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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.
2009 Project Description
The results of this work were reported at regional research conference, at an invited seminar at a peer institution, and have been submitted for publication to a peer-reviewed journal.
Sorbitol is the primary photosynthate and translocated carbohydrate in apple (Malus domestica Borkh.) and is converted to fructose by SORBITOL DEHYDOGENASE (SDH, EC 188.8.131.52) in fruit and other tissues. Studies were conducted to determine the expression of nine SDH genes, SDH activity, and sorbitol content of apple 1) in buds and floral tissues from dormancy to bloom, 2) in leaves and shoot tips of trees on two rootstocks, the moderately vigorous MM111 and the dwarfing M9, and 3) in shoot tips in response to application of prohexadione-Ca to suppress shoot growth and defoliation and girdling (D/G) to deprive the shoot tip of sorbitol.
In mature, orchard-grown trees, sorbitol was the main soluble carbohydrate in expressed xylem sap from dormancy to bloom at levels >3-6-fold those of glucose and fructose, the other major sugars present. Sorbitol levels increased from dormancy to its highest concentration at the half inch green stage and declined by bloom, while those of the other sugars increased. SDH activity per mg protein increased over 4-fold from dormancy to flowering. Three out of the nine known SDH genes, SDH1, SDH2 and SDH3, were expressed in immature and mature leaves and all buds from dormancy to bloom as well as in all floral organs, except that only SDH3 transcript was found in stamen tissue. Two genes, SDH6 and SDH9, were floral-tissue specific, SDH6 transcript was detected in all floral organs except stamens at full bloom, and SDH9 was only expressed in anthers with pollen. In buds and leaves of young, container-grown trees, SDH1 and SDH2 generally accounted for the majority of total SDH expression. There were generally no effects of rootstock on SDH expression, SDH activity, or sorbitol concentration in leaves, while apical shoot tips on M9 rootstock exhibited greater SDH activity than those on MM111 or lateral shoot tips on either rootstock, though SDH expression of apical and lateral shoot tips on M9 was lower than on MM111. Prohexadione-Ca reduced apical but not lateral shoot growth, increased apical but not lateral shoot tip sorbitol content, had no effect on SDH activity, and increased SDH1 expression of all shoot tips. D/G treatment reduced shoot growth, sorbitol content, and SDH activity, but increased SDH1 expression of apical shoot tips only and SDH2 expression of lateral shoot tips only.
This work indicated that sorbitol and other sugars are abundantly available from dormancy to bloom, that SDH activity increased during this period, and that SDH expression is at least in part developmentally regulated within the individual floral and leaf tissues. In shoot tips and leaves of young trees, SDH transcript level was not correlated with sorbitol availability or SDH activity, suggesting other factors have significant regulatory effects after SDH expression on SDH activity.