Research on Biochemistry of Herbicides: An Historical Overview

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-phenylcarbam ate or EPC). Warburg found that it strongly inhibited photosynthesis in Chlorella. EPC did not develop 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 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 of 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 inhibitors were subsequently divided into two groups: pure electron transport inhibitors (phenylureas, 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 bi-pyridyliums were shown to be reduced by PS I, hence, diverted electrons from the native acceptor. 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 engineering and biotechnology resulted in identification of the 32 kD a herbicide-binding protein and determination of its amino acid sequence. Crystallization and X-ray examination of the photosynthetic reaction center from Rhodopseudomonas by Michel et al. in the mid-1980s provided new models to account for interactions of herbicides with the D -1 protein. During the 1980s, herbicides were identified that interfered with biochemical machinery in chloroplasts that is not involved in electron transport and light harvesting: inhib