Spatial-temporal variations of proline and related amino acids reveal distinct nitrogenous utilization strategies in rice during detoxification of exogenous cyanide.

Cyanide (CN) pollution in agricultural systems impairs amino acid metabolism in rice plants, hence decreasing their quality and yield. Meanwhile, little is known about the effects of CN assimilation on the innate pool of proline (Pro) and its synthesis-related amino acids (Pro-AAs) in rice plants. In this study, a hydroponic experiment was carried out to investigate the effect of exogenous KCN on indigenous levels of Pro-AAs, i.e., Pro, glutamate (Glu), arginine (Arg), and ornithine (Orn) in rice seedlings fertilized with either nitrate (NO) or ammonium (NH) through the biochemical and RT-qPCR analysis. At the same KCN treatment concentration, the relative growth rate of NH-fed rice seedlings was considerably higher than that of NO-fed rice seedlings, but the residual concentration of CN in NH-fed rice tissues was lower than that of NO-fed rice tissues. Based on the UPLC and stoichiometry molar ratio calculations, it is evident that the Glu pathway contributed significantly to Pro synthesis in rice under KCN + NO treatments; whereas the Orn pathway governed the synthesis of Pro in rice under KCN + NH treatments. Moreover, transcriptional and bioinformatics analysis revealed that NH fertilization resulted in spatial-temporal differences in the genetic response in rice tissue during detoxification of CN compared with KCN + NO treatments. These findings suggested that the innate level of Pro serves as "a fishing float" to balance the flux between Pro and Pro-AAs in exogenous KCN-treated rice plants under different nitrogenous nutritional conditions.