Transport mechanisms of D-amino acids and their physiological implications in Arabidopsis thaliana

Abstract

Almost all proteinogenic L-amino acids (L-AA) have enantiomers called D-amino acids (D-AA) which do not differ just in their molecular spatial arrangement but also in their biological function. Since D-AA are part of the bacterial cell wall, they can be found in all kinds of tissues and materials, which are associated with bacteria. This is the reason why D-AA are found in soil and plants are able to actively take up D-AA by their roots. There have been several studies stated D-AA toxicity for plants. However, an explanation for the toxicity and plants handling with ubiquitous D-AA are still unknown. Here we show that growth inhibition of Arabidopsis thaliana seedlings treated with D-Met is connected to increased ethylene production. This ethylene related phenomenon has been described before as it was shown that D-Met interferes with ACC (1-aminocyclopropane-1-carboxylic acid) metabolizing (malonyl conjugating) enzymes resulting in an increased ethylene production. But additionally, we could establish the link between the ethylene magnitude and the activity of the main D-AA metabolizing enzyme the D-amino acid transferase (DAT1). We could reveal that the DAT1 protein has a great impact on the plants D-Met concentration since it transforms all tested D-AA, but preferably D-Met, directly into D-Ala and D-Glu and into their respective L-enantiomer indirectly. As a result, in the Col-0 wild-type the increased ethylene and malonyl-methionine production and thus growth reduction was almost negligible. By comparison, the dat1 loss of function mutants and the DAT1 defective Arabidopsis ecotype Landsberg erecta were severely affected upon 0.5 mM D-Met treatment. In addition, we could proof that exudation of D-AA occurs passively and comparable to their L-forms in amount and time. The rhizosphere is harbouring a great number of beneficial as well as detrimental bacteria. Plants transformation of D-AA into D-Ala and D-Glu, which are essential for bacterial peptidoglycan combined with their exudation could attract beneficial bacteria. On the other hand, biofilms on plant roots have also been reported to be regulated by D-AA. Thus, an influence on the bacterial rhizosphere community by D-AA released or controlled by the plant can be envisioned. However, the efflux regulation mechanism remains unclear. The here presented study displays the central role of the DAT1 enzyme in D-AA metabolism. Moreover, it reveals the relationship of this metabolic pathway and plants reduction in growth due to increased ethylene synthesis. Another outcome is the passive exudation of D-AA through plant roots suggesting a strategy to cope with D-AA overflow and a putative plant-microbe interaction.