Abstract
Free asparagine plays a central role in nitrogen storage and transport in many plant species due to its relatively high ratio of nitrogen to carbon. However, it is also a precursor for acrylamide, a Class 2a carcinogen that forms during high-temperature processing and cooking. The concentration of free asparagine was shown to increase by approximately 70% in rye grain in response to severe sulfur deficiency (F-test, p = 0.004), while the concentration of both free asparagine and free glutamine increased (by almost threefold and approximately 62%, respectively) in response to nitrogen application (F-test, p < 0.001 for free asparagine; p = 0.004 for free glutamine). There were also effects of nutrient supply on other free amino acids: The concentration of free proline, for example, showed a significant (F-test, p = 0.019) effect of nitrogen interacting with sulfur, with the highest concentration occurring when the plants were deprived of both nitrogen and sulfur. Polymerase chain reaction products for several genes involved in asparagine metabolism and its regulation were amplified from rye grain cDNA. These genes were asparagine synthetase-1 (ScASN1), glutamine synthetase-1 (ScGS1), potassium-dependent asparaginase (ScASP), aspartate kinase (ScASK), and general control non-derepressible-2 (ScGCN2). The expression of these genes and of a previously described sucrose non-fermenting-1-related protein kinase-1 gene (ScSnRK1) was analyzed in flag leaf and developing grain in response to nitrogen and sulfur supply, revealing a significant (F-test, p < 0.05) effect of nitrogen supply on ScGS1 expression in the grain at 21 days post-anthesis. There was also evidence of an effect of sulfur deficiency on ScASN1 gene expression. However, although this effect was large (almost 10-fold) it was only marginally statistically significant (F-test, 0.05 < p < 0.10). The study reinforced the conclusion that nutrient availability can have a profound impact on the concentrations of different free amino acids, something that is often overlooked by plant physiologists but which has important implications for flavor, color, and aroma development during cooking and processing, as well as the production of undesirable contaminants such as acrylamide.
Highlights
Free asparagine plays a central role in nitrogen storage and transport in many plant species due to its relatively high ratio of nitrogen to carbon (Lea et al, 2007)
A previous study with field-grown rye showed a significant effect of nitrogen availability on free amino acid, and free asparagine, concentration in rye grain, with increased nitrogen availability bringing about a rise in free asparagine concentration (Postles et al, 2013)
The analysis showed an effect of variety, with Festus accumulating significantly more (F-test, p = 0.006) free asparagine than Askari (Table 1D), there was no interaction effect between variety and nutrient treatments, so both varieties responded to nutrient availability in the same manner
Summary
Free asparagine plays a central role in nitrogen storage and transport in many plant species due to its relatively high ratio of nitrogen to carbon (Lea et al, 2007). Interest in its metabolism and accumulation has greatly increased recently due to its role in the formation of acrylamide (C3H5NO), a processing contaminant that is produced in the Maillard reaction, a series of non-enzymic reactions between reducing sugars such as glucose, fructose and maltose, and free amino acids (Halford et al, 2011). The Maillard reaction occurs at the high temperatures generated by frying, baking, roasting, or high-temperature processing, and is responsible for the production of melanoidin pigments and complex mixtures of compounds that impart the flavors and aromas that are associated with fried, baked, and roasted foods. Rye and almost certainly other cereals, free asparagine concentration is the determining factor for acrylamide-forming potential (reviewed by Curtis et al, 2014). In response to the CONTAM report the Commission is currently reviewing additional options for risk management measures
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