Protoporphyrinogen Destruction by Plant Extracts and Correlation with Tolerance to Protoporphyrinogen Oxidase-Inhibiting Herbicides.

Herbicidal damage by photobleaching diphenylether herbicides is the indirect result of inhibition of an enzyme in chlorophyll biosynthesis. The substrate of the inhibited enzyme, protoporphyrinogen, accumulates and is subsequently converted to protoporphyrin, a potent photoactive compound which causes light-dependent membrane damage. In the present study, we report characteristics of a factor in the soluble fraction of leaves which can decompose protoporphyrinogen to nonporphyrin products. This process may be important in protecting plants from herbicide damage, since it would interfere with accumulation of the phototoxic porphyrin, protoporphyrin. We found that this protoporphyrinogen destruction is associated with the protein fraction of the soluble leaf homogenate, suggesting its enzymatic nature. Protoporphyrinogen destruction is stable to mild heat, but is eliminated by boiling. Protoporphyrinogen destruction is present in the soluble leaf homogenate but is not localized within the stromal fraction of the chloroplast. The reductants dithiothreitol and beta-mercaptoethanol, but not glutathione, inhibit protoporphyrinogen destruction at high concentrations. In contrast, ascorbic acid markedly inhibits destruction even at low concentrations, suggesting a role for cellular ascorbic acid in protecting protoporphyrinogen from destruction, thereby enhancing herbicide action. Protoporphyrinogen destruction was least active in young cucumber leaves, a plant highly susceptible to herbicides. Higher levels of protoporphyrinogen destruction were found in leaves of broadleaf mustard and radish, two plants exhibiting herbicide tolerance. For cucumber, the extent of destruction increased with the age of the plant. These findings suggest a correlation between increased protoporphyrinogen destruction and herbicide tolerance in some plant species.