Physiologic, metabolomic, and genomic investigations reveal distinct glutamine and mannose metabolism responses to ammonium toxicity in allotetraploid rapeseed genotypes

Ammonium (NH4+) toxicity has become a serious ecological and agricultural issue owing to increasing soil nitrogen inputs and atmospheric nitrogen deposition. There is accumulating evidence for the mechanisms underlying NH4+-tolerance in rice and Arabidopsis, but similar knowledge for dryland crops is currently limited. We investigated the responses of a natural population of allotetraploid rapeseed to NH4+ and nitrate (NO3−) and screened one NH4+-tolerant genotype (T5) and one NH4+-sensitive genotype (S211). Determination of the shoot and root NH4+ concentrations showed that levels were higher in S211 than in T5. 15NH4+ uptake assays, glutamine synthetase (GS) activity quantification, and relative gene transcriptional analysis indicated that the significantly higher GS activity observed in T5 roots than that in S211 was the main reason for its NH4+-tolerance. In-depth metabolomic analysis verified that Gln metabolism plays an important role in rapeseed NH4+-tolerance. Furthermore, adaptive changes in carbon metabolism were much more active in T5 shoots than in S211. Interestingly, we found that N-glycosylation pathway was significantly induced by NH4+, especially the mannose metabolism, which concentration was 2.75-fold higher in T5 shoots than in S211 with NH4+ treatment, indicating that mannose may be a metabolomic marker which also confers physiological adaptations for NH4+ tolerance in rapeseed. The corresponding amino acid and soluble sugar concentrations and gene expression in T5 and S211 were consistent with these results. Genomic sequencing identified variations in the GLN (encoding GS) and GMP1 (encoding the enzyme that provides GDP-mannose) gene families between the T5 and S211 lines. These genes will be utilized as candidate genes for future investigations of the molecular mechanisms underlying NH4+ tolerance in rapeseed.