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

• Carbohydrate metabolism was studied in potatoes stored using commercial practices.
• Tuber sugar contents were stable during cooling and storage at 9 °C but not at 3 °C.
• Changes in expression of carbohydrate metabolism genes occurred in three phases.
• Little or no feedback regulation of gene expression by sugar contents was observed.
• Dramatic increases in sucrose below 5 °C drove reducing sugar accumulation.

Regulation of sugar metabolism in potato tubers stored at low temperatures has significant ramifications for producers and consumers of potato chips and French fries. Low-temperature storage reduces losses due to sprouting and disease but induces undesirable accumulation of the reducing sugars glucose and fructose. These sugars react with free amino acids during frying to produce dark-colored and bitter-tasting products that have elevated levels of acrylamide, a suspected human carcinogen. Elevated amounts of tuber glucose and fructose are produced when vacuolar acid invertase (VInv) 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 temperature-dependent regulation of sugar accumulation and expression of carbohydrate metabolism genes during and after the cooling of tubers to 9 or 3 °C. It was found that transcriptional control of genes central to carbohydrate metabolism occurred in three overlapping phases. First, early in cooling, slight increases in VInv and β-amylase expression occurred. Second, at lower temperatures, large decreases in AGPase and GBSS expression were observed. Finally, when tubers reached 3–5 °C, large increases in β-amylase expression were observed, coinciding with large increases in sucrose in all genotypes and large increases in glucose and fructose in tubers with high VInv expression. Suppression of VInv expression did not alter the expression of β-amylase, AGPase, GBSS, or other genes, indicating that there was little or no feedback from reducing sugars to transcriptional control of carbohydrate metabolism. Invertase activity of WT tubers stored at 9 °C was comparable to that of WT tubers stored at 3 °C, but reducing sugars were markedly higher at the lower temperature. Taken together, 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 β-amylase and decreased starch resynthesis by AGPase and GBSS, and by differences in VInv activity that persist throughout storage.