Editorial: Amino Acids in Plants: Regulation and Functions in Development and Stress Defense

For quite a long time, research on amino acid metabolism received only limited attention in the areas of plant physiology and biochemistry. Owing to the essential function of amino acids in protein synthesis, it was tempting to assume that plants use and metabolize amino acids in the same manner as microorganism or humans do. However, as fully autotrophic organisms, plants face fundamentally different challenges compared to organisms that make a living on plant- (or algae-) produced biomass. Furthermore, plants produce literally hundreds of non-proteinogenic amino acids or amino acid-derived secondary metabolites and thus require a very distinct regulatory system to coordinate the needs of primary and secondary metabolism. The distribution of amino acid metabolism between roots and aboveground organs or among the different subcellular compartments adds another level of complexity further distinguishing plants from prokaryotes or animals. From the perspective of human nutrition, the amino acids that we cannot synthesize by ourselves are the most interesting. Accordingly, the efforts to understand the routes and regulation of Lys, Met, and Trp biosynthesis has boosted amino acid research in plants, since these three amino acids are often contained in limiting amounts in staple crops. From an agricultural perspective, amino acid biosynthetic pathways that are exclusively found in plants were analyzed and used for the development of herbicides. At present, the threats of climate change and groundwater contamination by excess nitrogen fertilization call for a re-focusing of our efforts. Optimal productivity is currently achieved by massive application of nitrogen-containing fertilizers and is yet limited by the efficiency of the plants in taking up, distributing, storing and assimilating the nitrogen, with all these processes being governed by amino acid metabolism and transport. Similarly, many defense responses of plants against biotic or abiotic stress involve metabolic adjustments in amino acid metabolism to counteract detrimental environmental impacts directly or to provide precursors for defense compounds. Several amino acids emerge as important signaling molecules that orchestrate plant growth and development by integrating the metabolic status of the plant with environmental signals, especially in stressful conditions. However, our knowledge about amino acid metabolism, the mechanisms that regulate the levels of free amino acids and the diverse functions of amino acids in plants is far from complete. This Research Topic aims to collect contributions from different facets of amino acid metabolism, transport or signaling to bring together and integrate into a comprehensive view the latest advances in our understanding of the multiple functions of amino acids in plants.