Oxidative Stress in Photodynamic Herbicidal Action of 5-Aminolevulinic Acid

Summary

Tetrapyrrole biosynthesis can be manipulated to induce the accumulation of photodynamic porphyrins for herbicidal action. Chemicals used as photodynamic herbicides are, 5-aminolevulinic acid (ALA), the porphyrin precursor; diphenyl ether and allied compounds, protoporphyrinogen oxidase inhibitors; and modulators of the heme and chlorophyll biosynthetic pathways such as 2, 2′-bipyridyl and 1, 10 phenanthroline. Protox inhibitors cause maximum accumulation of protoporphyrin IX whereas ALA-based photodynamic herbicides induce overaccumulation of Mg-tetrapyrroles. Cucumber plants were sprayed with 20 mM solution of ALA, the precursor of tetrapyrroles, and then incubated in darkness for 14 h. Upon transfer to light (2000; μmol m⁻² s⁻¹), the plants died after 6 h of exposure due to photodynamic damage and their Photosystem II (PS II) and Photosystem I (PS I) photochemical reactions were impaired. Thylakoid membranes prepared in darkness from control and 2 mM ALA-treated plants were illuminated (250 μmoles m⁻² s⁻¹) in the presence of scavengers of active oxygen species. The singlet oxygen scavengers histidine and sodium azide protected the thylakoid membrane linked function of PS II from photodynamic damage. However, the hydroxyl radical scavenger formate and the Superoxide radical scavengers Superoxide dismutase and 1, 2-dihydroxybenzene-3, 5-disulfonic acid failed to protect the PS II reaction. Non-phototransformable protochlorophyllide was the most abundant pigment in the thylakoid membranes isolated from ALA-treated plants and acted as a type II photosensitizer. Superoxides produced by the control and treated thylakoid membranes in light were abolished by diuron suggesting that type I photosensitization reaction due to protochlorophyllide is nearly absent in ALA-treated plants. However, Superoxide produced by the photosynthetic electron transport chain need to be dissipated by detoxifying enzymes, Superoxide dismutase, ascorbate peroxidase and glutathione reductase. Due to photodynamic reactions Superoxide dismutase activity was not affected whereas the ascorbate peroxidase and glutathione reductase activities were impaired suggesting that besides singlet oxygen which is the primary and immediate cause of photodynamic damage, impairment of two enzymes responsible for detoxification of Superoxide generated by univalent reduction of oxygen by the photosynthetic electron transport chain would substantially contribute to the death of the plant. Because of its environmental safety, ALA and allied compounds have the potential of becoming important commercial herbicides.