Accumulation of Photodynamic Tetrapyrroles Induced by Acifluorfen-Methyl

Abstract

Treatment with acifluorfen-methyl (AFM), methyl 542-chloro-4-Itrifluoromethyll phenoxy)-2-nitrobenzoate, inhibited protochlorophyllide synthesis in dark-held, 5-amino levulinic acid-fed, excised cotyledons of cucumber (Cucumis sativus L.). Protochlorophyllide and protoporphyrin IX levels in AFM-treated cotyledons were inversely related and dependent on AFM concentration; as the herbicide dose increased, protoporphyrin IX levels also increased with a concomitant loss of protochlorophyllide. Significant protoporphyrin IX accumulation was induced by concentrations of AFM from the linear region of the membrane disruption dose response curve. The patterm of precursor accumulation seen in HPLC chromatograms from extracts of AFM-treated tissue indicate that Mg insertion into the tetrapyrrole ring is inhibited, suggesting interference with Mg-chelatase. An inhibitor of 5-amino levulinic acid synthesis, gabaculine (3-amino-2,3-dihydrobenzoic acid), completely blocked the membrane disruption activity of AFM in illuminated cotyledons. Protoporphyrin IX accumulating in AFM-treated tissues may serve as the primary photosensitizer for initiating lipid peroxidation.As a consequence of the rapid induction of lipid peroxidation, illumination ofplant tissues treated with p-nitro DPEI herbicides results in the disruption ofa wide range ofbiological constituents and functions (18,26). This broadly destructive activity by the DPE herbicides has made it difficult to monitor specific interactions between these compounds and light-dependent in vivo physiological processes. These effects lead to one of the most readily visible manifestations of the treatment of whole plants with DPE herbicides: the bleaching of the foliar tissue as a consequence of pigment photooxidation.In an earlier study (15) greening was induced in AFM-treated cucumber cotyledons without triggering a loss in plasmalemma membrane integrity under a regime oflow intensity, intermittent illumination. Yet, even under these conditions, we observed significant reductions in the levels of Chl accumulated in AFMtreated tissue. These results suggested the possibility that, in addition to the pigment bleaching in DPE-treated plant tissue which occurs under continuous illumination, AFM might also directly inhibit pigment biosynthesis in a manner not related to photooxidation.We have since found further evidence that the DPE herbicides directly inhibit Chl synthesis prior to the formation of Pchl(ide).' Abbreviations: DPE, diphenyl ether; AFM, acifluorfen-methyl; ALA, 6-amino levulinic acid; Pchl(ide), a mixture of Pchlide and Pchlide ester; proto IX, protoporphyrin IX; Mg-proto IX, Mg-protoporphyrin IX; Mgproto IX Me, Mg-protoporphyrin IX monomethyl ester; LA, levulinic acid: LSD (0.05), least significant difference at the 5% probability level.

Action Mechanism of DLH-1777, a Novel 4-Pyridone-3-carboxamide Herbicide

“…The results of our present study indicate that DLH-1777 has a mode of action consistent with the recent mode1, [13][14][15][16][17] and that the herbicidal property of light-dependent, bleaching and peroxidizing herbicides comes from their ability to induce the accumulation of Proto, a porphyrin, in treated plants. It is Cotyledons were treated with 10 a of DLH-1777 (B) and oxyfluorfen (C) for 3, 6 and 8 hr in the dark and extracted with acetone/0.1 M NH4OH (9/1, v/v), believed that Proto, under light, induces the formation of singlet oxygen, which strongly oxidizes membrane lipids5,10-12) and causes rapid bleaching by triggering oxidative photodestruction of chloroplast membranes and pigments.25, 26) Not only DPE herbicides such as acifluorfen13-17) and oxyfluorfen17, 27, 28) but non-DPE herbicides observed to act in a similar fashion to DPEs (heterocyclic 5-membered ring compounds such as oxadiazon, 29) chlorphthalim, 17, 27, 28) S-23142, 30) and TNPPethyl31) and the pyridine derivative LS 82-55613) that relatively resemble DLH-1777 in structure) have been reported to cause high levels of Proto accumulation in the treated plants.…”

A Single Precursor Protein for Ferrochelatase-I fromArabidopsis Is Imported in Vitro into Both Chloroplasts and Mitochondria

“…Recently, ferrochelatase activity has been found associated with the plasma membrane, which also contains measurable activity of the previous enzyme of the tetrapyrrole synthesis pathway, protoporphyrinogen oxidase (41). The product of this enzyme, protoporphyrin IX, is extremely photoreactive, as evidenced by the effectiveness of the diphenylether herbicides which cause this compound to accumulate (42,43). In treated plants in the light, reactive oxygen species are formed which then cause lipid peroxidation and rapid cell death (44).…”

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

“…The PM enriched fraction exhibited less than 10% of the Cyt oxidase activity of the mitochondria (Table II), ruling out significant mitochondrial contamination. Contamination of the PM enriched fraction with fragments of plastid membrane was evaluated with the use of carotenoids as a specific marker for plastid membranes (29). The carotenoid content of the plastid fraction from the barley root homogenate (prepared by low speed centrifugation as indicated in "Materials and Methods") was compared with the carotenoid content of the PM enriched fraction.…”