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Research on Biochemistry of Herbicides: an Historical Overview Donald E Research on Biochemistry of Herbicides: An Historical Overview Donald E. Moreland USDA/ARS and Department of Crop Science, North Carolina State University, Raleigh, NC 27695-7620, U.S.A. Z. Naturforsch. 48c, 121-131 (1993); received October 10, 1992 Herbicide Targets, Photosynthesis, Amino Acid Biosynthesis, Lipid Biosynthesis, Diphenyl Ethers Otto Warburg, the father of cellular bioenergetics, seems to have been the first investigator to report on inhibition of a plant biochemical reaction by a progenitor of a selective herbicide. The year was 1920 and the compound was phenylurethane (ethyl N-phenylcarbamate or EPC). Warburg found that it strongly inhibited photosynthesis in Chlorella. EPC did not de­ velop into a commercial herbicide, but the isopropyl derivatives (propham and chlorpropham) which were introduced in the late 1940s became selective herbicides. The phenylureas (monu- ron and diuron) were introduced in the early 1950s and shortly thereafter, interference with the Hill reaction by both phenylureas and phenylcarbamates was reported. During the latter part of the 1950s, into the 1960s, and even now, additional herbicidal chemistry was and is being announced that interferes with the Hill reaction. Duysens, in 1963, identified the site of action o f diuron, i.e., on the acceptor side of PS II. Corwin Hansch, in 1966 introduced the SAR or QSAR concept in which inhibitory action of Hill inhibitors was related to various chemical and physical parameters. Because of differential responses to partial, thylakoid-associated reactions, the Hill inhibi­ tors were subsequently divided into two groups: pure electron transport inhibitors (phenyl­ ureas, s-triazines, triazinones, and uracils) and inhibitory uncouplers (acylanilides, dinitro- phenols, benzimidazoles, dinitroanilines, and benzonitriles). The inhibitory uncouplers (dino- seb-types), unlike the diuron-types, uncoupled photophosphorylation by interacting with the coupling factor complexes in both chloroplasts and intact mitochondria. Additionally, the bi- pyridyliums were shown to be reduced by PS I, hence, diverted electrons from the native ac­ ceptor. Field observations of triazine resistance were reported in 1970 and resistance was subse­ quently demonstrated at the thylakoid level. Application of the techniques of genetic engineer­ ing and biotechnology resulted in identification of the 32 kDa herbicide-binding protein and determination of its amino acid sequence. Crystallization and X-ray examination of the photo­ synthetic reaction center from Rhodopseudomonas by Michel et al. in the mid-1980s provided new models to account for interactions of herbicides with the D-l protein. During the 1980s, herbicides were identified that interfered with biochemical machinery in chloroplasts that is not involved in electron transport and light harvesting: inhibition of lipid biosynthesis by aryloxyphenoxypropionates and cyclohexanediones, aromatic amino acid bio­ synthesis by glyphosate, branched chain amino acid biosynthesis by sulfonylureas and imida- zolinones, carotenoid biosynthesis by pyridazinones, and porphyrin biosynthesis by diphenyl- ethers and oxadiazoles. The current status of research in most, if not all, of the above areas was reported through oral and poster presentations at this Omiya Symposium. Introduction Inhibitors of many different types have been used to elucidate almost all of the biochemical Whereas the target sites of the majority of the major commercial families of herbicides is now pathways associated with animal and plant metab­ olism. Herbicides and research related to their known, this was not true in 1980. At that time, the interactions of herbicides with PS II were the best biochemical mechanisms of action have helped to known. Developments have progressed dramati­ unravel details of several of the biochemical path­ cally since then. Where, when, and how did bio­ ways in plants. Limitations of space will prevent chemical research with herbicides develop? These detailed documentation of observations and re­ are the questions that I hope to chronologically ports. For some observations, reports were pub­ identify in this contribution. lished within a year or two of each other by differ­ ent groups of investigators who had worked inde­ pendently. Hence, it is difficult to impartially Verlag der Zeitschrift für Naturforschung, D-W-7400 Tübingen identify which laboratory did what and when in 0939-5075/93/0300-0121 $01.30/0 specific terms. Of primary importance, however, Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung This work has been digitalized and published in 2013 by Verlag Zeitschrift in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der für Naturforschung in cooperation with the Max Planck Society for the Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Advancement of Science under a Creative Commons Attribution Creative Commons Namensnennung 4.0 Lizenz. 4.0 International License. 122 D. E. Moreland • Historical Overview of Herbicide Interactions are the contributions that have been made to our tained inhibition of photosynthesis and respiration storehouse of knowledge on the biochemistry of in Chlorella cells by a series of ethyl esters of alkyl plants through site-of-action studies with herbi­ and aryl carbamates [2], The ho concentrations of cides by a large number of researchers. I have pri­ the test compounds for inhibition of respiration marily considered observations made with herbi­ were approximately three times that required to in­ cides until about the mid-1980s and have included hibit photosynthesis. During the 1930s and 1940s, contributions made at the molecular level (primary many investigators subsequently included the ure­ responses) as well as the extrapolation of these ob­ thanes as inhibitors in their studies on photosyn­ servations to the expression of toxicity (lethality) thesis, much like diuron and atrazine have been at the whole plant level (secondary responses). used since the late 1950s. Historically, effects of various chemicals on photosynthesis and respiration have been studied since around 1878 when chloroform was described Herbicides - The Early Years as reversibly inhibiting photosynthesis at concen­ Effects of phenylurethane on plant growth and trations that did not affect respiration [1]. Early development was first described by Friesen in 1929 studies included alkaloids (quinine, strychnine, [3] who reported on the morphological aberrations morphine), diethyl ether, alcohols, cyanide, hy- produced on wheat and oat seedlings. The effects droxylamine, dinitrophenol, various gases (hydro­ noted included a delay of germination, develop­ gen sulfide, carbon monoxide, sulfur dioxide, and ment of cellular enlargement, and interference nitrous oxides), sodium azide, o-phenanthroline, with pigmentation, i.e., the seedlings had a blue- and urethanes. Inhibitors such as cyanide, hy- green color. Ten years later, Lefevre [4] compared droxylamine, and hydrogen sulfide were referred the antimitotic activity of phenylurethane to that to as catalyst poisons and as such were considered of colchicine in wheat seedlings. to form a complex with metal atoms associated In the interval between the two observations, with enzyme prosthetic groups. Others, including IAA was isolated and identified in 1934 [5], but dinitrophenol were considered to probably react had been synthesized in 1886 by German chemists. with specific groups on catalytically active pro­ In the late 1930s and early 1940s, scientists per­ teins [1], Effects of most of the compounds had formed a large number of experiments in both Eu­ previously been explored on animal systems. rope and the U.S. in an attempt to identify the Hence, information was available on the general mechanisms of action of auxins and compounds types of physiological alterations that were in­ that possessed auxin-like activity. The compounds duced. This was especially true for the dinitro- were applied to a wide range of plant species and phenols and the urethanes. large numbers of physiological and morphological Chloroform and ether were originally classified effects were observed. Findings from much of this as narcotics and as such were considered to act by research were either not published or publication blocking active surfaces. In animal studies with was delayed because of the events that surrounded homologous series of organic compounds, the effi­ World War II. Templeman and Sexton [6, 7], who ciency of narcotization was known to increase were associated with ICI at Jealott’s Hill in the with the length of the carbon chain, and thus to U.K., reported on field observations conducted in parallel surface activity. Urethane (ethyl carba­ 1940 and 1941 with phenoxyacetic acids. The com­ mate or the ethyl ester of carbamic acid) and phe- pounds were toxic to dicotyledonous, but not to nylurethane (ethyl ester of N-phenylcarbamic monocotyledonous plants. The formative effects acid, EPC) also were classified as narcotics. Ure­ induced by the phenoxyacetic acids had been re­ thane and phenylurethane had been used in animal ported earlier by Zimmerman and Hitchcock [8]. studies and veterinary medicine as anesthetics, and Eventually, 2,4-D was developed for use in the had been shown to possess antineoplastic proper­ U.S. and MCPA for use in Europe. The discovery ties. of the phenoxyalkanoates revolutionized agricul­ Warburg, whom many consider to be the father tural practices. Success achieved in their use prob­ of cellular
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