Sulfonylurea resistance weeds

The development of herbicide-resistant weeds has also been an influence on the selection of herbicides used on field corn or soybean. Weed resistance now affects nearly every decision a farmer makes about herbicide selection either a farmer is trying to control resistant weeds or is selecting herbicides that may reduce the possibility of weed populations becoming resistant. The adoption of the imidazolinone- and sulfonylurea-tolerant com hybrids mentioned above was in part a response to the presence of atrazine-tolerant pigweeds or kochia in many fields. However, a recent decrease in die use of imidazolinone and sulfonylurea herbicides can also be attributed to the development of populations of weeds that have become resistant to these herbicides. 

Although the ALS inhibitor herbicides have been used for approximately 20 years, the number of resistant weed biotypes for this group now exceeds those for all other types of herbicides. Singh and Shaner (1995) and Boutsalis (2001) reported that a total of five chemical families or herbicide classes are commercially marketed as inhibitors of ALS, and that these herbicides comprise more than 50 active ingredients for selective use in many different crops. They include sulfonylureas, imidazolinones, triazolopyrimidines, sulfonylamino-carbonyl-triazolinones, and pyrimidinyl (thio)benzoates. 

The first ALS-resistant weeds were reported in 1987 when prickly lettuce (Mallory-Smith, 1990 Mallory-Smith et al, 1990b) and kochia (Primiani et al, 1990) control failures occurred in Idaho and Kansas, respectively, after 5 consecutive years of chlorsulfuron use. The kochia biotype proved to be cross-resistant to six other ALS-inhibitor herbicides, including sulfonylureas and imidazolinones. Within 5 years, sulfonylurea-resistant kochia had been identified at 832 sites in 11 states of the United States and in three Canadian provinces (Saari et al, 1994). ALS inhibitor-resistant kochia and Russian thistle have become widespread problems in cereal-producing regions of northwestern United States and Canada. The mobility of these tumble weeds as plants with mature seeds or pollen carried by wind has undoubtedly contributed to the rate at which resistance has spread. 

Burnet, M.W.M., J.T. Christopher, J.A.M. Holtum, and S.B. Powles (1994). Identification of two mechanisms of sulfonylurea resistance within one population of rigid ryegrass (Lolium rigidum) using a selective germination medium. Weed Sci., 42 468 173. 

Christoffoleti, P.J., P. Westra, and F. Moore III (1997). Growth analysis of sulfonylurea-resistant and -susceptible kochia (Kochia sco-paria). Weed Sci., 45 691-695. 

Lovell, S.T., L.M. Wax, M.J. Horak, and D.E. Peterson (1996b). Imidazolinone and sulfonylurea resistance in a biotype of common waterhemp (Amaranthus rudis). Weed Sci., 44 789-794. 

Stallings, G.P., D.C. Thill, C.A. Mallory-Smith, and B. Shafii (1995b). Pollen-mediated gene flow of sulfonylurea-resistant kochia (Kochia scoparia). Weed Sci., 43 95-102. 

Many of the characteristics which combine to make ALS an excellent target for engineering beneficial herbicide resistance in crop plants may also lead to the proliferation of herbicide-resistant weeds. These characteristics include the following sulfonylurea herbicide resistance is a semi-dominant trait that is carried on a nuclear gene(s) ALS is the single primary site of action there are multiple positions in ALS that can be mutated to confer herbicide resistance mutant ALS enzymes can possess full catalytic activity. The latter property results in engineered crop plants that are fit, but can equally well result in weed biotypes that are fit. 

The properties of the herbicides that target ALS can also contribute to the emergence of resistant weeds. There are several classes of compounds which target ALS, including the sulfonylureas, the imidazolinones and the triazolopyrimidines. Within these classes are a number of herbicides that are used at rates that kill a high proportion of the weeds, thus increasing the likelihood that resistant biotypes will... 

The remainder of this chapter will describe the mode of action, crop selectivity mechanisms and environmental properties of the sulfonylurea herbicides. Other issues including resistant weeds and recropping intervals will also be discussed. 

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. 

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. 

Baumgartner, J.R., K. Al-Khatib, and R.S. Currie (1999). Cross-reference of imazethapyr-resistant common sunflower (Helianthus annuus) to selected imidazolinone, sulfonylurea, and triazolopyrimidine herbicides. Weed Technol. 13 489 -93. 

Primiani, M.M., J.C. Cotterman, and L.L. Saari (1990). Resistance of kochia (Kochia scoparia) to sulfonylurea and imidazolinone herbicides. Weed Technol., 4 169-172. 

Saari, L.L., J.C. Cotterman, and M.M. Primiani (1990). Mechanisms of sulfonylurea herbicide resistance in the broadleaf weed Kochia scoparia. Plant Physiol., 93 55-61. 

The evolution of resistance is not necessarily a slow process, as was first believed. Experience has shown, for example with sulfonylurea herbicides, that resistance can develop very quickly, in some cases fully evident after only 3 years of use of a particular herbicide family. That the genetic composition of a weed population can change so quickly is a sobering thought. 

FIGURE 4 Results from a field test of herbicide-resistant tobacco lines, conducted in North Carolina in 1987 in conjunction with Northrup King Co. The row of plants to the right of center contains elite tobacco lines that were transformed with the mutant tobacco ALS gene from the herbicide-resistant Hra line, while the row to the left of center has non-transformed control plants. After treatments with the sulfonylurea herbicide chlorimuron ethyl at 4X normal field application rate, the weeds were killed, the non-transformed controls were severely injured and remained stunted, and the transformed plants remained as vigorous as the unsprayed controls. 

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. 

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). 

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