Identification of Sorbitol Transporters Expressed in the Phloem of Apple Source Leaves

Abstract

Sorbitol is a major photosynthetic product and a major phloem-translocated component in Rosaceae (e.g. apple, pear, peach, and cherry). We isolated the three cDNAs, MdSOT3, MdSOT4, and MdSOT5 from apple (Malus domestica) source leaves, which are homologous to plant polyol transporters. Yeasts transformed with the MdSOTs took up sorbitol significantly. MdSOT3- and MdSOT5-dependent sorbitol uptake was strongly inhibited by xylitol and myo-inositol, but not or only weakly by mannitol and dulcitol. Apparent K(m) values of MdSOT3 and MdSOT5 for sorbitol were estimated to be 0.71 mM and 3.2 mM, respectively. The protonophore, carbonyl cyanide m-chlorophenylhydrazone (CCCP), strongly inhibited the sorbitol transport. MdSOT3 was expressed specifically in source leaves, whereas MdSOT4 and MdSOT5 were expressed in source leaves and also in some sink organs. MdSOT4 and MdSOT5 expressions were highest in flowers. Fruits showed no or only weak MdSOT expression. Although MdSOT4 and MdSOT5 were also expressed in immature leaves, MdSOT expressions increased with leaf maturation. In addition, in situ hybridization revealed that all MdSOTs were expressed to high levels in phloem of minor veins in source leaves. These results suggest that these MdSOTs are involved in sorbitol loading in Rosaceae.

The genome of the domesticated apple (Malus × domestica Borkh.)

“…Compared with other plant genomes, apple has considerably more copies of key genes related to sorbitol metabolism. These include aldose 6-P reductase (A6PR), which is rate-limiting for sorbitol biosynthesis, sorbitol-dehydrogenase (SDH), which converts sorbitol to fructose in the fruit 25 , and sorbitol transporter PcSOT2, which is specific to Rosaceae fruit 26,27 . In total, A r t i c l e s there are 71 sorbitol metabolism genes in apple; in other species, the number ranges between 9 and 43 (Supplementary Tables 7 and 26, and Supplementary Fig.…”

The high-quality draft genome of peach (Prunus persica) identifies unique patterns of genetic diversity, domestication and genome evolution

“…Integrating the most recent Rosaceae molecular phylogeny 18 with data on sorbitol content 19 , it is evident that leaf sorbitol synthesis and accumulation are restricted to the subfamily Spiraeoideae (for example, apple, peach and cherry), whereas in the subfamilies Rosoideae and Dryoideae, this polyol is comparatively absent 20,21 . Accordingly, in Spiraeoideae, a previous study 22 described sorbitol transporters (SOT) that substantially increase sorbitol uptake, and in cherry, two SOT-encoding genes are known to have a major role in sorbitol accumulation 23 . Besides those encoding transporters, other key genes in sorbitol metabolism encode A6PR (aldose 6-P reductase, which is rate limiting for sorbitol biosynthesis) and SDH (sorbitol dehydrogenase), which converts the alcohol into sugars in fruits 24,25 .…”

Analyses of Expressed Sequence Tags from Apple

“…Sorbitol is then produced through dephosphorylation of sorbitol 6-P by an as yet unidentified 61-kD phosphatase (Zhou et al, 2003;EC 3.1.3.50). The sorbitol is then transported to sink tissues by a family of specialized sorbitol transporters (18 NR sequences) for unloading (Watari et al, 2004). Once in the sink tissue such as fruit, sorbitol dehydrogenase (EC 1.1.1.14; 23 NR sequences) converts sorbitol to Fru (Loescher et al, 1982), thereby reentering the pool of available sugars.…”