Herbicide binding resistance

Erickson, J.M., M. Rahire, J.D. Rochaix, and L. Mets (1985). Herbicide resistance and cross-resistance Changes at three distinct sites in the herbicide-binding protein. Science, 228 204-207. 

Vermass, W.F.J. and C.J. Arntzen (1984). Synthetic quinones influencing herbicide binding and photosystem II electron transport. The effects of triazine-resistance on quinone binding properties in thylakoid membranes. Biochim. Biophys. Acta., 725 483 -91. 

The high efficacy of triazine herbicides and their repetitive use in crops and noncrop situations has resulted in the selection of weeds that are resistant to these herbicides or are not well controlled at the lower rates now being used. In most instances, triazine resistance is due to an alteration in the herbicide-binding site in PS II. Despite the widespread occurrence of triazine resistance, these herbicides are still widely used, even in fields in which triazine-resistant biotypes are known to occur. The rate of increase in the selection for triazine-resistant weed species depends in part on the integration of alternative weed control strategies, in addition to the use of triazine herbicides, for control of these weed species. Due to their resistance mechanism, many triazine-resistant weeds are less competitive than their susceptible counterparts. 

Smeda, R.J., PM. Hasegawa, P.B. Goldsbrough, N.K. Singh, and S.C. Weller (1993). A serine-to-threonine substitution in the triazine herbicide-binding protein in potato cells results in atrazine resistance without impairing productivity. Plant Physiol., 103 911-917. 

Triazine Resistance We attempted to answer the previous four questions using data and examples derived from the study of the best documented case of herbicide resistance, triazine resistance. Two kinds of mechanisms may be responsible for this triazine resistance first is the presence of detoxification metabolic pathways, as seen in corn (11). This also may occur in weed populations, especially Panicoideae, but a low heritability makes its study complex. The second mechanism of triazine resistance is the loss of herbicide binding at the level of the chloroplast. 

This approach has been used primarily to study photosynthesis, for example to introduce tolerance to a variety of stress conditions. Many studies have been successful in verifying the effea of single amino acid modifications on herbicide binding affinity. High affinity binding to the D1 protein is a useful property for the detection of herbicides. A fluorescence biosensor based on mutants resistant to various herbicide subclasses was developed, it makes possible to distinguish between subclasses of herbicides (e.g., triazines from urea and phenolic type herbicides). ... 

The specificity of the whole cells and isolated photosynthetic materials was obtained by applyir the knowledge available on the relationships between herbicide binding activity and the structure of the D1 protein. For example, distinctions among classes of chemicals could be achieved through mutations in amino acid residues of D1 which can impart resistance to individual triazine herbicides. Realisation of new, sophisticated transduction systems based on printed electrodes, fluorescence and chemiluminescence as well as alternative systems such as the reconstimtion of Qp site in overexpressed DI protein utilizing chromophore quinones to enhance sensitivity and specificity for detected signals. 

The present paper is a discussion of the photosystem II herbicides and their mechanisms of action. Among the topics covered are the green plant photosystems, photochemistry and electron transfers within photosystem II, requirements for herbicidal activity, mechanisms of action, herbicide selectivity and resistance, herbicide-binding proteins, and theoretical studies of herbicidebinding site interactions. 

For the particular case of triazine-resistant weed biotypes found in areas of the world where there has been frequent use of triazine herbicides, the resistance has been traced to a lowered binding affinity at the PS II herbicide binding site (17,19,25,26). 

They suggest that modification of the herbicide binding site which confers triazine resistance also makes photochemical electron transport much less efficient. The alteration resulted in a lowered capacity for net carbon fixation and lower quantum yields in whole plants of the resistant types of Senecio vulgaris L. 

Target-site resistance is due to reduced or lost ability of the herbicide to bind to its target protein. This is usually an enzyme with a crucial function in metabolic pathways or the component of an electron transport system. As a further possibility, target-site resistance could also be caused by an overproduction of the herbicide-binding protein. 

Cross-resistance means that a single resistance mechanism causes resistance to several herbicides. The term target-site cross-resistance is used when these herbicides bind to the same target site, whereas nontarget-site cross-resistance is due to a single nontarget-site mechanism (e.g., enhanced metabolic detoxification) that entails resistance across herbicides with different modes of action. 

The triazine herbicides bind to this site and thus inhibit the photosynthetic electron flow. In the resistant mutants triazine binding is markedly reduced. As an example, the concentration of atrazine needed to obtain a 50% inhibition of photosynthetic electron flow in isolated chloroplasts of Chenopodium album was found to be at least 430 x higher for chloroplasts from an atrazine-resistant mutant than for chloroplasts from wUd-type plants [22]. 

In 1999 Masabni and Zandstra reported on a mutant of Portulaca oleracea with a resistance pattern to PS II inhibitors that was different to most triazine resistant weeds [28], This mutant was resistant to the phenylureas linuron and diuron, but also cross-resistant to atrazine and other triazines. Sequencing of the D1 protein revealed that in the resistant biotype the serine 264 was replaced by threonine and not by glycine. This was the first report on a serine 264 to threonine mutation on a whole plant level. It was proposed that the serine-to-threonine mutation modified the conformation of the herbicide binding niche at the D1 protein in a way, which resulted in reduced binding of phenylureas and triazines as well. 

Another novel mutant was identified, when field accessions of Poa annua with resistance to PS II inhibitors, collected in Western Oregon, were analyzed after amplification of the herbicide-binding region (933 base pair fragment) of the chloroplast psbA gene using PGR. 

Taken together, both approaches have allowed the prediction of the herbicide binding to the CT domain. In particular, it was possible to determine amino acid changes responsible for herbicide resistance to AOPP and/or CHD analogues and localize the amino acid directly involved in the binding of herbicides, but only for this domain [50]. 

The Herbicide Resistant Mutant T1 from Rhodopseudomonas viridis Altered Herbicide Binding and Three-Dimensional Structure... 

In order to study the herbicide binding sites and the structure-function relationship of the D protein in photosystem II we have selected different mutants resistant to DCMU, Atrazine and loxynil in a unicellular cyanobacteria Syneohooyst is 6714. We have determined the Dj sequence in each mutant and analyzed by different techniques (fluorescence, oxygen, thermoluminescence) the electron transfer in photosystem II of the different strains. We have also studied the electron transfer in resistant and susceptible Chenoipodium album. 

The data presented here indicates that LY181977 inhibits photosynthetic electron transport through photosystem II. LY181977 has some interaction with that portion of the herbicide binding site which confers atrazine resistance in mutant DCMU4. 

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