**2. Weed resistance to herbicides photosystem II inhibitors, triazines**

The triazine herbicides were discovered in the J.R. Geigy Ltd. laboratories, an international chemical company founded in 1952 and based in Basel, Switzerland [33]. Generally, in the latter half of the twentieth century, triazines have played a significant role in the promotion of the crop production. Atrazine is one of the most used triazine herbicides in agriculture for con‐ trol of annual monocots (*Setaria* sp., *E. crus‐galli*, *Digitaria sanguinalis*) and dicot weed species (*Amaranthus* sp., *Chenopodium* sp., *Cirsium arvense*, *D. stramonium*, *Sonchus* sp., *Xanthium stru‐ marium*, etc.) and is the most widely used herbicide in maize, orchards and sorghum crops. Triazines specifically inhibit photosystem II (PS II) in plants and in all organisms with oxygen‐ evolving photosystems. Generally, they prevent electron transfer by displacing plastoquinone (QB) from a specific binding site on the D1 protein subunit of PS II [34, 35].

The intensive use of triazines resulted in two important cases: appearance of atrazine‐resistant weed species, leading to the increased use of herbicide mixtures or alternative herbicides. The first confirmed atrazine‐resistant weed species [6] helped identify the herbicide‐binding D1 protein in PS II. After the psbA gene was found and sequenced [36], the psbA gene from an atrazine‐tolerant *Amaranthus* was then sequenced [37]. Based on their findings, the resistance is due to a chloroplast genome mutation of the psbA gene, which codes the D1 protein. The molecular analysis showed that resistance is due to the substitution of serine 264 to glycine (Ser‐264‐Gly) in many weed species [38–42]. The substituted urea herbicides, as PS II inhibi‐ tors [43] also bind in a niche on the D1 protein, but not at the identical site as the triazines.

A schematic diagram of the folding of the herbicide‐binding site on the D1 protein [44], updated with further amino acids in triazine resistance, is given in **Figure 2** [45]. From total of 345 amino acids in the D1 protein, around 60 are part of the herbicide and QB‐binding site. Arrows indicate possible mutations (such as Val‐219, Ala‐251, Phe‐255, Gly‐256, Ser‐264

*Cross‐resistance* **(CR)** means that a single‐resistance mechanism causes resistance to several herbicides. CR can be conferred by a single gene or by two or more genes influencing a single mechanism. There are two types of CR: target‐site cross‐resistance (TS‐CR) and non‐target‐site cross‐resistance (NTS‐CR). The most common type of CR is TS‐CR where an altered target site confers resistance to many or all of the herbicides that inhibit the same enzyme, for example, Trp‐574‐Leu amino acid substitution within the ALS gene was found in two populations of *Cyperus iria* after exposition to bispyribac‐sodium, halosulfuron, imazamox and penoxsulam [29]. On the other hand, NTS‐CR is type of herbicide resistance in which a mechanism other than resistant enzyme target sites is involved (e.g. reduced absorption, translocation, or

**Figure 1.** The route of the herbicide after the application, and the possible mechanisms of resistance in plant. After application: (1) herbicide absorption/penetration, (2) translocation, (3) accumulate at the target protein location, and (4) binding to the target protein, (5) disruption of the biosynthesis pathways or cell structures, and/or generation of cytotoxic molecules. NTSR mechanisms: (A) reduction in herbicide penetration, (B) altered translocation of the herbicide away from the target protein, (C) enhanced detoxification of the herbicide, or (D) enhanced neutralization of cytotoxic molecules generated by herbicide action. TSR mechanisms: (E) target protein overproduction, and/or (F) structural mutations that modify the 3D structure and electrochemical properties of the target protein. Structural mutations can have no, moderate or strong negative effects on the stability of herbicide binding to the target protein, which results in (F‐a) no, (F‐b) moderate or (F‐c) marked reduction in herbicide sensitivity at the protein level, respectively; or can (F‐d) increase the stability of herbicide binding to the target protein, which results in an increase in herbicide sensitivity at the

*Multiple‐resistance* is a situation where two or more resistance mechanisms are present within the same plant, often due to sequential selection by herbicides with different modes of action (e.g. resistance of *Lolium* sp. populations to glyphosate and ACC‐ase inhibitors, as well as resistance to glyphosate and ALS inhibitors were confirmed by multiple‐resistance [31]).

enhanced herbicide detoxification) [30].

protein level (downloaded from Ref. [32]).

10 Herbicide Resistance in Weeds and Crops

**Figure 2.** The amino acid sequence of the herbicide‐binding D1 protein in the PS II. This work of Michel and Deisenhofer was honoured with the Nobel Prize. (downloaded from Ref. [45]).


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

and Leu‐275) in herbicide‐resistant plants and algae or amino acids tagged by herbicides azi‐ doderivatives (Met‐214 by azidoatrazine) [45].

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

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, respectively) [71].


**Table 3.** Confirmed mechanisms of resistance to herbicide PS II inhibitors in some weed species.
