**3. Weed resistance to herbicide ALS inhibitors**

and Leu‐275) in herbicide‐resistant plants and algae or amino acids tagged by herbicides azi‐

**Table 2.** The first confirmed cases of weed species that have developed resistance to different herbicides site of action

**Number of weed species Total**

20 32 11 7 4 74

1 6 13 4 4 28

/ / 1 3 / 4

/ / 2 18 16 36

/ 11 62 53 33 159

/ 5 21 14 8 48

Total 21 54 110 99 65

**1970–1979 1980–1989 1990–1999 2000–2009 2010–2016**

Currently, resistance to herbicides that target photosynthesis at PS II has been documented in 74 weed species for triazines (C1/5 group), 28 in C2/7 and only 4 in C3/6 according to the data in the **Table 2** [7]. Except the usual amino acid substitution Ser‐264‐Gly in the D1 protein, reduced absorption, translocation and/or detoxification have been reported very often for

However, diverse chemical groups of herbicides PS II inhibitors (according to HRAC: C1—triazineas, triazinones, triazolinone, pyridazinones, phenyl‐carbametes, uracils; C2—amides, ureas; C3—benzothiadiazinones, nitriles, phenyl pyridazines) bind to over‐ lapping, but not identical sites on the D1 protein [43]. Several different amino acid sub‐ stitutions that confer resistance to herbicide PS II inhibitors have been identified in or near the QB‐binding niche such as: Ser‐264‐Thr in *Portulaca oleracea* [71], Ser‐264‐Gly and Val‐219‐Ile in *P. annua* and *K. scoparia* [64, 68, 70], Asn‐266‐Thr in *S. vulgaris* [73] as well as Ser‐264‐Gly, Ala‐251‐Val and Leu‐218‐Val in *C. album* [41, 59]. In addition, dependence of herbicides, interaction between herbicides, specific amino acid substitution, varying levels of cross or negative cross‐resistance have been reported for different mutations in the D1 protein [64]. Resistance ratios for *P. oleracea* a Ser‐264‐Thr mutant were 8 and >6 for linuron and diuron, respectively; >800 for atrazine; and >20 for terbacil. Linuron resistant *P. oleracea* was negatively cross‐resistant to pyridate and bentazon (0.75 and 0.5,

doderivatives (Met‐214 by azidoatrazine) [45].

respectively) [71].

**Herbicide groups according the site of action**

12 Herbicide Resistance in Weeds and Crops

Inhibitors PS II (C1/5)

Inhibitors PS II (C2/7)

Inhibitors PS II (C3/6)

Inhibitors ESPSP enzyme

Inhibitors AHAS enzyme

Inhibitors ACC‐ase

according to decades.

resistance to triazines in many weed species (**Table 3**).

Herbicide inhibitors of acetoacetate synthase (ALS) and acetohydroxyacid synthase (AHAS) belong to several chemical classes: sulfonylurea (SU), triazolopyrimidines (TPs), pyrimidinyl(thio) benzoates, sulfonylaminocarbonyltriazolinones, imidazolinones (IMIs). The first commercial SU herbicide was chlorsulfuron, which was introduced by DuPont in 1982 for weed control in small grain crops. The SUs are highly active herbicides, effective at use rates as low as 2 g a.i. ha−1 [74]. Almost simultaneously, researchers at American Cyanamid discovered a structurally distinct family of herbicides, the IMIs, which were also shown to inhibit the ALS enzyme [75]. Since then, three additional chemical classes of ALS inhibitors have been discovered. Those products provide both pre‐emergent and post‐emergent control of many serious monocot and dicot weed species in many crops.

ALS is the first enzyme in the branched‐chain amino acid pathway, which catalyzes the first steps in amino acid biosynthesis such as valine, leucine and isoleucine [76]. The first case of resistance to ALS inhibitors (chlorsulfuron) was reported within 5 years after the introduction of SU herbicides, in 1987 in the United States [77]. Herbicide‐resistant weed evolution is more common for ALS inhibitors compared to herbicides of other groups. Currently, 159 weed species have evolved resistance to ALS‐inhibiting herbicides [7] according to decades that could be seen in **Table 2**. Weed resistance to ALS inhibitors is due to an alteration of the gene encoding the ALS enzyme. The positions in ALS from various sources (plant, yeast, bacteria) where mutations are known to confer resistance to one or more herbicides distributed across the α, β and γ domain of the protein (**Figure 3**) [78]. Weed species or genera with high incidence of target‐site ALS resistance include *Amaranthus* spp., *K. scoparia* and *Papaver rhoeas*, among others. Studies have shown that mutations of eight amino acid residues are known to be involved in causing weed resis‐ tance: Ala‐122, Pro‐197, Ala‐205, Asp‐376, Arg‐377, Trp‐574, Ser‐653 and Gly‐654 (**Table 4**).

**Figure 3.** ALS mutations conferring herbicide resistance. Arrows point to positions in the sequences of ALS from different sources (plant, yeast, bacteria) where spontaneous or induced mutations result in an herbicide‐insensitive enzyme. Colours designate substitutions occurring in more than one species (downloaded from Ref. [78]).



case of resistance to ALS inhibitors (chlorsulfuron) was reported within 5 years after the introduction of SU herbicides, in 1987 in the United States [77]. Herbicide‐resistant weed evolution is more common for ALS inhibitors compared to herbicides of other groups. Currently, 159 weed species have evolved resistance to ALS‐inhibiting herbicides [7] according to decades that could be seen in **Table 2**. Weed resistance to ALS inhibitors is due to an alteration of the gene encoding the ALS enzyme. The positions in ALS from various sources (plant, yeast, bacteria) where mutations are known to confer resistance to one or more herbicides distributed across the α, β and γ domain of the protein (**Figure 3**) [78]. Weed species or genera with high incidence of target‐site ALS resistance include *Amaranthus* spp., *K. scoparia* and *Papaver rhoeas*, among others. Studies have shown that mutations of eight amino acid residues are known to be involved in causing weed resis‐ tance: Ala‐122, Pro‐197, Ala‐205, Asp‐376, Arg‐377, Trp‐574, Ser‐653 and Gly‐654 (**Table 4**).

**Amino acid substitutions Other mechanisms of resistance**

Detoxification [86]

detoxification [87, 88]

**Weed species Mechanism of resistance**

14 Herbicide Resistance in Weeds and Crops

*Amaranthus retroflexus* L. Ala‐122‐Thr, Pro‐197‐Leu, Ala‐205‐

*Amaranthus powellii* S. Warts. Ala‐122‐Thr, Asp‐376‐Glu, Arg‐377‐

*Amaranthus hybridus* L. Ala‐122‐Thr, Asp‐376‐Glu, Arg‐377‐

*Amaranthus blitoides* S. Wats Pro‐197‐Ser, Arg‐377‐His

*Ambrosia artemisiifolia* L. Arg‐377‐His

Val, Asp‐376‐Glu, Arg‐377‐His, Trp‐574‐Leu, Ser‐653‐Thr [80]

enzyme. Colours designate substitutions occurring in more than one species (downloaded from Ref. [78]).

**Figure 3.** ALS mutations conferring herbicide resistance. Arrows point to positions in the sequences of ALS from different sources (plant, yeast, bacteria) where spontaneous or induced mutations result in an herbicide‐insensitive

His, Ser‐653‐Thr

His, Ser‐653‐Asn

*Amaranthus tuberculatus* (Moq.) Sauer Arg‐377‐His, Ser‐653‐Asn/Thr Altered enzyme activity,

*Amaranthus palmeri* (S.) Warts. Arg‐377‐His, Ser‐653‐Asn Altered enzyme activity [87, 89]


**Table 4.** Confirmed mechanisms of resistance to herbicide ALS inhibitors in some weed species.

The most different amino acid substitutions in α‐domain at position Pro‐197 have been linked in confirmed weed‐resistant species such as: *K. scoparia* (Pro‐197‐Ser/Thr/Leu/ Ala/Gln/Arg), *Descurainia sophia* (Pro‐197‐Ser/Thr/Leu/Ala/His/Tyr), *P. rhoeas* (Pro‐197‐ Ser/Thr/Leu/Ala/His/Arg), *L. rigidum* (Pro‐197‐Ser/Leu/Ala/Gln/Arg), *Apera spica‐venti* (Pro‐197‐Ser/Thr/Ala/Asn), etc. Also, the substitution of Trp‐574‐Leu confers resistance to several weed species (*A. retroflexus*, *C. iria*, *D. sophia*, *C. canadensis*, *K. scoparia*, *P. annua* etc.) and the levels of resistance are all high against SUs, IMIs and TPs (cross‐resistance) [29, 79–83]. Generally, the low number of confirmed weeds resistant to ALS inhibitors is due to altered enzyme activity, reduced translocation and detoxification. Additionally, many weed populations resistant to ALS inhibitors have developed multiple‐resistance to other chemical classes with different modes of action (e.g. auxinic herbicides, EPSPS inhibitors, ACC‐ase inhibitors) [31, 84, 85].
