Effect of Diphenyl Ether Herbicides on Oxidation of Protoporphyrinogen to Protoporphyrin in Organellar and Plasma Membrane Enriched Fractions of Barley

Abstract

In barley (Hordeum vulgare L.) root cells, activity for oxidizing protoporphyrinogen to protoporphyrin (protoporphyrinogen oxidase), a step in chlorophyll and heme synthesis, was found both in the crude mitochondrial fraction and in a plasma membrane enriched fraction separated by a sucrose gradient technique utilized for preparing plasma membranes. The herbicides to increase greatly the intracellular levels of the photodynamic tetrapyrrole, protoporphyrin, causing light dependent phytotoxic effects (2-4, 6, 14-16, 18, 22-24, 26, 29). DPE herbicides also inhibit Protox, which is the last enzyme of the common branch of the heme and Chl biosynthetic pathways in plants and oxidizes protoporphyrinogen to protoporphyrin (1,5,12,20,21,27,30

Cellular Localization of Protoporphyrinogen-Oxidizing Activities of Etiolated Barley (Hordeum vulgare L.) Leaves (Relationship to Mechanism of Action of Protoporphyrinogen Oxidase-Inhibiting Herbicides)

“…Our data are in general agreement with the model of Jacobs et al (1991), in which it was hypothesized that plastid Protox is inhibited by AFM, whereas a PM-associated Protox-like activity is unaffected. This hypothesis helps to explain how inhibition of an enzyme can cause the product to accumulate in vivo.…”

“…Much of the porphyrin fluorescence appeared to be associated with the PM. Enzymic data of Jacobs et al (1991) supported this conclusion. They found that, although AFM strongly inhibited etioplast Protox in vitro, a Protox-like activity of a PM-enriched fraction was almost unaffected by the herbicide.…”

“…A modified version of the scheme of Jacobs et al (1991) has been drawn to reflect the results of this study (Fig. 3).…”

“…An objective of this paper is to confirm with more purified PM preparations than those used by Jacobs et al (1991) that a PM-associated Protox activity exists. In the previous paper, root PM and crude leaf etioplast preparations were used.…”

Level of protoporphyrinogen oxidase activity tightly correlates with photodynamic and defense responses in oxyfluorfen-treated transgenic rice

“…[4][5][6][7] The acute toxicity of oxyfluorfen is a result of the accumulation of Protogen IX, which is oxidized in the cytoplasm to Proto IX by a nonspecific plasma membrane-bound peroxidase. 8,9) Proto IX in the cytoplasm is not metabolized by the porphyrin biosynthetic pathway because Mg-chelatase and Fe-chelatase, which use Proto IX as a substrate, are located in chloroplasts and mitochondria. Cytoplasmic Proto(gen) IX, a potent photosensitizer, absorbs light that is used in detrimental reactions.…”