This work has been digitalized and published in 2013 by Verlag Zeitschrift für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution 4.0 International License. Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Creative Commons Namensnennung 4.0 Lizenz. Mode of Evolved Photooxidant Resistance to Herbicides and Xenobiotics Marcel A. K. Jansen, Christo Malan*, Yoseph Shaaltiel**, and Jonathan Gressel Department of Plant Genetics, The Weizmann Institute of Science, Rehovot, IL-76100, Israel Z. Naturforsch. 45c, 463-469 (1990); received November 9, 1989 Paraquat, Acifluorfen, Atrazine, Photoinhibition, Superoxide Dismutase A few species have evolved resistance to paraquat after repeated selection. As paraquat still inhibited NADP reduction, we hypothesized that resistance might be due to (a) detoxification of the paraquat-generated active oxygen species and (b) that resistant plants would have some cross resistance to other xenobiotic oxidants as well as to photoinhibition, which we subse quently demonstrated. The levels of plastid isozymes of the oxygen detoxification pathway: (CuZn) superoxide dismutase, ascorbate peroxidase and glutathione reductase were genetical ly higher in the resistant than in the sensitive biotype of Conyza bonariensis through the F2 generation. Resistance was suppressed by chelators of copper and/or zinc. Intact chloroplasts from resistant plants had less membrane damage with and without paraquat, than those from sensitive plants. Resistant Conyza plants recover from paraquat inhibition of photosynthesis in 3 -4 h in high light, whereas sensitive plants died. Both resistant and sensitive plants recov ered from paraquat in 3 -4 h in low light intensities. Paraquat-resistant Conyza plants were cross-tolerant to S 0 2, atrazine, acifluorfen and to photoinhibition. Drought-tolerant maize inbreds were cross-tolerant to paraquat, SO, and acifluorfen (compared to sensitive lines) and they also possessed higher levels of (Cu/Zn) superoxide dismutase and glutathione reductase. The tolerance to oxidant stresses in Conyza and maize increases with plant age, suggesting that the shift to resistance is a constitutive, earlier expression of the genes normally expressed later in development. Introduction Weed species have evolved resistance to para quat under field conditions. This may seem sur prising as paraquat is a contact herbicide that im mediately and irreversibly adsorbs to soil colloids. Thus, it lacks residual activity. Single annual ap plications of paraquat should not exert the same selection pressure as single annual applications of atrazine having high residual activity. Indeed, para- quat-resistant populations only evolved where paraquat was applied 3-10 times per year, for 6 or more years, giving it the same effective selection pressure as annual atrazine applications. Paraquat interacts with photosystem I, generat ing superoxide. No cases of paraquat resistance are known to be due to PS I target-site mutations. This may mean that the binding site of paraquat (if it does directly interact with a protein) is even more conserved than the 32 kDa protein that in teracts with atrazine. It may also mean that para- Present addresses: * Research Center for Plant Biotechnology, P/bag X293, Pretoria 0001, South Africa. ** Migal Galilee Technology Center, Kiryat Shmona, IL-10200, Israel. Reprint requests to J. Gressel. Verlag der Zeitschrift für Naturforschung. D-7400 Tübingen 0341-0382/90/0500-0463 $01.30/0 quat interacts with more than PS I; indeed para quat can interact with mitochondrial electron transport, as well as directly interact with mam malian enzymes. PS I in isolated thylakoids of paraquat-resistant plants is totally susceptible to paraquat [1,2]. This has been construed by some as implying that re sistance could not be at the level of the chloroplast. Various groups have found by petiole feeding of labelled paraquat, that the radioactivity in resist ant plants was localized near veins and cell walls. The distribution was more diffuse in susceptible plants [3-5]. This was interpreted as paraquat being sequestered and/or its translocation blocked before reaching chloroplasts, even though photo synthesis was not measured in vivo. Unfortunately, the contention that paraquat was sequestered was never demonstrated by showing that the radiolabel was in paraquat and not in a metabolite. Label from paraquat should eventually be sequestered, but the question is whether this can be a primary (first occurring) reaction in resistant plants. The first time points for sequestration were always hours after application. Thus, any different feature between resistant and sensitive plant appearing be fore this time is semantically more “primary”. We will describe evidence below that paraquat quickly gets to the chloroplast and that isolated in-