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

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

The Minus-End-Directed Kinesin OsDLK Shuttles to the Nucleus and Modulates the Expression of Cold-Box Factor 4

The transition to terrestrial plants was accompanied by a progressive loss of microtubule minus-end-directed dynein motors. Instead, the minus-end-directed class-XIV kinesins expanded considerably, likely related to novel functions. One of these motors, OsDLK (Dual Localisation Kinesin from rice), decorates cortical microtubules but moves into the nucleus in response to cold stress. This analysis of loss-of-function mutants in rice indicates that OsDLK participates in cell elongation during development. Since OsDLK harbours both a nuclear localisation signal and a putative leucin zipper, we asked whether the cold-induced import of OsDLK into the nucleus might correlate with specific DNA binding. Conducting a DPI-ELISA screen with recombinant OsDLKT (lacking the motor domain), we identified the Opaque2 motif as the most promising candidate. This motif is present in the promoter of NtAvr9/Cf9, the tobacco homologue of Cold-Box Factor 4, a transcription factor involved in cold adaptation. A comparative study revealed that the cold-induced accumulation of NtAvr9/Cfp9 was specifically quelled in transgenic BY−2 cells overexpressing OsDLK-GFP. These findings are discussed as a working model, where, in response to cold stress, OsDLK partitions from cortical microtubules at the plasma membrane into the nucleus and specifically modulates the expression of genes involved in cold adaptation

Phylogenetic and comparative gene expression analysis of barley (Hordeum vulgare) WRKY transcription factor family reveals putatively retained functions between monocots and dicots

WRKY proteins belong to the WRKY-GCM1 superfamily of zinc finger transcription factors that have been subject to a large plant-specific diversification. For the cereal crop barley (Hordeum vulgare), three different WRKY proteins have been characterized so far, as regulators in sucrose signaling, in pathogen defense, and in response to cold and drought, respectively. However, their phylogenetic relationship remained unresolved. In this study, we used the available sequence information to identify a minimum number of 45 barley WRKY transcription factor (HvWRKY) genes. According to their structural features the HvWRKY factors were classified into the previously defined polyphyletic WRKY subgroups 1 to 3. Furthermore, we could assign putative orthologs of the HvWRKY proteins in Arabidopsis and rice. While in most cases clades of orthologous proteins were formed within each group or subgroup, other clades were composed of paralogous proteins for the grasses and Arabidopsis only, which is indicative of specific gene radiation events. To gain insight into their putative functions, we examined expression profiles of WRKY genes from publicly available microarray data resources and found group specific expression patterns. While putative orthologs of the HvWRKY transcription factors have been inferred from phylogenetic sequence analysis, we performed a comparative expression analysis of WRKY genes in Arabidopsis and barley. Indeed, highly correlative expression profiles were found between some of the putative orthologs. HvWRKY genes have not only undergone radiation in monocot or dicot species, but exhibit evolutionary traits specific to grasses. HvWRKY proteins exhibited not only sequence similarities between orthologs with Arabidopsis, but also relatedness in their expression patterns. This correlative expression is indicative for a putative conserved function of related WRKY proteins in mono- and dicot species

Comparative Mutant Analysis of Arabidopsis ABCC-Type ABC Transporters: AtMRP2 Contributes to Detoxification, Vacuolar Organic Anion Transport and Chlorophyll Degradation

The enormous metabolic plasticity of plants allows detoxification of many harmful compounds that are generated during biosynthetic processes or are present as biotic or abiotic toxins in their environment. Derivatives of toxic compounds such as glutathione conjugates are moved into the central vacuole via ATP-binding cassette (ABC)-type transporters of the multidrug resistance-associated protein (MRP) subfamily. The Arabidopsis genome contains 15 AtMRP isogenes, four of which (AtMRP1, 2, 11 and 12) cluster together in one of two major phylogenetic clades. We isolated T-DNA knockout alleles in all four highly homologous AtMRP genes of this clade and subjected them to physiological analysis to assess the function of each AtMRP of this group. None of the single atmrp mutants displayed visible phenotypes under control conditions. In spite of the fact that AtMRP1 and AtMRP2 had been described as efficient ATP-dependent organic anion transporters in heterologous expression experiments, the contribution of three of the AtMRP genes (1, 11 and 12) to detoxification is marginal. Only knockouts in AtMRP2 exhibited a reduced sensitivity towards 1-chloro-2,4-dinitrobenzene, but not towards other herbicides. AtMRP2 but not AtMRP1, 11 and 12 is involved in chlorophyll degradation since ethylene-treated rosettes of atmrp2 showed reduced senescence, and AtMRP2 expression is induced during senescence. This suggests that AtMRP2 is involved in vacuolar transport of chlorophyll catabolites. Vacuolar uptake studies demonstrated that transport of typical MRP substrates was reduced in atmrp2. We conclude that within clade I, only AtMRP2 contributes significantly to overall organic anion pump activity in viv

D-Amino Acids in Plants: Sources, Metabolism, and Functions

Although plants are permanently exposed to d-amino acids (d-AAs) in the rhizosphere, these compounds were for a long time regarded as generally detrimental, due to their inhibitory effects on plant growth. Recent studies showed that this statement needs a critical revision. There were several reports of active uptake by and transport of d-AAs in plants, leading to the question whether these processes happened just as side reactions or even on purpose. The identification and characterization of various transporter proteins and enzymes in plants with considerable affinities or specificities for d-AAs also pointed in the direction of their targeted uptake and utilization. This attracted more interest, as d-AAs were shown to be involved in different physiological processes in plants. Especially, the recent characterization of d-AA stimulated ethylene production in Arabidopsis thaliana revealed for the first time a physiological function for a specific d-AA and its metabolizing enzyme in plants. This finding opened the question regarding the physiological or developmental contexts in which d-AA stimulated ethylene synthesis are involved in. This question and the ones about the transport characteristics of d-AAs, their metabolism, and their different physiological effects, are the focus of this review