Inhibitors of Protoporphyrinogen Oxidase
Protoporphyrinogen oxidase (PPO) is an enzyme in the chloroplast of the plant cell that oxidizes protoporphyrinogen to produce protoporphyrin IX. This product is important because it is a precursor molecule for both chlorophyll (needed for photosynthesis) and heme (needed for electron transfer chains) (Figure: Protoporphyrin lX).
However, inhibitors of the oxidase enzyme do more than merely block the production of chlorophyll and heme. When the enzyme is inhibited, the protoporphyrinogen substrate accumulates and is slowly oxidized by the high concentrations of O2 being produced in the chloroplast, producing protoporphyrin IX. This spontaneous production of the product may seem to bypass the inhibitor and let the cell function normally, but it has dire consequences.
Protoporphyrin is a very effective photosensitizing molecule. Normally, the concentrations of protoporphyrin in the cell are kept very low and it is channeled from its production site in the chloroplast to other locations in the cell where it is needed for heme biosynthesis. In the presence of the herbicide inhibitors, the concentrations of protoporphyrin increase and begin to accumulate throughout the cell.
As with most of the herbicides discussed in this course, if the plant is maintained in the dark or in dim light, the effects of the herbicide are not observed. However, when exposed to light the protoporphyrin released in the cell is excited to the triplet state with a high efficiency and interacts with molecular O2 to produce singlet oxygen. Singlet oxygen is toxic to cells because it is much more destructive than molecular oxygen in the normal triplet state. Favorite targets of singlet oxygen include the double bonds of fatty acids and amino acids.
Membranes, sites with high concentrations of unsaturated fatty acids, are particularly vulnerable to peroxidation (molecular damage from free radicals). The plasma membrane of the plant cell is considered to be the vulnerable component most impacted by the photodynamic damage from herbicides that inhibit protoporphyrinogen oxidase. Here is a velvetleaf plant that has been sprayed with Blazer, a PPO inhibitor (Figure: Blazer Damage). Notice the necrotic (localized dead tissue) lesions that have formed in response to the herbicide. Inhibition of PPO has caused release of protoporphyrin throughout the plant cell. In the presence of light singlet oxygen is produced, causing rapid cell death.