Inhibition photosynthetic electron transport

Bugg al. ( ) obtained evidence that indicated the site of photosynthetic electron transport inhibition by nitrofluorfen [2-chloro-l-(4-nitrophenoxy)-4-( trif luoromethyDbenzene], was associated with the plastoquinone-Cyt f region between PS I and PS II. This is in agreement with previous research conducted on the mechanism of action of DPE s (13, 23-27). However, in view of the above data from cucumber, these results are probably not indicative of the primary herbicidal site of action. 

Reaction of 232 with 4-substituted l,3-oxazol-5(4/7)-one 247 led to diacylhydrazines 248 or to imidazole derivatives 249 depending on the reaction temperature (Scheme 24). l,2,4-Triazole-3-thione 250 was obtained by a two-step sequence from 232 with phenyl isothiocyanate and subsequent base-catalyzed cyclization of thiosemicarbazide 251. The effects of hydrazones 241-246 on inhibition of photosynthetic electron transport in spinach chloroplasts and chlorophyll content in the antialgal suspensions of Chlorella vulgaris were investigated <2005CEC622>. 

Triazines inhibit photosynthesis in all organisms with oxygen-evolving photosystems. They block photosynthetic electron transport by displacing plastoquinone from a specific-binding site on the D1 protein subunit of photosystem II (PS II). This mode of action is shared with several structurally different groups of other herbicides. The elucidation of the mechanism of the inhibitory action is followed in this review. 

Tischer, W. and H. Strotmann. (1977). Relation between inhibitor binding by chloroplasts and inhibition of photosynthetic electron transport. Biochim. Biophys. Acta., 460 113-125. 

Van Rensen, J.J.S., D. Wong, and Govindjee (1978). Characterization of the inhibition of photosynthetic electron transport in pea chlo-roplasts by the herbicide 4,6-dinitro-o-cresol by comparative studies with 3-(3,4-dichlorophenyl)-l,l-dimethylurea. Z. Naturforsch., 33C 413-120. 

Triazine (e.g., atrazine, simazine) and substituted urea (e.g., diuron, monuron) herbicides bind to the plastoquinone (PQ)-binding site on the D1 protein in the PS II reaction center of the photosynthetic electron transport chain. This blocks the transfer of electrons from the electron donor, QA, to the mobile electron carrier, QB. The resultant inhibition of electron transport has two major consequences (i) a shortage of reduced nicotinamide adenine dinucleotide phosphate (NADP+), which is required for C02 fixation and (ii) the formation of oxygen radicals (H202, OH, etc.), which cause photooxidation of important molecules in the chloroplast (e.g., chlorophylls, unsaturated lipids, etc.). The latter is the major herbicidal consequence of the inhibition of photosynthetic electron transport. 

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). 

Although hydrogenase linked H2 production does not require ATP utilization, normal aerobic fixation of atmospheric CO2 does. As will be discussed below, when CO2 fixation does not occur (as is the case under anaerobic, sulfur-deprived condi tions), the accumulation of ATP molecules in the stroma inhibits ATPase function. This results in the non dissipation of the proton gradient and causes the build-up of the proton motive force. It has been shown that, under these conditions, photosynthetic electron transport is down regulated917 and consequently reductant is not available for efficiently producing H2.140... 

Plastoquinone is one of the most important components of the photosynthetic electron transport chain. It shuttles both electrons and protons across the photosynthetic membrane system of the thylakoid. In photosynthetic electron flow, plastoquinone is reduced at the acceptor side of photosystem II and reoxidized by the cytochrome bg/f-complex. Herbicides that interfere with photosynthesis have been shown to specifically and effectively block plastoquinone reduction. However, the mechanisms of action of these herbicides, i. e., how inhibition of plastoquinone reduction is brought about, has not been established. Recent developments haVe brought a substantial increase to our knowledge in this field and one objective of this article will be to summarize the recent progress. 

Tischer and Strotmann ( 7), the binding constant corresponds to the inhibition constant, i. e. the I,. value (the concentration necessary for 50% inhibition of photosynthetic electron transport), provided the I. value is extrapolated to zero chlorophyll concentration. The value of 527 molecules of chlorophyll per molecule of bound inhibitor indicates that roughly one molecule of herbicide binds per electron transport chain, because about 400-600 molecules of chlorophyll are considered to be associated with each electron transport chain. 

Thus, the concentration necessary for 50% displacement roughly corresponds to the PIjq value. It is possible, therefore, to assay the pl value of a new compound just by examination of its displacement behaviour. It is no longer necessary to determine the pi value by testing the inhibition of a light-driven photoreduction. Another very potent inhibitor of photosynthetic electron transport, DBMIB (2,5-dibromo-3-methyl-6-isopropyl-l,4-benzoquinone) (13), almost completely fails to displace metribuzin from the membrane (Figure 3). This is due to the fact that DBMIB has a completely different site of action as compared to the photosystem II herbicides, i. e. it inhibits plastohydroquinone oxidation by acting at the cytochrome b /f-complex (13). 6... 

The acetanilides (propachlor, butachlor, alachlor and metolachlor) are used as preemergence herbicides against annual broad-leaved weeds and grasses. The acetanilides owe their herbicidal activity to the inhibition of photosynthetic electron transport and energy production. See Table 5.10. 

Its mode of action is similar to other photosynthesis inhibitors in that its activity is dependent on light. It inhibits photosynthetic carbon dioxide fixation and photosynthetic electron transport. 

The mode of action of the substituted ureas is relatively well known. It results in an inhibition of photosynthesis by blocking photosynthetic electron transport and photophosphorylation. 

Triazines act by interfering with photosynthesis and it seems clear that, like the substituted urea herbicides, the primary site of action is inhibition of the Hill reaction of photosynthetic electron transport. 

Bromoxinil or 3,5-dibromo-4-hydroxybenzonitrile Benzonitrile Inhibits photosynthetic electron transport, selective for certain annual broad leave weeds Cereals, maize, sorghum, turf 11-2... 

The natural product cyanobacterin has been found to inhibit photosynthetic electron transport in other organisms. A series of analogs of cyanobacterin were prepared as potential herbicides. Several of the analogs also inhibit the growth of the test photosynthetic organisms. The synthesis and structure-activity relationships of these analogs are discussed. 

A variety of herbicides kill plants by inhibiting photosynthetic electron transport. QUESTION 13.9... 

Almost all of the carbamate herbicides inhibit photosynthesis, as has been shown by the investigations of Moreland and Hill (1959). Asulam and terbutol do not inhibit photosynthetic electron transport in vitro, while the other carbamates do only in high concentrations not occurring in vivo. The conclusion of Corbett (1974), that the inhibition of photosynthesis is only a side-effect of these compounds, therefore seems justified. 

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