Herbicide safeners and synergists

Herbicide safeners, also termed antidotes in some texts, are chemicals that, when applied before or with herbicides, increase the tolerance of a cereal crop to a herbicide. This activity has been known since the 1970s and the safening effect is not seen on the weeds. A list of herbicide safeners available as commercial products is given in Table 4.2.

As some safeners may show structural homology with herbicides, it was previously thought that they competed with the herbicide molecule for the target site. We now know, however, that their protective effect results from a general enhancement of detoxification processes, including the induction of:

- P450 oxygenases

- Glutathione ^-transferases

Figure 4.7 A schematic representation of the different phases of herbicide metabolism in a plant cell (R, denotes the active herbicide molecule). Key: 1. hydrolase; 2. oxido-reductase; 3. glutathione S-transferase; 4. glucosyl transferase; 5. malonyl transferase.

- Glucosyltransferases

- Vacuolar transport

- Glutathione synthesis

- Glutathione peroxidase

- Sulphate assimilation

Since a crop has to show some degree of tolerance to the herbicide for it to be safened, it would appear that the process is a 'top-up' mechanism for detoxification routes that are already operative. Despite considerable interest in safeners, their activities have mainly been demonstrated in monocotyledonous crops, notably maize, wheat, sorghum and rice.

It is likely that the identification of genes encoding safener-inducible enzymes will allow their transfer to give genetically modified crops with an enhance ability to detoxify herbicides. This approach has already shown potential (Davies and Caseley, 1999), although further development will require a greater public acceptance of GM crops in future.

Table 4.2 Herbicide safeners available as commercial products (from Davies and Caseley, 1999).

Safener

Crop

Herbicide

Application method

Benoxacor (CGA 154281)

Cloquintocet-mexyl (CGA 184927)

Cyometrinil (CGA 43089)

Dichlormid (DDCA, R25788)

co-chci,

Cloquintocet-mexyl (CGA 184927)

Fenchlorazole-ethyl (HOE /t^00^^

70542)

Fenclorim (CGA 123407)

Fenclorim (CGA 123407)

Maize Wheat

Sorghum Maize

Wheat Rice

Metolachlor

Spray as mixture with herbicide

Clodinafop-propargyl Spray as mixture with herbicide

Metolachlor

Seed-treatment

EPTC, butylate, vernolate Pre-plant incorporated with herbicide

Fenoxaprop-ethyl

Pretilachlor

Spray as mixture with herbicide

Spray as mixture with herbicide (Continued)

Table 4.2 (Continued)

Safener

Crop

Herbicide

Application method

Flurazole (MON 4606) Fluxofenim (CGA 133205) ci-( Vc=n^-ch2Y

Mefenpyr-diethyl

MG 191

Naphthalic anhydride (NA)

Sorghum Sorghum Cereals

Alachlor

Metolachlor

Wheat, rye, triticale, barley Fenoxaprop-ethyl

Maize

Maize

Thiocarbamates

Seed-treatment

Seed-treatment

Halosulfuron-methyl Spray as mixture with herbicide

Spray as mixture with herbicide

Spray as mixture with herbicide

EPTC, butylate, vernolate Seed-treatment

Oxabetrinil (CGA 92194) <^>-c=n-o-ch2^ """] Sorghum

Metolachlor

Seed-treatment

Figure 4.8 Structures of tetcyclasis, 1-aminobenzotriazole and tridiphane.
Sorghum Safeners
Figure 4.9 A generalised summary of safener and synergist action.

In contrast to safeners functioning by stimulating herbicide degradation, herbicide synergists inhibit these enzyme systems, so that plant defence mechanisms are overcome and phytotoxicity rapidly ensues. Aminobenzotriazole and tetcyclasis, for example, are able to bind to the haem portion of the cytochrome P450 to prevent herbicide oxidations. Furthermore, tridiphane can inhibit GSTs to synergise atrazine, may form its own phytotoxic glutathione conjugate, and may also inhibit cytochrome P450-linked monooxygenases (Moreland et al., 1989) (Figure 4.8).

A generalised summary of safener and synergist action is presented in Figure 4.9.

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