Lethal Hydroxyl Radical Production in Paraquat-Treated Plants

Abstract

Bipyridinium herbicides, including paraquat and diquat, are believed to act by generating highly reactive, oxygen-centered free radicals within chloroplasts when treated plants are exposed to sunlight. This hypothesis has not yet been confirmed by direct chemical measurements of specific free radicals. We studied paraquat-treated plants using a new method able to detect and quantify formation of highly reactive and deleterious hydroxyl radicals (HO*), in which dimethyl sulfoxide (DMSO) is used as a molecular probe. DMSO is oxidized by HO* to form the stable, nonradical compound, methane sulfinic acid, which can be easily extracted from plant ffssue and measured spectrophotometrically. Initial experiments revealed formation of extraordinary numbers of hydroxyl radicals in light-exposed, paraquat + DMSOtreated plants, equivalent at least to the cumulative number of HO* radicals per gram of fresh tissue that would be produced by 10,000 rads of gamma irradiation. This appears to be the greatest production of hydroxyl radicals yet observed in a biological system and is quite sufficient to explain the rapid death of top growth in paraquat-treated plants.

Transgenic Tobacco Plants Overexpressing Chloroplastic Ferredoxin-NADP(H) Reductase Display Normal Rates of Photosynthesis and Increased Tolerance to Oxidative Stress

“…FNR antisense plants were not especially sensitive to this compound, presumably because their chloroplasts had impaired MV photoreduction (Hajirezaei et al, 2002), which is a mandatory step for the manifestation of toxicity (Babbs et al, 1989). As reported in the preceding overexpressing FNR in chloroplasts displayed wild-type rates of MV photoreduction.…”

“…Oxidative damage can also be inflicted by subjecting plants to the toxic effects of MV, an herbicide that propagates superoxide anion radicals through a redoxcycling reaction of the reduced form with molecular oxygen (Babbs et al, 1989). FNR antisense plants were not especially sensitive to this compound, presumably because their chloroplasts had impaired MV photoreduction (Hajirezaei et al, 2002), which is a mandatory step for the manifestation of toxicity (Babbs et al, 1989).…”

The Arabidopsis PARAQUAT RESISTANT2 gene encodes an S-nitrosoglutathione reductase that is a key regulator of cell death

“…In plant cells, paraquat mainly targets chloroplasts by accepting electrons from photosystem I and then reacting with oxygen to produce superoxide and hydrogen peroxide, which cause photooxidative stress [17,18]. Consistently, chloroplasttargeted overexpression of superoxide dismutase (SOD) genes confers paraquat resistance in a number of transgenic species [19][20][21].…”

“…Moreover, we have identified and characterized the par2-1 mutant, revealing an important function of GS-NOR1/HOT5/PAR2 in the regulation of ROS-mediated cell death. The par2-1 mutant was identified by its resistance to paraquat, which has been known to induce the generation of ROS and free radicals [15,17,18]. It has been estimated that paraquat treatment on light-grown ryegrass and duckweed produces excessive amount of hydroxyl radicals equivalent to that produced by 10 000 rads of γ irradiation, a dose that is able to induce rapid cell death in plants [17].…”

“…The par2-1 mutant was identified by its resistance to paraquat, which has been known to induce the generation of ROS and free radicals [15,17,18]. It has been estimated that paraquat treatment on light-grown ryegrass and duckweed produces excessive amount of hydroxyl radicals equivalent to that produced by 10 000 rads of γ irradiation, a dose that is able to induce rapid cell death in plants [17]. Thus, the killing effect of paraquat has been mainly attributed to the production of excessive ROS [15,17,18].…”