Photosystem inhibitor

Electron-transfer sensitization, 19 109 Electron transport, between photosystem inhibitors, 13 288 Electron-transport layer (ETL)... 

Photosystem II inhibitors, 13 288-294 Photosystem inhibitors, electron transport between, 13 288... 

Amicarbazone is the latest representative in the still economically important group of photosystem inhibitors. It belongs to the chemical class of carbamoyl triazolinones, was found and developed by Bayer AG and will be commercialized in the US in the com market and in sugarcane growing countries by Arysta Life-Sciences. 

Electron Transport Between Photosystem I and Photosystem II Inhibitors. The interaction between PSI and PSII reaction centers (Fig. 1) depends on the thermodynamically favored transfer of electrons from low redox potential carriers to carriers of higher redox potential. This process serves to communicate reducing equivalents between the two photosystem complexes. Photosynthetic and respiratory membranes of both eukaryotes and prokaryotes contain stmctures that serve to oxidize low potential quinols while reducing high potential metaHoproteins (40). In plant thylakoid membranes, this complex is usually referred to as the cytochrome b /f complex, or plastoquinolplastocyanin oxidoreductase, which oxidizes plastoquinol reduced in PSII and reduces plastocyanin oxidized in PSI (25,41). Some diphenyl ethers, eg, 2,4-dinitrophenyl 2 -iodo-3 -methyl-4 -nitro-6 -isopropylphenyl ether [69311-70-2] (DNP-INT), and the quinone analogues,... 

Photosystem II Inhibitors. The PSII complex usually is assumed to be that stmctural entity capable of light absorption, water oxidation, plastoquiaone reduction, and generation of transmembrane charge asymmetry and the chemical potential of hydrogen ions (41). The typical PSII complex... 

Photosystem II inhibitors, 13 288-294 plant growth regulator synthesis and function inhibitors, 13 304—307 Herbicide analysis methods, 13 312—313 Herbicide atomizer, 23 197 Herbicide binding, polypeptide... 

Photosynthetic electron flow, 13 287 Photosystem I (PSI), 13 286. See also PSI transport processes Photosystem I inhibitors, 13 286-288 Photosystem II (PSII), 13 286. See also PSII entries... 

Achnrne L, Pereda-Miranda R, Iglesias-Prieto R, Moreno-Sanchez R, Lotina-Hennsen B (1999) Tricolorin A, a Potent Natural Uncoupler and Inhibitor of Photosystem II Acceptor Side of Spinach Chloroplasts. Physiol Plant 106 246... 

Pfister, K. and C.J. Amtzen (1979). The mode of action of photosystem II-specific inhibitors in herbicide-resistant weed biotypes. Z. Naturforsch. Sect. C Biosci., 34 996-1009. 

Trebst, A. (1991). The molecular basis of resistance of photosystem II inhibitors. In J.C. Caseley, G.W. Cussans and R.K. Atkin, eds., Herbicide Resistance in Weeds and Crops. Boston Butterworth-Heinemann, pp. 145-164. 

Velthuys, B.R. (1981). Electron-dependent competition between plastoquinone and inhibitors for binding to photosystem II. Fed. European Biochem. Societies Lett., 126 277-281. 

Oettmeier, W., K. Masson, C. Fedtke, J. Konze, and R.R. Schmidt (1992). Effect of different photosystem II inhibitors on chloroplasts isolated from species either susceptible or resistant toward s-triazine herbicides. Pestic. Biochem. Physiol., 18 357-367. 

Mitsutake, K.-I., Iwasmura, H., Shimizu, R., Fujita, T. (1986) Quantitative structure-activity relationships of photosystem II inhibitors in chloroplasts and its link to herbicidal action. J. Agric. Food Chem. 34, 725-732. 

Arrhenius A, Gronvall F, Scholze M, Backhaus T, Blanck H. 2004. Predictability of the mixture toxicity of 12 similarly acting congeneric inhibitors of photosystem II in marine periphyton and epipsammon communities. Aquat Toxicol 86 351-367. 

In-vitro approach Data are available in abundance concerning metal effects on isolated chloroplasts (for a review, see Clijsters and Van Assche, 1985). All the metals studied were found to be potential inhibitors of photosystem 2 (PS 2) photosystem 1 (PS 1) was reported to be less sensitive. From the in-vitro experiments, at least two potential metal-sensitive sites can be derived in the photosynthetic electron transport chain the water-splitting enzyme at the oxidising side of PS 2, and the NADPH-oxido-reductase (an enzyme with functional SH-groups) at the reducing side of PS 1 (Clijsters and Van Assche, 1985). Moreover, in vitro, non cyclic photophosphorylation was very sensitive to lead (Hampp et al., 1973 b) and mercury (Honeycutt and Korgmann, 1972). Both cyclic and non-cyclic photophosphorylation were proven to be inhibited by excess of copper (Uribe and Stark, 1982) and cadmium (Lucero et al, 1976). 

An elegant example of this is the monitoring of herbicide residues via the photosynthetic electron transport (PET) pathway by utilising cyanobacteria or thylakoid membranes (5). For many herbicides the mode of action is as inhibitors of PET, often acting between the 2 photosystems as indicated in figure 3, and the result is a decrease in the photocurrent. 

Electron Transport Inhibitors. Electron transport is inhibited when one or more of the intermediate electron transport carriers are removed or inactivated. The site of action of most herbicidal electron transport inhibitors is considered to be associated closely with photosystem II. Consequently, reactions coupled to photosystem II are inhibited, such as basal electron... 

Inhibitory Uncouplers. Inhibitory uncouplers inhibit the reactions affected by both electron transport inhibitors and uncouplers. Hence, they inhibit basal, methylamine-uncoupled, and coupled electron transport with ferricyanide as electron acceptor and water as the electron donor, much like electron transport inhibitors. Coupled noncyclic photophosphorylation is inhibited and the phosphorylation reaction is slightly more sensitive than the reduction of ferricyanide. Cyclic photophosphorylation is also inhibited. NADP reduction, when photosystem II is circumvented with ascorbate + DPIP, is not inhibited however, the associated phosphorylation is inhibited. Inhibitory uncouplers act at both sites 1 and 2 (Figure 2). 

Figure 4. Interaction of structural elements of herbicidal inhibitors of photosystem 11 with a postulated receptor...

Figure 6. Structural elements of some heterocyclic inhibitors of photosystem II [adapted from Trebst and Harth (16)], Numbers...

As the mode of action has been treated in detail in Dr. Moreland s paper, I shall only summarize that the exact site of action of the inhibitor molecule seems to be at the watersplitting site of the photosystem. Inhibition of energy transfer in chloroplasts is, apparently, essential for the plant killing action. Chlorophyll is thought to be the principal... 

Photosystem II. Spinach and pea PSII particles coated on different Ti02 based electrodes were used for photocurrent measurements in the presence of PSII electron acceptor DMBQ. In all experiments, addition of DMBQ resulted in an increase in photocurrent which remained constant for long periods. In control experiments with no deposition of PSII on the electrodes, there was no change in the photocurrent pattern on addition of DMBQ. Addition of the PSII oxygen evolution inhibitor DCMU caused an immediate fall in photocurrent, suggesting that the electron transport to the Ti02 electrode is linked to water photolysis. 

Electron Flow through Photosystems I and II Predict how an inhibitor of electron passage through pheophytin would affect electron flow through (a) photosystem II and (b) photosystem I. Explain your reasoning. 

Fig. 4 Dynamics of in vivo chlorophyll fluorescence (Fo) and photosynthetic efficiency (Fv/Fm) of Phaeocystis globosa during viral infection as assessed by fluorometry. Open symbols represent uninfected cultures, while the filled symbols represent virally infected P. globosa. Maximum fluorescence (Fm) was obtained after addition of the photosystem II inhibitor DCMU (20 pM final concentration). Fv equals Fm-Fo. Data are expressed in relative units (r.u.)...

Isoproturon (Urea) (CH3)2CH y NHCON(CH3)2 Photosynthetic electron transport inhibitor at the photosystem II receptor site... 

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