Imidazolinones tolerant crops

Plants tolerant to imidazolinones have been produced by both transgenic and non-transgenic mechanisms. However, all of the imidazohnone-tolerant crops currently being sold have been developed by non-transgenic methods. The first imidazolinone-tolerant crop (maize) was introduced in 1992. Subsequently, four additional imidazolinone-tolerant crops (canola, rice, wheat and sunflower) have been commercialized [31]. All of the imidazohnone-tolerant crops are being sold under the CLEARFIELD trade name. 

From the imidazolinone family, four different molecules, imazapyr, imazapic, imazethapyr and imazamox, have been registered for weed control in various imidazolinone-tolerant crops in different regions of the world. These herbicides are applied alone or in combination with other imidazolinones or with other classes of herbicides for a broad spectrum, season-long weed control. A combination of different imidazolinone tolerance traits and multiple herbicide options provides an effective weed management tool for farmers around the world. 

In a similar fashion, the EIQ has been applied to Canadian canola cultivation in order to estimate the environmental impact of the adoption of herbicide tolerant canola, which includes transgenic varieties. In recent years, herbicide tolerant canola varieties (glyphosate-, glufosinate-, and imidazolinone-tolerant) have been adopted by Canadian fanners at a larges scale, accounting for 80% of the national canola acreage in 2000. The adoption of these herbicide tolerant varieties has been associated with a concomitant decrease in herbicide use on canola. The environmental impact of herbicide use on canola determined by the EIQ method decreased in correlation with the decreased amounts of herbicide applied to the crops, rather than this would be due to different intrinsic properties (EIQ) of the herbicides involved [23]. 

With the invention of the aceto-hydoxy-add synthesis inhibitors (AHAS) the dominance of herbiddes that ad as photosynthesis inhibitors was dramatically broken - as it was also by the development of genetically modified herbicide tolerant crops. These especially important areas of research and development, from the 1990s up to now, are exemplified in Chapters 3 and 7. The development of 12 new sulfonyl urea herbicides launched since 1995 and the invention of four development compounds of the same chemical class, after the introduction to the market of twenty compounds already between 1980 and 1995, refleds the importance of this biochemical mode of adion for the herbicide market as well as the different chemistries found to be active at this target, such as imidazolinones, triazolo-pyrimidines, pyrimidinyl-carboxylates, and sulfonylaminocarbonyl-triazolinones. 

These imidazolinone tolerance traits in different crops were developed by various methods. These methods included tissue culture selection (maize), pollen mutagenesis (maize), microspore selection (canola), seed mutagenesis (wheat and rice) and incorporation of resistance trait from a weedy relative (sunflower). Details of these methods have been previously reviewed [31, 32]. In all of these cases, the basis of tolerance is due to the presence of an altered form of AHAS that is resistant to inhibition by imidazolinones. The resistant enzyme is produced due to a single base pair change in the gene encoding the large subunit... 

A more recent factor affecting weed management has been the introduction of crops genetically altered for tolerance or resistance to herbicides. The first herbicide-tolerant field com (IMI hybrid corn) was developed as a way to reduce the effects of carryover from imidazolinone and sulfonylurea herbicides applied to soybean in a corn-soybean rotation. These hybrids also soon found use in areas where triazine use was restricted. 

Acetolactate synthase (ALS) is the enzyme target site of the sulfonyl-ureas. In common with the imidazolinone aryl carboxylates, these herbicides inhibit valine and isoleucine biosynthesis. The imidazolinones are exemplified by Assert (8), which consists of a mixture of m- and p-isomers. The selectivity of ALS inhibitors invariably can be accounted for by differential metabolism or uptake or related phenomena rather than by any significant inherent difference in the properties of the ALS enzymes of crop and weed species. Assert is another example of a herbicide activated in plants by deesterification to the phytotoxic acid, and susceptible species such as Avena fatua (wild oat) are unable to metabolize the molecule further. Facile ring-methyl hydroxylation to the m- and p-benzyl alcohols, however, followed by glycosylation confers tolerance to maize and wheat." ... 

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