D-Amino Acids in Plants: New Insights and Aspects, but also More Open Questions

Abstract

The relevance of the homochirality of proteinogenic amino acids for life is undisputed, but also to their D-enantiomers a growing number of biological functions could be assigned. When it comes to D-amino acids in plants, information was relatively scarce for a long time. Nowadays, also in this field, knowledge is growing which will be presented and discussed in this review. In this respect, it was shown that D-amino acids are taken up by plants from soil but could also be synthesized de novo. Investigations of plant D-amino acid metabolism as well as other studies revealed a central function of D-Ala in plants, which await further elucidation. Also other D-amino acids are shown to cause physiological effects in plants, ranging from nitrogen utilization over stress adaptation to chloroplast division, and indicate that D-amino acids are responsible for a variety of yet poorly understood or even undiscovered functions in plants.

d-Amino Acids Are Exuded by Arabidopsis thaliana Roots to the Rhizosphere

“…This observation led to the question of which processes may contribute to this d -AA reduction in the plants. Various enzymatic and nonenzymatic processes have been suggested elsewhere [ 26 ]. Among the putative enzymatic metabolizations of d -AAs, we tested the impact of d -AA-specific transamination.…”

“…Another possible mechanism for d -AA reduction to be tested was rhizodeposition [ 26 ]. It is a well-established fact that plants release proteinogenic AAs into their rhizosphere [ 28 ], but the root exudation of d -AAs has not been reported before.…”

“…Although it has been stated before that plants possess low capacity to metabolize d -AAs and lack d -AA oxidases [ 13 ], there is also a report biochemically characterizing a putative d -AA oxidase from corn [ 33 ]. Furthermore, the Arabidopsis genome harbors at least one putative d -AA oxidase gene [ 26 ].…”

“…This question takes the center stage of the present study. As one possibility for reducing the d -Ala and d -Glu contents in plants, rhizodeposition has been suggested [ 26 ]. In this study, it is shown that exogenously applied d -Ala and d -Glu is significantly reduced in Arabidopsis seedlings within 24 h. Furthermore, exudation of these and other d -AAs could be observed.…”

Chiral analysis of β-methylamino alanine (BMAA) enantiomers after (+)-1-(9-fluorenyl)-ethyl chloroformate (FLEC) derivatization and LC-MS/MS

“…Evidence has been provided concerning their utilization by plants as a nitrogen source, and there are indications of their participation in signaling processes, such as D-Ser binding to glutamate receptors which may inuence pathogen response. 23 Similar to protein amino acids, BMAA is an optically active biomolecule existing as L-and D-BMAA. According to a study by Andersson et al 17 performed in rats, the uptake of L-BMAA was much higher than the uptake of the D-enantiomer.…”