Resistance to Branched Chain Amino Acid Inhibitors in Weeds and Crops

Five years after the commercial release of the first SU herbicide, chlorosulfuron (Glean), DuPont removed this product from the market due to the rapid expansion of herbicide resistant kochia.  Chlorosulfuron is still sold in non-crop markets under the trade name of Telar®, primarily because of its activity on Canada thistle.  There are also a number of newer SU herbicides that are available for small grain weed control because these herbicides still control a range of important broadleaf and some grass weeds.

So why did resistance to ALS inhibitors appear so rapidly compared to other herbicide MOAs?  It has been suggested that the natural mutation rate conferring ALS resistance might be as high as 10^-6 for some weed species (probably kochia).  The other major factors contributing the to rapid spread of resistance are 1) very strong herbicide selection pressure 2) dominant trait resulting in highly resistant heterozygous individuals 3) pollen mediated gene flow allowing for resistance in plants that have never been exposed to the herbicide and 4) no fitness penalty associated with the resistance gene.  Single point mutations are sufficient to affect the binding of one or several ALS inhibitors, resulting in herbicide resistance.  There are four amino acids that are the most common sites for these mutations; alanine 122 (A122), proline 197 (P197), tryptophan 574 (W574), and serine 653 (S653).

Imidazolinone resistance is associated with point mutations at A122 and S653 (Table 6).  The A122 substitution with threonine (Thr) confers strong resistance because this amino acid makes important hydrophobic interactions with the isopropyl and methyl groups of the dihydroimidazolone ring.  The herbicide has lower affinity when a larger, polar amino acid like threonine is substituted.  The situation with S653 is very similar, the substitution of any larger amino acid limits imidazolinone binding because it obstructs the space associated with the aromatic ring.

Strong resistance to SU herbicides is associated with point mutations at P197 (Table 6).  Eight different point mutations at this location result in SU resistance, while only one proline to leucine substitution results in cross-resistance to both SUs and IMIs.  P197 appears to be positioned at the entrance to the active site channel at the end of an alpha-helix.  The aromatic ring of the SU contacts this amino acid, but it does not appear to interact with imazaquin (extraplated to IMIs in general).  It appears that any point mutation at P197 restricts SU molecules from reaching the active site channel, while only bulky amino acids would restrict IMIs.

The most carefully studied mutations occur at W574.  The substitution of leucine for tryptophan at positon 574 confers strong resistance to both SU and IMI herbicides.  This amino acid does two things, first it helps to define the shape of the channel and second it is an important anchor for both herbicide classes.  Therefore, point mutations at this location affect the binding of both SUs and IMIs.

The ALS inhibiting herbicides still provide excellent control of many important weed species in a wide range of crops; therefore, many companies have used techniques like ethylmethanesulfonate (EMS) mutagensis to produce a number of ALS tolerant crops.  These crops include corn, rice, soybean, canola, and wheat.  These new non-transgenic crops provide additional opportunities for selective weed management, while at the same time increasing the selection pressure on weed populations.  The point mutations resulting from a variety of selection techniques have all been identified in natural weed populations.

   Table 6.  Amino acid substitutions conferring resistance to SU and IMI herbicides. 

                   (Adapted from Tranel and Wright. 2002. Weed Sci. 50:700-712)

McMourt, J.A., S.S. Pang, J. King-Scott, L.W. Guddat and R.G. Duggleby. 2006. Herbicide-binding sites revealed in the structure of plant acetohydroxyacid synthase. PNAS 103:569-573.

Tranel P.J. and T.R. Wright. 2002. Resistance of weeds to ALS-inhibiting herbicides: What have we learned?.  Weed Sci. 50:700-712