Molecular basis of resistance to acetolactate synthase-inhibiting herbicides inSisymbrium orientaleandBrassica 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 Km (pyruvate) (the substrate concentration at which the reaction rate is half maximal) but a greater Vmax (the rate when the enzyme is fully saturated with substrate) for two of the resistant S orientale biotypes over susceptible levels. F1 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 F2 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 ALS‐resistant biotypes were amplified 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 identified 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. © 1999 Society of Chemical Industry

A novel Pro197Glu substitution in acetolactate synthase (ALS) confers broad-spectrum resistance across ALS inhibitors

“…Pro197 mutations confer resistance to SUs and TPs; Ala122, Ser653 and Gly654 mutations confer resistance to IMIs; and Asp376 and Trp574 mutations confer broad-spectrum resistance to all five classes of ALS inhibitors [12][13][14]. Although Pro197 mutations usually result in SU but not IMI resistance [29], resistance to IMI herbicides has also been observed in several weed species due to the expression of an insensitive form of ALS resulting from a substitution at position 197 [16,[30][31][32][33][34][35][36]. A new discovery now demonstrates that the cross-resistance pattern endowed by a target-site mutation depends on the substitution position, the specific substitutions, and, sometimes, the weed species [14].…”

“…The I50 of the TP herbicides florasulam and pyroxsulam for the S plants were 0.003 μM and 0.018 μM, respectively; however, the WRR04 plants had much higher I50 values of 1.10 μM and 6.05 μM, respectively. The estimated imazethapyr concentrations for the I50 were 0.45 μM for the S plants and 16.63 μM for the WRR04 plants. Based on the RI of I50, the ALS from the WRR04 plants is highly resistant to tribenuron (113-fold), pyrithiobac sodium (up to 1413-fold), florasulam (367-fold), pyroxsulam (336-fold), and imazethapyr (37-fold).…”

“…Among 12 resistance AASs (Ala, Arg, Asn, Gln, His, Ile, Leu, Lys, Met, Ser, Thr, and Trp) identified at Pro197 [12,13], the frequency of Pro197Ser is much higher than that of the others, and the frequency of mutations requiring only one nucleotide change is higher than that of those requiring two nucleotide changes [14]. To date, resistance-endowing AASs resulting from two nucleotide mutations have seldom been reported in weed species (e.g., Asn, Ile, Met, Lys, and Trp substitutions at Pro197 and the Tyr substitution at Ala122, each only reported once) [13][14][15][16][17]. This phenomenon may be related to the herbicide selection pressure, weed species, and fitness costs that are associated with ALS mutations.…”

Bastard Cabbage (Rapistrum rugosum L.) Resistance to Tribenuron-Methyl and Iodosulfuron-Methyl-Sodium in Spain and Alternative Herbicides for Its Control

“…The very high resistance factors found in the present populations suggest that the target-site mechanism might confer resistance. In several other Brassicaceae species, this mechanism has been confirmed to ALS-inhibiting herbicides in Sinapis alba [17,18], Sinapis arvensis [5,[26][27][28], Raphanus raphanistrum [29], Sisymbrium orientale and Brassica tournefortii [30], and Descurainia sophia [19,20,31]. However, despite being rarer, metabolism-based resistance to ALS-inhibiting herbicides has also The resistance factors calculated from the biomass of populations R1 and R2 were 41.6 and 26.1, respectively, but when calculated based on survival, they increased to 633 and 487 (Table 4), respectively.…”

Molecular basis of resistance to mesosulfuron-methyl in Japanese foxtail, <i>Alopecurus japonicus</i>

“…Pro 197 substitution usually results in high sulfonylurea resistance, and many substitutions have been reported for this amino acid residue. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] This study determined the resistance of A. japonicus populations to mesosulfuron-methyl. Its objectives were to (1) characterize the degree of resistance in resistant and susceptible populations, and (2) test whether the resistance mechanism is due to a modified ALS.…”