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.

Amy E Wiberley-Bradford,Paul C Bethke,Jiming Jiang,James S Busse

doi:10.1186/1756-0500-7-801

Abstract

BackgroundStoring potato tubers at low temperatures minimizes sprouting and disease but can cause an accumulation of reducing sugars in a process called cold-induced sweetening. Tubers with increased amounts of reducing sugars produce dark-colored, bitter-tasting fried products with elevated amounts of acrylamide, a possible carcinogen. Vacuolar invertase (VInv), which converts sucrose produced by starch breakdown to glucose and fructose, is the key determinant of reducing sugar accumulation during cold-induced sweetening. In this study, wild-type tubers and tubers in which VInv expression was reduced by RNA interference were used to investigate time- and temperature-dependent changes in sugar contents, chip color, and expression of VInv and other genes involved in starch metabolism in tubers during long-term cold storage.ResultsVInv activities and tuber reducing sugar contents were much lower, and tuber sucrose contents were much higher, in transgenic than in wild-type tubers stored at 3-9°C for up to eight months. Large differences in VInv mRNA accumulation were not observed at later times in storage, especially at temperatures below 9°C, so differences in invertase activity were likely established early in the storage period and maintained by stability of the invertase protein. Sugar contents, chip color, and expression of several of the studied genes, including AGPase and GBSS, were affected by storage temperature in both wild-type and transgenic tubers. Though transcript accumulation for other sugar-metabolism genes was affected by storage temperature and duration, it was essentially unaffected by invertase silencing and altered sugar contents. Differences in stem- and bud-end sugar contents in wild-type and transgenic tubers suggested different compartmentalization of sucrose at the two ends of stored tubers.ConclusionsVInv silencing significantly reduced cold-induced sweetening in stored potato tubers, likely by means of differential VInv expression early in storage. Transgenic tubers retained sensitivity to storage temperature, and accumulated greater amounts of sucrose, glucose and fructose at 3°C than at 7-9°C. At each storage temperature, suppression of VInv expression and large differences in tuber sugar contents had no effect on expression of AGPase and GBSS, genes involved in starch metabolism, suggesting that transcription of these genes is not regulated by tuber sugar content.

Highlights

Storing potato tubers at low temperatures minimizes sprouting and disease but can cause an accumulation of reducing sugars in a process called cold-induced sweetening
Data for Vacuolar acid invertase (VInv) expression in RNA interference (RNAi) and WT tubers at later times were not available, differences in expression and activity along the tuber axis were not reported, and effects of VInv suppression and related sugar accumulation on expression of other genes involved in starch metabolism were not investigated
Tubers stored at 3°C reached their final storage temperature in January, and beginning with the March sampling, a dramatic increase in sucrose contents occurred, with values rising to 6–7 mg g−1 fresh weight (FW) in WT and 14 mg g−1 FW in RNAi tubers

Summary

Introduction

Storing potato tubers at low temperatures minimizes sprouting and disease but can cause an accumulation of reducing sugars in a process called cold-induced sweetening. Tubers with increased amounts of reducing sugars produce dark-colored, bitter-tasting fried products with elevated amounts of acrylamide, a possible carcinogen. Color and taste depend largely on the abundance of reducing sugars, primarily glucose and fructose, in raw tubers. During frying, these reducing sugars react with free amino acids in a nonenzymatic Maillard reaction that results in darkcolored, bitter-tasting products [2,3]. These reducing sugars react with free amino acids in a nonenzymatic Maillard reaction that results in darkcolored, bitter-tasting products [2,3] Another product of the Maillard reaction is acrylamide, a neurotoxin and suspected carcinogen [4,5]. Up to 20% of the potatoes intended for processing in the United States are rejected because their reducing sugar contents are too high [9]

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