Inhibitors of Photosystem II

It has been estimated that perhaps as many as half of available herbicides have a mode of action that involves interaction with a few components in the electron transfer chain of Photosystem II. Remember that transfer of electrons from Photosystem II to Photosystem I is essential for the production of photosynthetic energy. A key step in this electron transfer chain is the reduction of plastoquinone by the D₁ protein in the thylakoid membrane. Herbicides with a mode of action involving this site act as inhibitors of plastoquinone binding. These herbicides bind to the D₁ protein and block the binding of PQ. By inhibiting the binding of PQ, the process of photosynthetic electron transfer is interrupted, and the synthesis of ATP and NADPH in the chloroplast is compromised. This results in an inability to fix CO₂ and produce the nutrients needed for the plant to survive. The block in electron transfer also causes an oxidative stress and the generation of radicals which cause rapid cellular damage.

Much effort has gone into the design of this class of inhibitors. Since the mode of action involves competition for a binding site within a membrane environment, the effectiveness of the herbicide will be greatly affected by small changes in its structure. Thus, small modifications in the molecular structure of a herbicide may cause differential sensitivity in different species of plants. Also, because the D₁ protein in different plant species will have slightly different sequences of amino acid residues, differential effectiveness is possible with the same herbicide molecule.

Plants also have detoxification systems that may greatly affect the response of different crops to herbicides.

For example, corn is relatively insensitive to atrazine because of an efficient detoxification system involving reaction with glutathione, a protective tripeptide, and transport to the cell’s vacuole. This is a velvetleaf plant that was sprayed with Atrazine (Figure: Atrazine). Notice that damage is starting to become apparent around the leaf periphery.