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Regulation of Expression and Activity of Sorbitol Dehydrogenase in Apple
Department of Horticulture
Sorbitol is the major photoassimilate in apple, and it is primarily converted to fructose by sorbitol dehydrogenase. Slow rates and/or inefficient utilization of sorbitol by apple fruit, as determined by patterns of sorbitol dehydrogenase expression and activity, can adversely affect yield, quality, and economic return. By understanding the mechanisms regulating expression and activity of sorbitol dehydrogenase in apple fruit, new horticultural and/or biochemical or genetic strategies could optimize the competitive position and resulting quality and yield of the fruit.
2011 Project Description
A study was performed to assess the effect of drought on SORBITOL DEHYDROGENASE (SDH, EC 126.96.36.199) expression and activity in apple buds. Apple trees on two different rootstocks were subjected to varying levels of drought and to antitranspirant treatments to determine effects on SDH, on water use efficiency, and on tree growth. A study was also performed to analyze the soluble carbohydrate dynamics in a spur or shoot bearing a flower and fruit from 1 week prior to bloom until 6 weeks afterwards. Samples from both studies are being analyzed and data compiled. In addition, interest in using Arabidopsis as a model for studying regulation of SDH expression led to a study of the role of the one SDH gene which it bears.
Using two independent Arabidopsis (Arabidopsis thaliana) sdh knock-out mutants, we have continued to study the role SDH plays in ribitol metabolism. Sorbitol content increased in both wild-type (WT) and mutant plant leaves during drought stress, but mutants showed a dramatically different phenotype, dying even if re-watered. The lack of functional SDH in mutant plants was accompanied by accumulation of foliar sorbitol and at least 10-fold more ribitol, neither of which decreased in mutant plants after re-watering.
In addition, mutant plants were uniquely sensitive to ribitol which prevented them from completing seed germination and inhibited seedling development in a concentration-dependent manner, effects not observed with other polyols or with ribitol-treated WT plants. Ribitol catabolism may occur solely through SDH in Arabidopsis, though at only 30% the rate of sorbitol.
The results indicated a role for SDH in metabolism of sorbitol to fructose and in ribitol conversion to ribulose in Arabidopsis during recovery from drought stress.
The finding that sorbitol dehydrogenase (SDH) metabolizes ribitol in Arabidopsis has not previously been reported. The role of SDH in plants for which sorbitol and ribitol are minor carbohydrates is not known. The study of these mutants can reveal this role.
Archbold, D.D., M. Nosarzewski, B. Wu, and P. Vuppalapati. 2011. Does availability of soluble carbohydrate reserves determine apple fruit set IXth International Symposium on Integrating Canopy, Rootstock, and Environmental Physiology in Orchard Systems, Geneva, NY. Acta Hort. 903: 795-801.