Herbicide atrazine-resistant mutant

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]. 

The negative cross-resistances in atrazine-resistant weeds include herbicides that act at or near the same site in photosystem II (DNOC and dinoseb) as well as herbicides acting on other photosystems (paraquat) or at totally different sites. There was negative cross-resistance to other tubulin binding herbicides in dinitroaniline resistant Eleucine indica (Table II), but not to six commercial herbicides on this weed (12). The negative cross-resistance to imazaquin (Table II) occurred in only one of 21 chlorsulfuron resistant mutants. The other mutants had varying levels of co-resistance to imazaquin. 

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. 

Fig.2 summarizes all the herbicide resistant mutants from R ps. viridis that have been characterized so far. T stands for selection with terbutryn (6) and A with atrazine (8). All replaced residues are located in the Qb site with one exception. In T3 additionally a second change occurred in the Qa site, valine M263 is replaced by phenylalanine. The use of atrazine and terbutryn in the selection procedure partially leads to the sub-... 

Figure 2 Summary of the herbicide resistant mutants from Rps. viridis. T stands for selection using terbutryn (6), A stands for selection using atrazine (8). Part of the sequence of the connecting loop between the fourth and fifth transmembrane helices of the L subunit from Rps. viridis wild type which forms the Qb site is shown...

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. 

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. 

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