Physiological consequences of mutation for ALS-inhibitor resistance

Abstract

Biochemical and physiological effects of target site resistance to herbicides inhibiting acetolactate synthase (ALS) were evaluated using sulfonylurea-resistant (R) and -susceptible (S) near isonuclearLactuca sativa‘Bibb’ lines derived by backcrossing the resistance allele fromLactuca serriolaL. intoL. sativa.Sequence data suggest that resistance inL. sativais conferred by a single-point mutation that encodes a proline197to histidine substitution in Domain A of the ALS protein; this is the same substitution observed in RL. serriola. Kmapp(pyruvate) values for ALS isolated from R and SL. sativawere 7.3 and 11.1 mM, respectively, suggesting that the resistance allele did not alter the pyruvate binding domain on the ALS enzyme. Both R and S ALS had greater affinity for 2-oxobutyrate than for pyruvate at the second substrate site. Ratios of acetohydroxybutyrate: acetolactate produced by R ALS across a range of 2-oxobutyrate concentrations were similar to acetohydroxybutyrate: acetolactate ratios produced by S ALS. Specific activity of ALS from RL. sativawas 46% of the specific activity from SL. sativa, suggesting that the resistance allele has detrimental effects on enzyme function, expression, or stability. ALS activity from R plants was less sensitive to feedback inhibition by valine, leucine, and isoleucine than ALS from S plants. Valine, leucine, and isoleucine concentrations were about 1.5 times higher in R seed than in S seed on a per gram of seed basis, and concentrations of valine and leucine were 1.3 and 1.6 times higher, respectively, in R leaves than in S leaves. Findings suggest that the mutation for resistance results in altered regulation of branched-chain amino acid synthesis.

Analysis of acetohydroxyacid synthase1 gene in chickpea conferring resistance to imazamox herbicide

“…Another observation from the in vitro assay showed that AHAS activity from imazamox-resistant CDC Cory was consistently (ϳ37%) less than AHAS activity from susceptible CDC Frontier. Similar variation (ϳ40%) in enzyme-specific activity between susceptible and tolerant cultivars was observed previously in prickly lettuce (Lactuca serriola), lettuce (Lactuca sativa), and black nightshade (Solanum ptyhanthum) (Ashigh and Tardif 2007;Eberlein et al 1999Eberlein et al , 1997. Results of our analyses suggest that although the mutation responsible for the resistance to imazamox has no effect on the transcription of the gene, there might be a possible fitness cost associated with the resistance to the herbicide, resulting in lower but consistent enzyme activity in chickpea CDC Cory.…”

Effect of Ahasl1‐1 and Ahasl1‐4 alleles on herbicide resistance and its associated dominance in sunflower

“…These results are in agreement with the location of the mutations, not at the active site but within the substrate‐access channel . Moreover, AHAS kinetic studies with sunflower and several other species have shown similar Km app values for resistant and susceptible genotypes …”

“…). Several works showed that herbicide resistance‐endowing AHAS gene mutations result in decreased AHAS activity in dicots . Increased or unchanged AHAS activity has been observed for a number of resistance mutations in other species .…”

“…The difference in AHAS activity observed between resistant and susceptible genotypes is therefore likely to be related to specific resistance mutations in different plant species, frequency of resistance alleles in the resistant weed population, and the genetic background of the genotypes under comparison. In this study, the near‐isogenic lines provide appropriate materials for studying the specific effects of the resistance alleles in the absence of confounding background effects …”

Pleiotropic effects of herbicide-resistance genes on crop yield: a review

“…For Lactuca , ALS activity was only 46% of that of the parental line after five back‐cross generations. However, the ALS mutant was less sensitive to feedback regulation and so branched amino‐acid concentration was 1.5 times higher in resistant seeds than in susceptible seeds . Unfortunately, that study did not collect data on growth and reproduction.…”