Herbicide-resistant weeds, binding

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. 

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. 

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. 

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). 

As the primary mechanism of action of the. -triazines involves inhibition of PS II electron transport, attention was also directed at analysis of chloroplast reactions in resistant weed biotypes (10, 11, 12). These studies can be summarized as follows (a) in aTl cases studied to date, there is a modification in the chloroplast membranes of resistant biotypes that changes the characteristics of s-triazine binding (b) this modification results in altered bincfTng characteristics of other classes of herbicides, (i.e., only slight resistance to ureas, but increased sensitivity to phenols) (see for review), and (c) the alteration of the herbicide receptor in resistant weeds is accompanied... 

The distinctly different behavior of the phenol-type herbicides following trypsin treatment suggests that different determinants within the PS II protein complex establish the "domains" that regulate the binding properties of these inhibitors. In spite of the fact that phenol-type herbicides will displace bound radiolabeled herbicides such as diuron, these inhibitors show noncompetitive inhibition (29, 30). At present, there are three lines of evidence which favor TH e involvement of two domains within the PS II complex that participate in creating the binding sites for these herbicides (a) isolated PS II particles can be selectively depleted of a polypeptide with parallel loss of atrazine sensitivity, but not dinoseb inhibition activity (33) (b) in resistant weed biotypes, chloroplast membranes that exhibit extreme triazine resistance have increased sensitivity to the phenol-type herbicides (13) and (c) experiments with azido (photoaffinity) derivatives of phenol and triazine herbicides result in the covalent labeling of different PS II polypeptides (, 31). 

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. 

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