Low CO(2) induces urea cycle intermediate accumulation in Arabidopsis thaliana

The non-proteinogenic amino acid ornithine links several stress response pathways. From a previous study we know that ornithine accumulates in response to low CO(2). To investigate ornithine accumulation in plants, we shifted plants to either low CO(2) or low light. Both conditions increased carbon limitation, but only low CO(2) also increased the rate of photorespiration. Changes in metabolite profiles of light- and CO(2)-limited plants were quite similar. Several amino acids that are known markers of senescence accumulated strongly under both conditions. However, urea cycle intermediates respond differently between the two treatments. While the levels of both ornithine and citrulline were much higher in plants shifted to 100 ppm CO(2) compared to those kept in 400 ppm CO(2), their metabolite abundance did not significantly change in response to a light limitation. Furthermore, both ornithine and citrulline accumulation is independent from sugar starvation. Exogenous supplied sugar did not significantly change the accumulation of the two metabolites in low CO(2)-stressed plants, while the accumulation of other amino acids was reduced by about 50%. Gene expression measurements showed a reduction of the entire arginine biosynthetic pathway in response to low CO(2). Genes in both proline biosynthesis and degradation were induced. Hence, proline did not accumulate in response to low CO(2) like observed for many other stresses. We propose that excess of nitrogen re-fixed during photorespiration can be alternatively stored in ornithine and citrulline under low CO(2) conditions. Furthermore, ornithine is converted to pyrroline-5-carboxylate by the action of δOAT.