Sugar metabolism, chip color, invertase activity, and gene expression during long-term cold storage of potato (Solanum tuberosum) tubers from wild-type and vacuolar invertase silencing lines of Katahdin

Authors: Wiberley-Bradford, Amy; Busse, James; JIANG, JIMING - University Of Wisconsin; Bethke, Paul

Submitted to: BMC Research Notes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/24/2014
Publication Date: 11/16/2014
Citation: Wiberley-Bradford, A.E., Busse, J.S., Jiang, J., Bethke, P.C. 2014. Sugar metabolism, chip color, invertase activity, and gene expression during long-term cold storage of potato (Solanum tuberosum) tubers from wild-type and vacuolar invertase silencing lines of Katahdin. BMC Research Notes. 7(1):801.

Interpretive Summary: High quality potato products, including potato chips and frech fries, are produced from high quality potato tubers. One of the most important determinants of quality for chip potatoes is the amount of glucose and fructose present in the tuber. When these two simple sugars are present in tubers at high concentrations, chips produced from them are dark-colored and bitter tasting. Glucose and fructose are produced when potato tubers are stored at low temperatures. Thus, although low-temperature storage is highly beneficial in that it minimizes spoilage of stored potatoes, most chip potato tubers are stored at relatively warm temperatures to keep sugar contents low. The activity of the enzyme (invertase) that forms glucose and fructose at low temperatures has been greatly reduced in variety Katahdin using molecular tools. In this study, characteristics of potatoes with this low enzyme trait are characterized during long-term storage at 3-9°C. These tubers produced lighter colored chips and had substantially reduced invertase enzyme activity and much lower glucose and fructose contents than control Katahdin through eight months of storage. Changes in gene expression for other enzymes that function in sugar metabolism or starch production were small or did not occur. These findings are important in that they indicate one way that potato tubers can maintain quality during cold storage. They also show that the approach used is highly specific for the target enzyme. Even enzymes with closely linked functions appear to be largely unaffected. These data will benefit potato breeders and others are working to produce higher quality potato varieties. They will also benefit scientists who are working in the areas of starch or sugar formation and use. Consumers and the environment will also benefit if a larger percentage of the harvested potato crop can be converted into a high quality product that the public enjoys rather than being lost to spoilage.

Technical Abstract: Storing potato tubers at low temperatures helps to minimize losses from sprouting and disease but often leads to an accumulation of reducing sugars in a process called cold-induced sweetening. When tubers with increased amounts of reducing sugars are processed at high temperatures, as by frying, they produce dark-colored, bitter-tasting products that contain elevated amounts of acrylamide, a neurotoxin and possible carcinogen. Transgenic potato tubers with decreased levels of vacuolar invertase (VInv), which converts sucrose from starch breakdown to the reducing sugars glucose and fructose, accumulate fewer reducing sugars during cold-induced sweetening than tubers with higher VInv levels. In this study, the characteristics of tubers in which the vacuolar invertase gene VInv was silenced using RNA-interference were investigated with regard to temperature-dependent sucrose and reducing sugar accumulation, chip color and expression of temperature-responsive genes during long-term cold storage. VInv transcript accumulation and enzyme activity in transgenic tubers were lower than those in control tubers at 3-9°C, but transcript accumulation of other sugar-metabolism genes was largely unaffected by invertase silencing. Chips made from transgenic tubers were paler in color than those from untransformed tubers, and transgenic tubers had lower reducing sugar contents. Tuber sucrose contents were higher in transformed lines than in controls. Responses of sugar contents and transcript levels to tuber storage temperature, time in storage and invertase expression shed light on the regulation of sugar metabolism in stored potato tubers.