Author
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Bethke, Paul |
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WIBERLEY-BRADFORD, AMY - University Of Wisconsin |
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Submitted to: Spudman
Publication Type: Trade Journal Publication Acceptance Date: 10/15/2015 Publication Date: 11/1/2014 Citation: Bethke, P.C., Wiberley-Bradford, A. 2014. The curse of invertase. Spudman. 52(8):22-23. Interpretive Summary: Technical Abstract: Among the greatest quality concerns for chip and fry processing potato tubers are cold-induced sweetening, sugar end defects, translucent ends, stem-end chip defect and high acrylamide-forming potential. These problems all result from elevated amounts of glucose and fructose, reducing sugars produced by a single enzyme, vacuolar invertase. Despite decades of work in potato breeding and storage management, invertase still causes problems for growers and processors everywhere chip and fry processing potatoes grow, reappearing annually like a curse. How is this possible, since we have known for 50 years that invertase causes cold-induced sweetening, and for fifteen years that it causes sugar end defects? What recent advances give hope that we can someday avoid the curse of invertase? The biochemical pathways linking starch metabolism with sucrose production and reducing sugar accumulation in potato tubers have been known for some time. Years of molecular and genetic studies indicated that multiple components of these pathways contribute to reducing sugar accumulation and affect processing quality. Despite this complexity, we have demonstrated that using genetic modification to decrease invertase activity to very low levels significantly reduced cold-induced sweetening, sugar end defects, stem-end chip defects and acrylamide-forming potential. This relatively straightforward approach, suppressing invertase activity with molecular tools, is being used commercially to develop fry processing and chipping potato varieties with reduced susceptibility to sugar-related defects. Can conventional potato breeding lower invertase activity to the level found in transgenic potatoes? This is extremely challenging, as potato breeders must select for many desired traits and against numerous negative traits in each generation, making rapid progress difficult. But over the past 50 years, there has been substantial progress in reducing invertase activity in potato cultivars. Breeding for cold-induced sweetening resistance has been particularly successful, in part because breeding programs can easily, reproducibly establish low-temperature storage conditions. Breeding efforts focused on chipping potatoes began in the late 1950s and resulted in dramatic reductions in invertase activity and reducing sugars. Combined with improvements in potato storage management, these advances have yielded cultivars that maintain reducing sugars at 0.03 milligrams per gram fresh weight or less. Fry processing clones were not subjected to the same selection pressure for low reducing sugar content as chip processing clones, so breeding for low invertase activity in fry processing varieties has trailed that in chip varieties. However, several recent clones have demonstrated that low reducing sugars and invertase activity will likely become increasingly common in fry processing varieties. Like cold-temperature storage, environmental stresses during tuber growth can increase post-harvest invertase activity. Establishing test plots that reproducibly impose heat, drought or pathogen stress, such as those that cause sugar end defect and stem-end chip defect, is not possible for most potato breeding programs, and direct selection for maintenance of low invertase activity in stressed tubers is difficult. This may be why some cultivars with good resistance to cold-induced sweetening remain sensitive to other defects, such as stem-end chip defect and sugar end defect. We may need better control over the factors that activate the invertase gene and increase invertase activity in response to environmental stresses such as high temperatures. This level of control will likely result from improved potato breeding methods. Many researchers in the potato breeding community are developing and implementing molecular markers linked to high-value chip and fry processing traits. Some of t |
