Integrated transcriptome and metabolome analysis reveals the physiological and molecular responses of allotetraploid rapeseed to ammonium toxicity

Ammonium (NH4+) and nitrate (NO3−) are the main nitrogen (N) sources accessible to plants. However, exclusive NH4+ may causes toxicity to crops. Transcriptomics in conjunction with metabolomics analysis in allotetraploid rapeseed under NH4+ stress showed that NH4+ taken up by roots was mainly transferred to leaves for assimilation. The concentrations of most amino acids, glucose, and glucose-6-phosphate increased whilst malate, succinate, and α-ketoglutarate concentrations decreased in NH4+-treated leaves. Moreover, NH4+ toxicity down-regulated genes related to photosynthesis and impaired the electron transport chain in chloroplasts. The expression of genes related to H2O2 decomposition, as well as the concentrations of monodehydro-ascorbic acid and ascorbic acid, reduced after exposure to NH4+-N, accompanied by higher O2 - and H2O2 accumulation. Taken together, our results indicated that the excessive accumulation of NH4+ in plants caused an imbalance of carbon and N metabolism, impaired photosystems and generated reactive oxygen species, which resulted in toxicity to rapeseed.