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 experi-ment 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 (NO3-) or ammonium (NH4+) through the biochemical and RT-qPCR analysis. At the same KCN treatment concentration, the relative growth rate of NH4+-fed rice seedlings was considerably higher than that of NO3--fed rice seedlings, but the residual concentration of CN- in NH4+-fed rice tissues was lower than that of NO3--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 + NO3- treatments; whereas the Orn pathway governed the synthesis of Pro in rice under KCN + NH4+ treatments. Moreover, transcriptional and bioinformatics analysis revealed that NH4+ fertilization resulted in spatial-temporal differences in the genetic response in rice tissue during detoxification of CN- compared with KCN + NO3- 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.