Molecular basis of resistance to acetolactate synthase-inhibiting herbicides inSisymbrium orientale andBrassica tournefortii

Abstract

Three Australian Sisymbrium orientale and one Brassica tournefortii biotypes are resistant to acetolactate synthase (ALS)-inhibiting herbicides due to their possession of an ALS enzyme with decreased sensitivity to these herbicides. Enzyme kinetic studies revealed no interbiotypic differences within species in K m (pyruvate) (the substrate concentration at which the reaction rate is half maximal) but a greater V max (the rate when the enzyme is fully saturated with substrate) for two of the resistant S orientale biotypes over susceptible levels. F 1 hybrids from reciprocal crosses between resistant and susceptible biotypes of S orientale showed an intermediate response to chlorsulfuron compared to the parental plants. ALS herbicide resistance in S orientale segregated in a 3:1 (resistant:susceptible) ratio in F 2 plants with a single rate of chlorsulfuron, indicating that resistance is inherited as a single, incompletely dominant nuclear gene. Two regions of the ALS structural gene known to vary in ALSresistant biotypes were ampli®ed and sequenced. Resistant S orientale biotypes NS01 and SS03 contained a single nucleotide substitution in Domain B, predicting a Trp (in susceptible) to Leu (in resistant) amino acid change. Two adjacent nucleotide substitutions (CCT to ATT) predicting a Pro (in susceptible) to Ile (in resistant) change in the primary amino acid sequence were identi®ed in Domain A of resistant S orientale biotype SS01. Likewise, a single nucleotide substitution at the same site in the resistant B tournefortii biotype predicts a Pro (in susceptible) to Ala (in resistant) substitution. No other interbiotypic nucleotide differences predicted amino acid changes in the sequenced regions, suggesting that the amino acid substitutions reported above are responsible for resistance to ALS-inhibiting herbicides in the respective biotypes.

An Isoleucine Residue within the Carboxyl-Transferase Domain of Multidomain Acetyl-Coenzyme A Carboxylase Is a Major Determinant of Sensitivity to Aryloxyphenoxypropionate But Not to Cyclohexanedione Inhibitors

“…Clearly, not only the position but the particular amino acid substitution at a given site is of importance, as illustrated by the Ile/Asn substitution conferring resistance to clodinafop in L. rigidum, whereas the Ile/Val does not. Similar conclusions have been drawn with studies on other enzymes, such as the extensively investigated acetolactate synthase (for review, see Boutsalis et al, 1999). Thus, a tridimensional model for ACCase CT domain is definitely needed to design new inhibitors able to block the CT activity of mutant, resistant ACCase alleles.…”

Acetolactate synthase inhibitor-resistant stinkweed (Thlaspi arvense L.) in Alberta

“…In contrast, Pro 197 His substitution in ALS inhibitor-resistant biotypes of prickly lettuce (Guttieri et al 1992;Eberlein et al 1997) conferred resistance to SUs, IMIs, and TPs, whereas a Pro 197 Ile substitution in Oriental/Indian hedge mustard (Sisymbrium orientale Torn.) conferred SU resistance and moderate IMI resistance (Boutsalis et al 1999). In kochia, Pro 197 substitution to either Thr, Ser, Arg, Leu, Gln, or Ala resulted in resistance to both SUs and IMIs (Guttieri et al 1995).…”

“…Resistant Oriental/Indian hedge mustard biotypes with ALS amino acid substitutions Pro 197 Ile or Trp 574 Leu also segregated in a 3:1 (HR:HS) ratio, indicating that resistance is inherited as a single incompletely-dominant nuclear gene (Boutsalis et al 1999), whereas resistance was inherited as a dominant monogenic trait in a Pro 197 target-site mutation in an HR biotype of corn poppy (Scarabel et al 2004). Heterozygosity in ALS inhibitor-resistant biotypes was also reported for Trp 574 in giant ragweed (Ambrosia trifida L.) (Patzoldt and Tranel 2002) and wild radish (Tan and Medd 2002), and for Pro 197 in kochia (Guttieri et al 1995) and wild radish (Yu et al 2003).…”

“…In weed biotype cases studied to date, ALS target-site resistance is controlled by a single dominant to semi-dominant nuclear-encoded gene (Boutsalis et al 1999;Foes et al 1999;Tranel and Wright 2002;Scarabel et al 2004;Tranel et al 2004). Consequently, HR alleles can disseminate by pollen, if the plants are cross-pollinated, or more frequently by seed movement.…”

Field and laboratory assessments of sugarcane mutants selected in vitro for resistance to imazapyr herbicide

“…To date, only one such mutation (Ala-559) in the als gene of sugarcane has been reported (Khruangchan et al 2011). Other possible target site alterations could include increased ALS enzyme basal activity due to higher als gene transcript levels or gene copies (Boutsalis et al 1999, Yu et al 2003. Enhanced enzymatic activity is also possible due to post-transcriptional regulation, increased mRNA stability and/or reduced enzyme degradation (increased half-life) (Yuan et al 2002).…”