Glutamine alleviates the toxicity of externally applied amino acids in Arabidopsis

Plants use amino acids not only as building blocks for protein synthesis, but also as nitrogen carriers between the various organs and as precursors of a plethora of specialized compounds that fulfill essential functions ranging from structural roles to interaction with other organisms. Feedback regulation of amino acid metabolic enzymes is well studied in plants and has been accepted as a main regulatory mechanism of pathway activity. Studies in Nicotiana sylvestris have shown that single amino acids provided at high concentrations are toxic to plants and plant cells, an inhibition that can be inexplicably reversed by addition of glutamine.

The present study established an Arabidopsis-based framework to further study the mechanisms controlling the activity of amino acid metabolic pathways. In this context, we determined with precision the toxicity of each amino acid on Arabidopsis plants grown in solid and liquid media. The level of toxicity varied between 0.3 and 4 mM, depending on the amino acid, with some amino acids found non-toxic at concentrations up to 20 mmol.l^-1. For some of the tested amino acids, the inhibition was relieved by adding amino acids from the same pathway, supporting the notion that the effect is a consequence of the feedback inhibition of the pathway. In good agreement with past reports from Nicotiana, we found that glutamine alleviated, or sometimes completely suppressed, the growth inhibition exerted by single amino acids. Suppression of valine toxicity was not due to decreased valine uptake by glutamine, showing that glutamine effect is due to another phenomenon. The involvement of Target of Rapamycin (TOR) was tested using the specific inhibitor AZD8055. TOR inhibition completely overrode the toxicity of Val and masked the effects of Gln, in good agreement with TOR being a high order regulator of the metabolism, but precluding any conclusion as to whether it is involved in the Gln-suppressing effect of Val toxicity.