Synthesis versus degradation: directions of amino acid metabolism during Arabidopsis abiotic stress response.

KEY MESSAGE: During abiotic stress low abundant amino acids are not synthesized but they accumulate due to increased protein turnover under conditions inducing carbohydrate starvation (dehydration, salt stress, darkness) and are degraded. Metabolic adaptation is crucial for abiotic stress resistance in plants, and accumulation of specific amino acids as well as secondary metabolites derived from amino acid metabolism has been implicated in increased tolerance to adverse environmental conditions. The role of proline, which is synthesized during Arabidopsis stress response to act as a compatible osmolyte, has been well established. However, conclusions drawn about potential functions of other amino acids such as leucine, valine, and isoleucine are not entirely consistent. This study reevaluates published datasets with a special emphasis on changes in the free amino acid pool and transcriptional regulation of the associated anabolic and catabolic pathways. In order to gain a comprehensive overview about the general direction of amino acid metabolism under abiotic stress conditions a complete map of all currently known enzymatic steps involved in amino acid synthesis and degradation was assembled including also the initial steps leading to the synthesis of secondary metabolites. Microarray datasets and amino acid profiles of Arabidopsis plants exposed to dehydration, high salinity, extended darkness, cold, and heat were systematically analyzed to identify trends in fluxes of amino acid metabolism. Some high abundant amino acids such as proline, arginine, asparagine, glutamine, and GABA are synthesized during abiotic stress to act as compatible osmolytes, precursors for secondary metabolites, or storage forms of organic nitrogen. In contrast, most of the low abundant amino acids are not synthesized but they accumulate due to increased protein turnover under conditions inducing carbohydrate starvation (dehydration, salt stress, extended darkness) and are degraded.