Résumé
Although organic nitrogen (N) compounds are ubiquitous in soil solutions, their potential role in plant N nutrition has been questioned. We performed a range of experiments on Arabidopsis thaliana genetically modified to enhance or reduce root uptake of amino acids. Plants lacking expression of the Lysine Histidine Transporter 1 (LHT1) displayed significantly lower contents of C-13 and N-15 label and of U-C-13(5),N-15(2) L-glutamine, as determined by liquid chromatography-mass spectrometry when growing in pots and supplied with dually labelled L-glutamine compared to wild type plants and LHT1-overexpressing plants. Slopes of regressions between accumulation of C-13-labelled carbon and N-15-labelled N were higher for LHT1-overexpressing plants than wild type plants, while plants lacking expression of LHT1 did not display a significant regression between the two isotopes. Uptake of labelled organic N from soil tallied with that of labelled ammonium for wild type plants and LHT1-overexpressing plants but was significantly lower for plants lacking expression of LHT1. When grown on agricultural soil plants lacking expression of LHT1 had the lowest, and plants overexpressing LHT1 the highest C/N ratios and natural N-15 abundance suggesting their dependence on different N pools. Our data show that LHT1 expression is crucial for plant uptake of organic N from soil.
Brief Summary We studied the potential role of organic nitrogen (N) for plant N nutrition by feeding dual-labelled glutamine to soil-grown Arabidopsis thaliana mutants with enhanced or impeded expression of the amino-acid transporter LHT1. Significant differences between the genotypes in root contents of labelled glutamine and of N-15 and C-13 validate that it is the glutamine per se that is taken up by the root and not some product derived from it by microbial activity. Our results demonstrate that a non-mycorrhizal plant accesses organic N in competition with soil microbes and that expression of root organic N transporters is decisive for the efficacy of this process.