Point herbicide resistance

Occurrence The appearance of herbicide resistant weeds was pre-dicted as early as 1950. Blackman (2) then pointed out the likenesses between mass selection for a given character in crops and selection of weeds after continuous herbicide treatments in fields Herbicide resistance has been reported worldwide in a wide variety of crops that involve herbicide families with different modes of action and several weeds species (1). 

The development of herbicide-resistant crop varieties as undertaken by American Cyanamid is an evolutionary process that proceeds through a series of decision points. Initially, the market potential for herbicide resistance in the crop is evaluated. After a project is assessed as worthwhile, a scheme to make the crop resistant is established and implemented. Once the scheme proves successful and resistance is introduced into the crop of interest, the trait is characterized to assess commercial utility. A method for delivering resistant crops to the marketplace is then identified. Finally, through close cooperation between the seed company and the chemical company, the herbicide-resistant crop becomes available commercially. 

Fig. 2.1.6. AHAS mutations conferring herbicide resistance. Arrows point to positions in the sequences of AHAS from plant Arabidopsis thaliana), yeast Saccharomyces cerevisiae), and bacterial (Escherichia coli, isozyme II) sources where spontaneous or induced mutations result in an herbicide-insensitive enzyme. Colors designate substitutions occurring in more than one species.

The reaction center of photosystem II (PSII) consists of three proteins The 32-kDa protein (32K, also referred to as D,), Dj, and cytochrome bjs, (1,2). It has several features in common with the reaction center from purple bacteria (3,4), including amino acid sequence homology in functional regions (3,5), arrangement of the transmembrane helices (6,7,8), and conservation of the binding sites for chlorophylls, pheophytins, quinones and a non-heme iron (3,4,6,7,9). Furthermore, 32K and the L-subunit of the bacterial reaction center are the site of triazine herbicide action (10-12), and point mutations at conserved residues in these proteins can confer herbicide resistance (3,13-15). 

The continuous use of a particular class of hobicide leads to a marked decrease in genetic diversity within populations. At the stone time, it selects plants possessing specific genetic traits conforing herbicide resistance. In addition, substitutions of amino acid residues confer resistance to SU herbicides. In this case, various point mutations occurred in the prolme codon and encoded several possible arttino acid substitutions. When we reviewed the data on genetic relationships and amino acid substitutions of the 69 plants, the emerged picture showed that resistant biotypes wo-e independent selected fiom these areas because of at least five different amino acid substitutions (i.e., 1 KTMr+KTNr 2 YCGr 3 YSSr 4 AKTr 5 ASTr). 

The one-point mutated ALS except for the mutation of serine at position 627 to isoleucine exhibited a high resistance to the SU herbicide, chlorsulfuron. On the contrary, the resistance level of these mutated ALS s to the IM herbicide, imazaquin, was lower than that of chlorsulfuron. The mutation of proline at position 171 to alanine and histidine did not confer resistance to imazaquin. On the odier hand, the resistance level of die proline mutated ALS s to the PC herbicides was moderate between diose of chlorsulfuron and imazaquin. These results were correlated to the cross-resistance pattern of the proline-mutated ALS of K. scoparia as already described. From these results, it is considered that rice mutated recombinant ALS s, especially the proline mutants, are useful as resistant enzyme models for the herbicide resistance management at newly developed or developing ALS-inhibiting herbicides. We are now preparing other kinds of proline mutants to confirm this idea. 

This point mutation is the result of a change of one amino-acid residue and the subsequent loss of the protein-herbicide affinity. Resistant chloroplasts are 1,000-fold more resistant than susceptible chloroplasts, when assayed in vitro. Whole plants also show at least a 200-fold increased resistance, which confers a clear, selective advantage in triazine treated fields, orchards and vineyards (13). Evidence that a specific gene is responsible for resistance was proven by the production of tolerant transgenic tobacco (12). Now, nearly 50 species are known to have at least one triazine resistant population and several million hectares in more than 15 countries are infected with triazine resistant weed populations (1). 

The same biotypes, Peldon A1 and Lines. El, are also resistant to the gramini-cide fenoxaprop, which is used for selective control of A. myosuroides and other grass weeds in cereals, mainly wheat. On a whole plant level. Lines. El was more resistant than Peldon Al. The selectivity of this herbicide has been attributed to rapid detoxification by GST-catalyzed conjugation in the cereal species. In both resistant A. myosuroides biotypes GST activities towards fenoxaprop were found to be increased to a similar degree, when compared with a susceptible biotype. This was due to increased expression of a constitutive GST and to expression of two novel GST isoenzymes. Furthermore, glutathione levels were increased in the resistant biotypes, in Peldon more than in Lines. The data pointed to an involve-... 

As wheat is the major crop for feeding people, it would be unwise to overlook the potential of cross-resistance to wheat herbicides evolving at many points over... 

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