Resistance to sulfonylureas

Haughn, G., Smith, J., Mazur, B. Sommerville, C. (1988). Transformation with a mutant Arabidopsis acetolactate synthase gene renders tobacco resistant to sulfonylurea herbicides. Molecular and General Genetics 211, 266-71. 

Christopher et al. (1992) reported that a chlorsulfuron-resistant rigid ryegrass in Australia was resistant to most other sulfonylurea and imidazolinone ALS inhibitors. However, a common cocklebur biotype resistant to several imida-zolinone herbicides was not resistant to sulfonylurea herbicides (Saari et al., 1994). It is, therefore, difficult to generalize as to patterns of resistance within the five classes of ALS inhibitors. Weed biotypes resistant to one herbicide will usually show some level of resistance to most herbicides within the same class, and may in addition show some resistance to ALS inhibitors in other classes. 

These cloned Arabidopsis and tobacco ALS genes have been used as hybridization probes to isolate ALS genes from other crop species and to isolate ALS genes from plants selected for resistance to sulfonylurea herbicides. 

Two selection strategies have been used to obtain plants resistant to sulfonylurea herbicides. 

In the second approach, herbicide-resistance mutations in the Arabidopsis ALS gene were studied in E. coli. To do this, wild type and mutant Arabidopsis genes were functionally expressed in E. coli, such that the plant genes complemented a branched chain amino acid auxotrophy in the bacteria (Smith et al. 1989, PNAS in press). ALS enzyme assays on extracts prepared from E. coli expressing the mutant Arabidopsis gene indicated that the mutant enzyme is resistant to sulfonylurea herbicides but is sensitive to the imidazolinone herbicide imazaquin. This selective... 

Research work in the US potentially offers herbicide-resistant sunflower varieties that have been developed by non-biotechnological techniques. Express , being developed by DuPont and Pioneer, shows resistance to sulfonylurea herbicide. Another variety, Clearfield , is resistant to imazamox. The key decision on these products depends on the decision under Section 18 of the Environmental Protection Agency. Should Section 18 label be approved, Micogen CO. already has enough Clearfield seeds to plant 15000 acres next year. Other companies will have Clearfield seeds ready in 2003. Express is expected to be commercially available in a few years (The Sunflower, December, 2001). 

In the ALS of a Lactuca serriola biotype, highly resistant to SUs and moderately resistant to IMIs, Prol97 was substituted by His. The pyruvate binding domain on the ALS enzyme was not found to be altered by the mutation [57]. From Ko-chia scoparia it was reported that several substitutions of Prol97 by another amino acid (Thr, Arg, Leu, Gin, Ser, Ala) will confer resistance to sulfonylureas [58]. In the same spedes, it was found later that a substitution of Trp574 by Leu will also cause resistance to sulfonylureas and in addition cross-resistance to imidazoh-nones [59]. The latter substitution was also detected in resistant biotypes of several other dicotyledonous weed species. 

In a biotype of Amarantkus retrojlexus from Israel, resistance was caused by a change of Prol97 to Leu. This biotype exhibited cross-resistance to sulfonylureas, imidazolinones, triazolopyrimidines and to pyrithiobac-sodium in vwo and on the ALS enzyme level [60]. In mutations of Amarantkus rudis, Ser653 was found to be exchanged by Thr or Asn. These were only resistant to imidazolinones [61]. 

Resistance. Pest resistance to chemical control has a long history, but currently its scope and speed of development are accelerating 1531 No class of compound is unaffected. A striking example is emergence of weeds resistant to sulfonylureas — and implementation of resistance management strategies involving mixtures and short-residual compounds — soon after their introduction (Brown and Kearney, this volume). 

As mentioned above, there is a key exception to the rule that sulfonylurea herbicide selectivity is based on metabolic inactivation. This exception is the case of plants which, through mutation, selection, or genetic engineering methods, have acquired a gene coding for a herbicide-resistant form of the ALS enzyme. Crop species which have been selected or engineered for broad resistance to sulfonylureas by this mechanism include soybeans (28), tobacco (17, Ig), and canola (22), with similar efforts proceeding in other crops (40 ... 

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