Herbicides sensitivity

Y.-F. Tsay, J. L. Schroeder, K. A. Feidmann, and N. M. Crawford, A herbicide sensitive gene CHLI of Arahidopsis encodes a nitrate-inducible nitrate transporter. Cell 72 705 (1993). 

Mattoo, A.K., U. Pick, H. Hoffmann-Falk, and M. Edelman (1981). The rapidly metabolized 32000-dalton polypeptide of the chloro-plast is the proteinaceous shield regulating photosystem II electron transport and mediating diuron herbicide sensitivity. Proc. Natl. Acad. Scl, 78 1572-1576. 

The sulfonylureas, an extremely potent class of herbicides, act by inhibiting acetolactate synthase (ALS), which is the first common enzyme in the biosynthetic pathways leading to the branched chain amino acids. Two other unrelated classes of herbicides also act by interfering with this enzyme. We have cloned and characterized the genes encoding ALS from several higher plants. The ALS genes isolated from herbicide sensitive and herbicide resistant plants have been compared, and several mutations which confer the herbicide resistant phenotype have been identified. 

Figure 2. Assays of tobacco transformed with Arabidopsis ALS genes. Tobacco containing either the herbicide sensitive (S) or resistant (R) ALS gene was tested for the ability of transformed callus to grow in the presence of herbicide (slashed boxes) and the herbicide resistance of enzyme activity in plant extracts (solid boxes). Each measurement is expressed as a percentage of the value that was obtained in the absence of herbicide.

The first approach employed selection of mutations in the yeast ALS gene to uncover additional sites of herbicide-resistance mutations. Forty-one independently isolated spontaneous mutations in the yeast ALS gene were characterized by DNA sequencing. These mutations revealed 24 different amino acid substitutions that occur at 10 different sites ranging from the amino to the carboxy ends of the protein. The amino acids at these 10 sites are highly conserved among natural herbicide-sensitive ALS enzymes the amino acid residues present in the wild type herbicide-sensitive yeast enzyme have been found in all wild type plant ALS enzymes that have been sequenced. 

Herbicide sensitivity in thylaXoids from metribuzin resistant Cblamyioaonaa reinhardii was investigated in the system described by... 

The mutation of two residues that are located in this binding site and affect herbicide sensitivity disrupts the structure of the domain. 

Table 9.5 I50S and R/S I50 ratios for ACC from herbicide-sensitive and...

Although the results of these extensive experiments are not easily interpreted, the hypotheses were made (a) that PChlide is the most important and ubiquitous photodynamic species caused to accumulate by ALA-based treatments, (b) that MV PChlide is a more effective photodynamic pigment than DV PChlide in DDV/LDV and DMV/LDV species, and (c) that both DDV/LDV and DMV/LDV species are highly susceptible to a mixture of Mg-PPIX and Mg-PPIXME (14) The results are difficult to interpret because equimolar levels of different porphyrins were not produced and the combinations of porphyrins produced by different modulators varied with species. Potential differences in tolerance to toxic oxygen species between species were not considered. Others have attempted to explain differential sensitivity to porphyrin-generating herbicides between species (15) and between herbicide-sensitive biotypes within species (16) by differences in ability to detoxify toxic oxygen species. As with other herbicides, penetration of the leaf cuticle by ALA and/or DP can also play a role in differences in efficacy of this herbicide combination (17). 

Herbicide Sensitivity The adventitious mutation of Tyr L222 to Gly (5) or Cys (Takahashi and Wraight, unpublished) is known to induce resistance to triazine herbicides. The site-directed mutation to Phe is structurally more conservative, but also induces considerable resistance (>100X) to terbutryne (Fig. 2). 

Renger [2] introduced the use of trypsin as a selective structural modifier capable of removing the herbicide sensitivity of PS2. It has been confirmed that this occurs through D1 cleavage at ARG 238 within the Qb/herbicide binding region [3]. However a preferential cleavage in the D2 protein at ARG 234 was also found [3]. 

Differential herbicide sensitivity at the site of action is a second possible selectivity mechanism, with good precedence in the aryloxyphenoxy and cyclo-hexanedione grass herbicides (2Q, 51). However, several studies now clearly indicate that the inherent sulfonylurea crop selectivities listed in Table 1 are not based on differential sensitivity at the site of action. ALS preparations isolated fix>m crops and weeds are equally sensitive to several selective sulfonylurea herbicides (28.32.33). Although data have not appeared for all of the compounds shown in Table 1, we conclude that differential active site sensitivity is not a general sulfonylurea selectivity mechanism. The clear exceptions to this generalization are the cases of genetically-altered plants (crops and we s) which have acquired through mutation or deliberate transformation a resistant form of the ALS enzyme (see below). 

Wilkinson R E 1980 Ecotypic variation of Tamarix pentandra epicuticular wax and possible relationship with herbicide sensitivity. Weed Sci 28 110-113... 

Sethoxydim (IpM) inhibited the biosynthesis of fatty acids in isolated oat chloroplasts (measured via [14C]acetate incorporation into the total fatty acid fraction) to ca. 60 %, whereas in the isolated chloroplasts of the dicotyledonous pea plants their synthesis was not affected by sethoxydim. About 50 % inhibition of fatty acid synthesis by IpM sethoxydim was detected in experiments with chloroplasts isolated from Poa pratensis, a herbicide-sensitive grass. In chloroplasts isolated from Poa annua, which as a whole plant is tolerant against sethoxydim [7], fatty acid biosynthesis was also blocked (to ca. 45 %) by IpM sethoxydim (Fig. 2). This indicates a basic difference in the mechanism of tolerance for Pisum sativum and Poa annua, respectively. In Poa annua, the tolerance is apparently based on... 

Contrarily to expectations, an increase (resp. decrease) in herbicide affinity was associated to a decrease (resp. increase) in the herbicide sensitivity, except in the case of DCMU sensitivity of PL-depleted R-TM (Tab. I). This suggests that at least in the S-biotype, (some) acyl lipids in the outer monolayer interact with the 32 Kd H-B protein so as to maintain... 

PPT application results in a 50% decrease in GS levels in leaf tissues glutamate dehydrogenase levels are enhanced but, as intracellular ammonia levels rise, the amination reaction catalyzed by glutamate dehydrogenase insufficiently compensates for the inhibition of GS. Nitrate reductase is reduced, but proteolytic activity is unaffected. A wide-ranging survey of GS levels in plants reported a 70-fold variation in extractable GS levels there was also considerable variation in the ratios of the cytosolic and chloroplastic isozymes. The kinetics of inhibition by PPT, however, were very similar, and there was no apparent correlation between natural GS levels and herbicide susceptibility. PPT uptake and transport therefore seem to be the determining factors in natural herbicide sensitivity. 

Herbicides also inhibit 5- (9/-pymvylshikiniate synthase, a susceptible en2yme in the pathway to the aromatic amino acids, phenylalanine, tyrosine and tryptophan, and to the phenylpropanes. Acetolactate synthase, or acetohydroxy acid synthase, a key en2yme in the synthesis of the branched-chain amino acids isoleucine and valine, is also sensitive to some herbicides. Glyphosate (26), the sulfonylureas (136), and the imida2oles (137) all inhibit specific en2ymes in amino acid synthesis pathways. 

Chlorophenoxy acids are relatively polar pesticides which are usually determined by LC because volatile derivatives have to be prepared for GC analysis. This group of herbicides can be detected by multiresidue methods combined with automated procedures for sample clean-up, although selectivity and sensitivity can be enhanced by coupled-column chromatographic techniques (52). The experimental conditions for Such analyses are shown in Table 13.1. 

Schmitt-Jansen M, Altenburger R (2005) Predicting and observing responses of algal communities to photosystem Il-herbicide exposure using pollution-induced community tolerance and species-sensitivity distributions. Environ Toxicol Chem 24 304... 

With the acceptable concentrations of herbicides in drinking water being taken to very low levels by some regulatory authorities (e.g., the EC), there has been interest in very low levels of atrazine present in some samples of groundwater and in drinking water. This finding illustrates the point that mobility of pesticides becomes increasingly evident as sensitivity of analysis improves. 

TAetection of the highly potent impurity, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the herbicide 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), necessitated an environmental assessment of the impact of this contaminate. Information was rapidly needed on movement, persistence, and plant uptake to determine whether low concentrations reaching plants, soils, and water posed any threat to man and his environment. Because of the extreme toxicity of TCDD, utmost precautions were taken to reduce or minimize the risk of exposure to laboratory personnel. Synthesis of uniformly labeled C-TCDD by Muelder and Shadoff (I) greatly facilitated TCDD detection in soil and plant experiments. For unlabeled experiments it seemed wise to use only small quantities of diluted solutions in situations where decontamination was feasible and to rely on the sensitivity afforded by electron capture gas chromatography... 

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