1. D-Amino Acids in Plants
L-amino acids are primary metabolites and building blocks of proteins, and therefore matter of intensive research as essential molecules for all living organisms. However, the D-enantiomers are also regularly found throughout the complete kingdom of life (e.g. as elements of the bacterial cell wall, as parts of toxins, or as co-agonists of neurotransmitters). Nevertheless, knowledge about them in plants is rather scarce, although there is growing evidence that D-amino acids (D-AAs) fulfil different physiological functions also in plants (Fig. 1).
We started to study the metabolism of D-AAs in plants, when we found that plants react differentially to D-AA application. Plants are able to actively take up D-AAs, release them to their rhizosphere, and to degrade them enzymatically. In the course of these studies, we identified and characterized a central enzyme of D-amino acid metabolism in Arabidopsis, the plastidic D-AA transaminase AtDAT1. This enzyme turned out to be responsible for the phenomenon of “D-AA stimulated ethylene production”, which we are currently investigating in more detail.
D-AAs also play a role in the biogenesis of chloroplasts. According to the endosymbiotic theory, plastids evolved from bacterial ancestors with cell walls consisting of peptidoglycan with D-Ala and D-Glu. As previously shown in mosses, we found strong indications that chloroplast envelopes of higher plants also contain elements such as peptidoglycan (Fig. 2). In future, we will investigate biosynthesis of peptidoglycan in plants and the physiological functions of its incorporation into the plastidic envelope.