Growth inhibition of Lemna gibba plantlets by the bleaching herbicide, SC‐0051 (2‐(2‐chloro‐4‐methanesulfonylbenzoyl)‐1,3‐cyclohexanedione)) was alleviated by the addition of homogentisic acid to the growth medium. Homogentisic acid is a key intermediate in the biosynthesis of tyrosine‐derived plant quinones as well as in tyrosine metabolism. The herbicide prevented the incorporation of radioactivity from [14C]tyrosine into lipophilic plant metabolites and, in rat liver extracts, the herbicide inhibited the conversion of tyrosine to homogentisic acid. The enzyme p‐hydroxyphenylpyruvate dioxygenase (EC 1.13.11.27) from both Zea mays seedlings and liver tissues, was found to be subject to strong inhibition by SC‐0051. Inhibition of plant quinone biosynthesis is a new mode of herbicidal action. One of the consequences of quinone depletion in plants in vivo is apparently an indirect inhibition of phytoene desaturation. The enzyme phytoene desaturase itself, however, is not afflicted by the herbicide.

The purpose of modern industrial herbicides is to control weeds. The species of weeds that plague crops today are a consequence of the historical past, being related to the history of the evolution of crops and farming practices. Chemical weed control began over a century ago with inorganic compounds and transitioned to the age of organic herbicides. Targeted herbicide research has created a steady stream of successful products. However, safeners have proven to be more difficult to find. Once found, the mode of action of the safener must be determined, partly to help in the discovery of further compounds within the same class. However, mounting regulatory and economic pressure has changed the industry completely, making it harder to find a successful herbicide. Herbicide resistance has also become a major problem, increasing the difficulty of controlling weeds. As a result, the development of new molecules has become a rare event today.

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The bar and pat genes, isolated from different Streptomyces species, both encode a phosphinothricin acetyltransferase (PAT) and are widely applied in plant genetic engineering. The genes were expressed in Escherichia coli and the corresponding proteins were purified and used for functional and structural comparison. Both proteins are homodimers regardless of whether they are expressed in microorganisms or in plants. They have comparable molecular weights and show immuno-cross-reactivity to their respective polyclonal antisera. The enzymes have a similar substrate affinity towards L-phosphinothricin and do not acetylate any of the other L-amino acids tested. In model digestion experiments using simulated human gastric fluids, their enzymatic activity is decreased within seconds, accompanied by a rapid and complete breakdown of both proteins. These data demonstrate the structural and functional equivalence of the PAT proteins, which is also reflected in the comparable performance of transgenic plants carrying the bar or pat gene.

Growth inhibition of plants and bacteria by the experimental herbicide Hoe 704 (2‐methylphosphinoyl‐2‐hydroxyacetic acid) was alleviated by the addition of the branched‐chain amino acids to growth media. Hoe 704 caused a massive accumulation of acetoin and acetolactate, indicating its direct interference with the branched‐chain amino acid biosynthetic pathway. The second enzyme of this pathway, acetolactate reductoisomerase (EC 1.1.1.86), was found to be subject to strong inhibition by Hoe 704. The inhibition was time‐dependent and competitive with the enzyme's substrate, acetolactate. This report establishes acetolactate reductoisomerase as a new target for a herbicidal compound.