (Suc), monosaccharides, and polyols] are essential molecules that provide energy and building blocks for growth and development, and constitute osmotic, nutrient, and signal molecules (Ruan, 2014)
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
40 consensus gene sets containing major facilitator superfamily (MFS) domains were selected from the “Gold Delicious” apple genome database, with an EXP cutoff

Summary

Introduction

Soluble sugars [i.e., sucrose (Suc), monosaccharides, and polyols] are essential molecules that provide energy and building blocks for growth and development, and constitute osmotic, nutrient, and signal molecules (Ruan, 2014). The loading and the unloading of the conducting complex, and the allocation of sugars into source and sink cells, are controlled by sugar transporters that mediate the movement of Suc (sucrose transporter: SUT) (Ruan, 2014), reducing monosaccharides (monosaccharide transporter or hexose transporter: MST or HT) (Büttner, 2010; Slewinski, 2011), or sugar alcohols [sorbitol (Sor), mannitol, xylitol, et al.] (Noiraud et al, 2001; Gao et al, 2005; Fan et al, 2009)

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