Elucidating the mechanisms of senescent sweetening in stored potato tubers
Barrera-Gavira, Jose M.
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Senescent sweetening is a storage disorder that typically occurs following medium to long-term storage of potato tubers in the presence of sprout suppressors at moderate storage temperatures. It represents a significant issue for the processing industry where reducing sugar accumulation results in problems of dark fry colour. Furthermore, the Maillard reaction between reducing sugars and asparagine results in the accumulation of the potential neurotoxin and carcinogen acrylamide in processed products. At present almost nothing is known regarding the mechanisms promoting senescent sweetening which differs from cold-induced sweetening in that it is not reversible by transfer of tubers to higher temperatures. In the present work we set out to test the hypothesis that oxidative damage caused during long term storage is linked to senescent sweetening. A marked difference in storage induced reducing sugar accumulation was observed between a sweetening resistant and a sweetening sensitive cultivar. However, markers of oxidative damage and activities of antioxidant enzymes did not exhibit any specific correlation with reducing sugar accumulation indicating that oxidative damage and senescent sweetening may not be linked. To identify the underlying biochemical causes of sugar accumulation GC/MS was used to quantify a range of primary metabolites in sweetened and unsweetened tubers. Few differences were observed in metabolite profiles however, labelling with [ 13C] glucose indicated a greater capacity for sucrose synthesis in the sweetening resistant compared with the sweetening sensitive cultivar. In addition, differences in specific activity of carbohydrate metabolism enzymes as well as microarray data suggest starch re-synthesis and alternative metabolic sinks for carbon as potential traits linked to sweetening resistance. Moreover, we identified GPT2 as a potential candidate gene associated with the accumulation of sugars during long-term storage. These findings will lead to a better understanding of the mechanisms, processes and genes involved in senescent sweetening and will provide insights into improved storage management in the short-term and the development of senescent sweetening resistant cultivars in the longer term.