Cloning, Expression, and Characterization of Sorbitol Transporters from Developing Sour Cherry Fruit and Leaf Sink Tissues

Abstract

The acyclic polyol sorbitol is a primary photosynthetic product and the principal photosynthetic transport substance in many economically important members of the family Rosaceace (e.g. almond [Prunus dulcis (P. Mill.) [Pyrus communis]). To understand key steps in long-distance transport and particularly partitioning and accumulation of sorbitol in sink tissues, we have cloned two sorbitol transporter genes (PcSOT1 and PcSOT2) from sour cherry (Prunus cerasus) fruit tissues that accumulate large quantities of sorbitol. Sorbitol uptake activities and other characteristics were measured by heterologous expression of PcSOT1 and PcSOT2 in yeast (Saccharomyces cerevisiae). Both genes encode proton-dependent, sorbitol-specific transporters with similar affinities (K m sorbitol of 0.81 mm for PcSOT1 and 0.64 mm for PcSOT2). Analyses of gene expression of these transporters, however, suggest different roles during leaf and fruit development. PcSOT1 is expressed throughout fruit development, but especially when growth and sorbitol accumulation rates are highest. In leaves, PcSOT1 expression is highest in young, expanding tissues, but substantially less in mature leaves. In contrast, PcSOT2 is mainly expressed only early in fruit development and not in leaves. Compositional analyses suggest that transport mediated by PcSOT1 and PcSOT2 plays a major role in sorbitol and dry matter accumulation in sour cherry fruits. Presence of these transporters and the high fruit sorbitol concentrations suggest that there is an apoplastic step during phloem unloading and accumulation in these sink tissues. Expression of PcSOT1 in young leaves before completion of the transition from sink to source is further evidence for a role in determining sink activity.Sorbitol (an acyclic polyol) and Suc are the primary photosynthetic products and the major phloemtranslocated components in a number of economically important taxa in the family Rosaceae, in particular in the subfamilies Pomoideae (e.g. (Loescher and Everard, 1996). Sorbitol is often the dominant translocated photosynthetic product. In mature apricot (Prunus armeniaca) leaves, for example, 65% to 75% of the translocated carbon is sorbitol (Bieleski and Redgwell, 1985). In these species, understanding the factors involved in facilitating and regulating sorbitol transport, including export from the leaf, long-distance distribution via the phloem network, and import into various sink tissues, is at least as important as those of Suc and other sugars.In the past 10 years, sugar transporters have been extensively studied in various sink and source tissues with the isolation of two distinct families of sugar carriers: the disaccharide transporters that primarily catalyze Suc transport and the monosaccharide transporters that mediate transport of the hexose sugars (for review, see Weise et al., 2000; Williams et al., 2000). Active uptake of Suc across the plasma membrane in all known cases involves an H ϩ -Suc symporter (Lemoine, 2000). The biochemical properties of the Suc transporters ...

Analysis of Transport Activity of Arabidopsis Sugar Alcohol Permease Homolog AtPLT5

“…However, this inhibition was interpreted to be an artifact of heterologous expression in yeast and a high specificity for sorbitol was attributed to PcSOT1 (15). The analysis of AtPLT5 from Arabidopsis in this study indicated that transported substrates include a wide range of sugars and sugar alcohols.…”

“…A second group of four genes is most similar to the plant vacuolar inositol permeases (Mitr1 and Mitr2) first characterized from Mesembryanthemum (13). AtPLT5 (At3g18830) is a member of a third group of 6 genes that is most closely related to the celery mannitol permease AgMAT1 (14), and the sorbitol permeases PcSOT1 and PcSOT2 from cherry fruit (15) and PmPLT1 and PmPLT2 from Plantago (16). Based on localization data, these sugar alcohol permease homologs appear to have two main functions: 1) loading sugar alcohols into the phloem for long-distance transport (14,16) and 2) uptake of sugar alcohols into sink cells.…”

“…A number of sugar alcohols inhibited mannitol uptake as did glucose, indicating that potentially AgMAT1 also transports hexoses. Other homologs have only been tested for the ability to transport sugar alcohols (15,16), and the substrate specificity of transporters in this family has remained in question.Whereas celery and Plantago are known to transport mannitol and sorbitol, respectively, in the phloem and sour cherry fruit accumulate sorbitol during ripening, Arabidopsis does not translocate or accumulate either sugar alcohol in significant quantities. Therefore, it is likely that the sugar alcohol permease homologs in Arabidopsis have different physiological roles than those proposed for homologous transporters in celery, Plantago, and sour cherry (14 -16).…”

“…A number of sugar alcohols inhibited mannitol uptake as did glucose, indicating that potentially AgMAT1 also transports hexoses. Other homologs have only been tested for the ability to transport sugar alcohols (15,16), and the substrate specificity of transporters in this family has remained in question.…”

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.…”