Sorbitol Metabolism and Sink-Source Interconversions in Developing Apple Leaves

Abstract

In apple (Mais domestica Borkh.) sorbitol is the primary product of photosynthesis, the major translocated form of carbon, and a common fruit constituent and storage compound. Previous work on sorbitol metabolism has revealed a NADPH-dependent aldose 6-phosphate reductase (A6PR) in green tissues, and a NAD-dependent sorbitol dehydrogenase in nongreen tissues. Results here show a decrease in sorbitol dehydrogenase activity and an increase in A6PR activity as leaves developing in the spring undergo the transition from sink to source. Sorbitol dehydrogenase activity reached a minimum as A6PR peaked. These changes were related to increases in leaf carbohydrate levels, especially sorbitol, and to increases in rates of net photosynthesis. Studies conducted in the autumn on senescing leaves also showed changes in enzyme activites, leaf carbohydrate levels, and photosynthesis. At this time, however, sorbitol dehydrogenase increased in specific activity, whereas A6PR activity, leaf carbohydrates, and photosynthetic rates all decreased substantially. Other experiments showed differences in the ability of young and mature leaves to metabolize sorbitol and in the distribution of sorbitol enzymes in leaves at transitional developmental stages. The results suggest that sorbitol metabolism in apple is tightly controlled and may be related to mechanisms regulating partitioning or source and sink activity.Changes in the activities of enzymes responsible for synthesis and degradation of translocatable carbohydrates have been studied in several plants. Justification for such studies is due in part to the role these enzymes may play in the capacity of a photosynthesizing leaf to export carbohydrates. Equally important is the role these enzymes must play in mechanisms determining sink activity. Studies of the factors controlling activities of the enzymes involved could lead to a better understanding of control processes in photosynthesis, translocation, and storage (9).For both theoretical and practical reasons, developing leaves represent interesting models for studying metabolism of translocatable carbohydrates. Theoretically, information on transformation of a leaf from a heterotrophic, importing organ to an autotrophic, exporting organ should also provide insight into those mechanisms associated with onset of export. From a practical viewpoint a leaf can contribute to plant yield only when it is autotrophic and has begun to export.

Carbon Fluxes in Mature Peach Leaves

“…Therefore, did sucrose after the end of pulse labeling. The fact that e detected radiosucrose, which was heavily labeled, may and sorbitol may blade and that not be metabolized in the mature leaf blade (2,12) gives rise idual tissues (19) to the hypothesis that the synthesis of labeled sorbitol from )tal radioactivity labeled hexoses originates partly from sucrose, as suggested se of herbaceous by the present results. The futile cycle, consisting of a cyclic (22) …”

“…The catabolic pathways of sorbitol involve sorbitol dehydrogenase (NADdependent, EC 1.1.1.14 [17] or NADP-dependent [29]) or sorbitol oxidase (28) in sink tissues. These enzymes are not expressed in mature leaves (2,12,15). Therefore, the fate of sorbitol in mature leaves is either translocation or storage.…”

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

“…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 . We found that in contrast to other species with sequenced genomes, apple and peach SDH and SOT are large gene families (Supplementary Table 17).…”