A proteomic and targeted metabolomic approach to investigate change in Lolium perenne roots when challenged with glycine

A combined proteomic and isotope tracer approach was used to investigate the impact of supplying N as glycine to roots of Lolium perenne. Initially, ammonium nitrate was supplied to all plants, after which half received glycine as their sole N source, while the remainder continued to receive ammonium nitrate. Plants supplied with glycine acquired less N than those receiving the mineral source, resulting in reduced root nitrate concentrations. The amino acid complement of roots was also strongly affected by the form of N supplied, and N-15 labelling indicated that the biochemical fate of acquired N in roots was dependent on the form of N available for uptake. Proteomic analysis of Lolium roots indicated that 6% of 627 root proteins resolved on 2D gels changed in abundance in response to the form of N applied. Multivariate analysis of protein abundance clearly discriminated the proteomes of L. perenne roots as a function of treatment applied. Seven affected proteins were identified (mostly by protein homology with sequenced species), including methionine adenosyltransferase, an enzyme involved in glycine metabolism. Although some changes in root amino acid and protein complements were due to responses to reduced N supply, both the distinct fate of N-15 tracers and the abundances of identified proteins could be attributed specifically to the form of N available to roots. The results demonstrate the potential of targeted proteomic approaches to identify functioning of plants where more traditional methods cannot resolve multipie, co-incident biological interactions and element fluxes.