Research on Biochemistry of Herbicides: An Historical Overview

Otto Warburg, the father o f cellular bioenergetics, seems to have been the first investigator to report on inhibition o f a plant biochemical reaction by a progenitor o f 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 (monuron 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 o f 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 o f action o f diuron, i.e., on the acceptor side o f PS II. Corwin Hansch, in 1966 introduced the SAR or QSAR concept in which inhibitory action o f Hill inhibitors was related to various chemical and physical parameters. Because o f 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, dinitrophenols, benzimidazoles, dinitroanilines, and benzonitriles). The inhibitory uncouplers (dinoseb-types), unlike the diuron-types, uncoupled photophosphorylation by interacting with the coupling factor complexes in both chloroplasts and intact mitochondria. Additionally, the bipyridyliums were shown to be reduced by PS I, hence, diverted electrons from the native ac­ ceptor. Field observations o f triazine resistance were reported in 1970 and resistance was subse­ quently demonstrated at the thylakoid level. Application o f the techniques o f genetic engineer­ ing and biotechnology resulted in identification o f the 32 kDa herbicide-binding protein and determination o f its amino acid sequence. Crystallization and X-ray examination o f the photo­ synthetic reaction center from Rhodopseudomonas by Michel et al. in the mid-1980s provided new models to account for interactions o f 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 o f lipid biosynthesis by aryloxyphenoxypropionates and cyclohexanediones, aromatic amino acid bio­ synthesis by glyphosate, branched chain amino acid biosynthesis by sulfonylureas and imidazolinones, carotenoid biosynthesis by pyridazinones, and porphyrin biosynthesis by diphenylethers and oxadiazoles. The current status o f research in most, if not all, o f the above areas was reported through oral and poster presentations at this Omiya Symposium.