Temperature-dependent regulation of sugar metabolism in wild-type and low-invertase transgenic chipping potatoes during and after cooling for low-temperature storage

Authors: WIBERLEY-BRADFORD, AMY - University Of Wisconsin; Busse, James; Bethke, Paul

Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2015
Publication Date: 5/1/2016
Publication URL: http://handle.nal.usda.gov/10113/61895
Citation: Wiberley-Bradford, A.E, Busse, J.S., Bethke, P.C. 2016. Temperature-dependent regulation of sugar metabolism in wild-type and low-invertase transgenic chipping potatoes during and after cooling for low-temperature storage. Postharvest Biology and Technology. 115:60-71. doi.org/10.1016/j.postharvbio.2015.12.020.

Interpretive Summary: A long-standing goal of the potato industry is to maintain processing quality of potato tubers destined for chip and fry manufacture during low-temperature storage. Low-temperature storage minimizes sprouting and disease but causes an undesirable accumulation of reducing sugars in a process called cold-induced sweetening. In this study, wild-type tubers and tubers in which the vacuolar invertase (VInv) gene was silenced using RNA interference were used to increase our understanding of sugar accumulation, invertase activity, and expression of temperature-responsive genes during and after cooling of tubers for low-temperature storage. A novel feature of this study is that tubers were cooled slowly, as is done in commercial practice, and tubers were stored long-term under conditions that maintained or degraded processing quality. Taken together, the data presented here give insights into a more nuanced view of cold-induced sweetening that encompasses effects of storage temperature and time in storage. The data show that transcriptional control of genes central to carbohydrate metabolism occurs in three overlapping phases during cooling, and as a result, reducing sugar accumulation is controlled both by temperature-regulated changes in sucrose accumulation and by differences in invertase activity that persist in storage even when differences in VInv expression are no longer observed. These data provide insights into why different varieties of potato have different storage temperature requirements. By separating responses that occur at different temperatures and times in storage, we are better able to establish methods to breed for successful storage of processing potatoes at cold temperatures. In the long run, this will benefit the potato industry by assuring a more consistent supply of high quality potatoes and by reducing waste caused by spoilage. Consumers will benefit though improvements in product quality.

Technical Abstract: Regulation of sugar metabolism in cold-stored potato tubers has significant ramifications for potato chip and French fry producers and consumers. Though low-temperature storage reduces losses due to sprouting and disease, it induces accumulation of the reducing sugars glucose and fructose. These react with free amino acids during frying to produce dark-colored and bitter-tasting products that have elevated levels of acrylamide, a suspected carcinogen. Elevated amounts of tuber glucose and fructose are produced when vacuolar acid invertase (VInv, EC 3.2.1.26) hydrolyses sucrose derived from starch degradation. In this study, wild-type tubers and tubers in which VInv expression was reduced by RNA interference were used to study the regulation of sugar metabolism during and after the cooling of tubers for low temperature storage. It was found that transcriptional control of genes central to carbohydrate metabolism occurred in three overlapping phases. First, soon after cooling began, increases in VInv and ß-amylase expression occurred. Next, in the later stages of cooling, large decreases in AGPase and GBSS expression were observed. Finally, when tubers reached 3-5oC, large increases in ß-amylase expression and tuber sucrose contents were observed, as were large increases in glucose and fructose contents in tubers with high VInv expression. These data support a model in which reducing sugar accumulation is controlled both by temperature-regulated changes in sucrose accumulation, resulting from increased starch degradation by ß-amylase (EC 3.2.1.2) and decreased starch resynthesis by AGPase (EC 2.7.7.27) and GBSS (EC 2.4.1.242), and by differences in VInv activity that persist throughout storage.