Grass roots chemistry: meta -Tyrosine, an herbicidal nonprotein amino acid

Abstract

Fine fescue grasses displace neighboring plants by depositing large quantities of an aqueous phytotoxic root exudate in the soil rhizosphere. Via activity-guided fractionation, we have isolated and identified the nonprotein amino acid m-tyrosine as the major active component. m-Tyrosine is significantly more phytotoxic than its structural isomers o-and p-tyrosine. We show that mtyrosine exposure results in growth inhibition for a wide range of plant species and propose that the release of this nonprotein amino acid interferes with root development of competing plants. Acid hydrolysis of total root protein from Arabidopsis thaliana showed incorporation of m-tyrosine, suggesting this as a possible mechanism of phytotoxicity. m-Tyrosine inhibition of A. thaliana root growth is counteracted by exogenous addition of protein amino acids, with phenylalanine having the most significant effect. The discovery of m-tyrosine, as well as a further understanding of its mode(s) of action, could lead to the development of biorational approaches to weed control.allelopathy ͉ festuca ͉ rhizosphere ͉ root ecology ͉ Arabidopsis R oot exudation of small molecules plays a major role in plant ecosystems and is often associated with the development of competitive advantage through allelopathy (1, 2). Juglone, a highly phytotoxic naphthoquinone produced by black walnut (Juglans nigra L.), and sorgoleone, a substituted quinone from sorghum (Sorghum spp.), are two classic examples of potently active allelochemicals deposited via the plant's living root system ( Fig. 1) (3). Elucidation of the structures and mode of action of previously unknown root-derived phytotoxins could lead to new biorational approaches to weed control.Because of their stress tolerance and disease resistance, fescue (Festuca spp.) grasses are commonly used in landscape, roadside, and pasture settings, as well as for conservation purposes (4, 5). The unusual ability of many fine leaf fescue species to outcompete other plants is well known, and previous investigations suggested that fescue root exudates have phytotoxic properties (4). Here, we report the isolation, identification, and biological activity of m-tyrosine, a potent, structurally unusual broadspectrum phytotoxin exuded by the roots of some fine leaf fescue grasses. Results and DiscussionIn an initial field evaluation of 80 fine fescue cultivars, 8 cultivars with strong weed suppressive potential were identified and their allelopathic potential in laboratory settings was confirmed (4). Based on both field and laboratory results, we selected ''Intrigue,'' a common Chewing's fescue cultivar (Festuca rubra L. ssp. commutata), for further studies.To identify the allelopathic compound(s) contained in Intrigue root exudates, we developed an activity-guided separation scheme based on the inhibition of lettuce (Lactuca sativa L.) radicle elongation in a filter paper-based assay. By using this assay, we compared the phytotoxicity of root surface washes (hexanes, dichloromethane, methanol, and water) prepared from 2-w...

Translation quality control is critical for bacterial responses to amino acid stress

“…These tRNA aminoacylation levels are comparable to those previously reported in other bacterial systems using similar methods, as is the observed increase in deacylated tRNA upon starvation (20)(21)(22). When the strains were grown under amino acid stress conditions in the presence of physiologically relevant levels of the natural noncognate PheRS substrate m-Tyr (3,23,24), levels of aa-tRNA Phe were 23% higher in the editing-deficient strain than in the wild type. These results also revealed that the cellular ratio of aminoacylated to deacylated tRNA increases significantly in the absence of PheRS editing, particularly under amino acid stress conditions ( Fig.…”

The emergence of grass root chemical ecology

“…3-5, and see Table 1). The article by Bertin et al (6) in this issue of PNAS adds significantly to this growing body of supportive literature.…”

“…Does the plant avoid accumulation of the compound by secreting it almost as quickly as it is produced, in a manner similar to that of Sorghum species that produce the allelochemical sorgoleone only in root hairs that secrete it rapidly (13)? Apparently this is not the mechanism, because Bertin et al (6) indicate that although the dry weight of the root exudate consists of up to 43% m-tyrosine, it is also a relatively abundant metabolite of the root. Is the compound sequestered into intracellular or intercellular locations where it can do little or no harm?…”

“…Bertin et al (6) have circumstantial evidence that the mode of action of mtyrosine is similar to that of some other nonprotein amino acids. That is, it substitutes for at least one protein amino acid (apparently phenylalanine in this case) during translation, resulting in dysfunctional proteins.…”

Multiple Quality Control Pathways Limit Non-protein Amino Acid Use by Yeast Cytoplasmic Phenylalanyl-tRNA Synthetase

“…Given that the discrimination against o-Tyr is only ϳ24,000 under typical growth conditions and ϳ6,900 in the presence of 2.0 mM paraquat (much lower discrimination than against m-Tyr), kinetic discrimination against o-Tyr-AMP by SccytoPheRS may play a role in Phe codon quality control at biologically relevant levels, particularly in conditions that favor oxidation of intracellular Phe. Alternatively, the import of exogenous o-Tyr is a plausible scenario, as structurally similar m-Tyr has been shown to be produced and exported by certain plant species in order to shape the ecology of their environments and inhibit the growth of competitor species (22). Production and export of non-protein amino acids is an effective strategy for chemically shaping a competitive environmental niche, provided the producer has the means to prevent the cytotoxic incorporation of these species (for review, see reviewed see Ref.…”