Herbicide safeners: Tools for improving the efficacy and selectivity of herbicides

Safeners (also known as antidotes) are chemical or biological agents that reduce the toxicity of herbicides to crop plants by a physiological or molecular mechanism. Commercialized safeners are mainly chemical compounds that enhance the tolerance of selected grass crops such as maize (Zea mays L.), grain sorghum [Sorghum bicolor(L.) Moench], rice (Oryza sativa L.), and wheat (Triticum aestivum L.) to chloroacetanilide, thiocarbamate, sulfonylurea, imidazolinone, and aryloxyphenoxypropionate herbicides. In practice, safeners are applied either to the crop prior to planting (seed safeners) or to the soil together with the herbicide, formulated as a prepackaged mixture. Safeners act as ''bioregulators'' controlling the amount of a given herbicide that reaches its target site in an active form. A safener-induced enhancement of the metabolic detoxification of herbicides in protected plants is the most apparent mechanism for the action of all commercialized safeners. Herbicide-detoxifying enzymes such as glutathione transferases (GST), cytochrome P450 monooxygenases (CytP450), esterases, and UDP-glucosyltransferases are induced by herbicide safeners. At the molecular level, safeners appear to act by activating or amplifying genes coding for these enzymes (e.g., GST).