Herbicides are small molecules that inhibit specific molecular target sites
within plant biochemical pathways and/or physiological processes. Inhibition
of these sites often has catastrophic consequences that are lethal to
plants. The affinity of these compounds for their respective target sites
makes them useful tools to study and dissect the intricacies of plant
biochemical and physiological processes. For instance, elucidation of the
photosynthetic electron transport chain was achieved in part by the use of
herbicides, such as terbutryn and paraquat, which act on photosystem II and
I, respectively, as physiological probes. Work stemming from the discovery
of the binding site of PS II–inhibiting herbicides was ultimately awarded
the Nobel Prize in 1988. Although not as prestigious as the seminal work on
photosynthesis, our knowledge of many other plant processes expanded
significantly through the ingenious use of inhibitors as molecular probes.
Examples highlight the critical role played by herbicides in expanding our
understanding of the fundamental aspects of the synthesis of porphyrins and
the nonmevalonate pathway, the evolution of acetyl-coenzyme A carboxylase,
cell wall physiology, the functions of microtubules and the cell cycle, the
role of auxin and cyanide, the importance of subcellular protein targeting,
and the development of selectable markers.