Herbicide-resistant crop varieties

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. 

Nevertheless, more and more genetically modified varieties (mainly insect- and herbicide-resistant crops) become listed in the Common EU Catalogue of varieties, indicating that the cultivation of those GM varieties could accelerate in the near future. GM varieties need to be authorised in accordance with Directive 2001/18/EC (former 90/220/EEC) before they are included in this Common Catalogue and potentially marketed in the EU. By doing so, this directive intends to provide a freedom of choice between GM and non-GM products, not only for... 

This approach should be used to enhance herbicide selectivity in crop varieties, to avoid carry-over injury, for specific and limited special problems, and for minor acreage and high value crops. A major objective of developing herbicide resistant crops should be to provide more flexibility in control of resistant weeds. 

Herbicide-resistant plant varieties have proven to be valuable experimental tools In determining the molecular mode of action of herbicides (2,8). In addition, such varieties are likely to be an Important source of selectable markers for use In plant molecular genetics and In the engineering of resistant crop species ( ) The earliest herbicide-resistant blotypes described arose spontaneously from weed populations which had been repeatedly exposed to a herbicide (10,11). More recently, mutagenesis and selection on defined media have been used to Isolate herbicide-resistant or herbicide-tolerant mutants of higher plants from populations of cells In tissue culture (4,5.7,12). While the use of plant tissue culture has proven useful for Isolation of some types of mutants, the... 

Triazine herbicides provide selective weed control in crops such as corn, sorghum, and sugarcane. In addition, some members of the triazine family are used for weed control in orchards, horticultural, and perennial crops, etc. A unique selective use of triazine herbicides is in triazine-tolerant rapeseed. Although triazine herbicides provide control of a wide variety of grass and broadleaf weeds, the long-term, widespread, and repetitive use of triazine herbicides in crop and noncrop situations has led to the selection of many triazine-resistant weeds. The physiological and biochemical basis of triazine selectivity between crops and weeds and of resistance to triazine herbicides in weeds is well understood. 

Glyphosate is toxic to plants and free-living microorganisms because it inhibits aromatic amino acid biosynthesis. On the other hand, it is extremely nontoxic to humans and animals because humans derive their amino acids from the diet. Additionally, it is broken down in the soil, so it is non-persistent. The only problem with glyphosate herbicides is that they will kill crop plants as readily as weeds. Recently, genetically engineered crop varieties have been introduced which are resistant to the herbicide, allowing weeds to be killed preferentially. 

Cloned herbicide resistant ALS genes have been used to transform both homologous and heterologous plant species. ALS genes can be modified in vitro in order to achieve selective resistance toward broad or narrow classes of inhibitors. The modified genes can be introduced into a variety of commercial crops. 

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). 

There is much controversy, at least in Europe, concerning genetic modification of plants. The three major crops affected so far are maize, soyabean and rapeseed. All of these, in addition to their other uses, are sources of oil. The reasons for modification in all these cases are related to herbicide tolerance and resistance to insects. For the varieties generally available at present, there is no known difference from non-modified strains with respect to fatty acid composition, oil yield, tocopherol level, or the level of any other minor oil constituent. 

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