Photosystem I inhibitors

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... 

Many commercial herbicides kill weeds by interfering with the action of photosystem II or photosystem I. Inhibitors of photosystem II block electron flow, whereas inhibitors of photosystem I divert electrons from the terminal part of this photosystem. Photosystem II inhibitors include urea derivatives such as diuron and triazine derivatives such as atrazine. These chemicals bind to the Qg site of the D1 subunit of photosystem II and block the formation of plastoquinol (QH2). 

There are no commercial herbicide resistant crops that function by increased expression of the protein target, although some level of plant resistance has been reported for glyphosate, glufosinate, some DPEs and inhibitors of hydroxy-phenylpyruvate dioxygenase. Similarly, cellular sequestration of the herbicide from the target has been reported with some DPEs, auxins and photosystem I inhibitors, but none have been developed commercially [3]. 

Some genera seem more prone than others to rapidly evolve resistance, as they have evolved resistance to more than one herbicide in many areas. These include Lolium spp, Conyza = Erigeron spp among others. There is even one case of sequentially appearing resistance to herbicides with different sites of action. Paraquat (a photosystem I inhibitor) was used to eliminate atrazine-resistant Conyza in vineyards, but resistance to the paraquat used to control the triazine-resistant weed then evolved [32]. 

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,... 

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. 

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. 

For cyclic electron flow, an electron from the reduced form of ferredoxin moves back to the electron transfer chain between Photosystems I and II via the Cyt bCyclic electron flow does not involve Photosystem II, so it can be caused by far-red light absorbed only by Photosystem I — a fact that is often exploited in experimental studies. In particular, when far-red light absorbed by Photosystem I is used, cyclic electron flow can occur but noncyclic does not, so no NADPH is formed and no O2 is evolved (cyclic electron flow can lead to the formation of ATP, as is indicated in Chapter 6, Section 6.3D). When light absorbed by Photosystem II is added to cells exposed to far-red illumination, both CO2 fixation and O2 evolution can proceed, and photosynthetic enhancement is achieved. Treatment of chloroplasts or plant cells with the 02-evolution inhibitor DCMU [3-(3,4-dichlorophenyl)-l, 1-dimethyl urea], which displaces QB from its binding site for electron transfer, also leads to only cyclic electron flow DCMU therefore has many applications in the laboratory and is also an effective herbicide because it markedly inhibits photosynthesis. Cyclic electron flow may be more common in stromal lamellae because they have predominantly Photosystem I activity. 

Paraquat (1,1 -dimethyl-4-4 -bipyridinium) is an inhibitor of photosystem 1. Paraquat, a dication, can accept electrons from photosystem I to become a radical. This radical reacts with O2 to produce reactive oxygen species such as superoxide and hydroxyl radical (OH ). Such reactive oxygen species react with double bonds in membrane lipids, damaging the membrane (Section 18.3.6). 

A number of other herbicides interfere with photosynthesis in specific ways. Amitrole inhibits biosynthesis of chlorophyll and carotenoids. The affected plants present a bleached appearance before they die because of the loss of their characteristic pigments. Another herbicide, atrazine, inhibits the oxidation of water to hydrogen ion and oxygen. Still other herbicides interfere with electron transfer in the two photosystems. In photosystem II, diuron inhibits electron transfer to plastoquinone, whereas bigyridylium herbicides accept electrons by competing with the electron acceptors in photosystem I. The inhibitors active in photosystem I include diquat and paraquat. The latter substance attained some notoriety when it was used to interfere with an... 

Neither photosystem I nor II were implicated in desaturation, as light is not required. Supporting this concept are the observations that neither DCMU nor atrazine (which prevent reduction of PQ at the Qb site of PS II, fig. 4) inhibited desaturation (fig. 2). Because PQ is involved in "reverse" electron transport, the effects of inhibitors of its function were determined. DBMBIB, UHDBT, BPA, stigmatellin (STG), and DNP-INT interfere with oxidation of reduced PQ by blocking the Qz site (fig. 4), and in fact the first three were able to Inhibit desaturation (fig. 3 and table 5). Both DNP-INT and STG may bind at an overlapping but more... 

The other major class of herbicides involved with photosynthesis are compounds that act as divertors of electron flow at photosystem I (PSI). After the initial discovery of diquat and paraquat in the late 1950s, other related compounds have had limited use. One of these, morfamquat, has now been withdrawn. Thirty years later, there are no new chemicals or herbicide families to rival paraquat and diquat at this particular target site. We have, however, an increased understanding of the nature of the damaging radical species that are generated as a result of the action of these compounds. With these herbicides, as also with the electron transport inhibitors, toxic oxygen species have a major role. 

Chlamydomonas cells were labeled with [2 S]-sulfate in the presence or absence of protein-synthesis inhibitors as described (Nechushtai, Nelson 1981). Photosystem I reaction center was then isolated and applied on SDS-polyacrylamide gels,that were stained with coomassie blue (A), or exposed to X-ray film (B). The complex was isolated from labeled untreated cells (lane 1) and from cells labeled in the presence of 2mHchloramphenicol (lane 2), 10()Lig/ml cycloheximide (lane 3)f and 100yug/ml cycloheximide for 1 hour incubation prior to the labeling (lane 4). 

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... 

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... 

Evidence in support of mechanism 1 is (i) the displacement of ubiquinone by the inhibitor orthophenanthroline in the analogous photosystem found in purple photosynthetic bacteria (56) and (ii) the similarity in size and shape between the flat polar component of the PS II herbicides and the quinone head of plastoquinone. Mechanism 1 can be studied directly through competitive displacement reactions between PS II herbicides and PQ. 

Bromoxynil is a special case because the parent compound bromoxynil-octanoate, although being a baseline toxicant, is highly toxic due to its high hydrophobicity (Fig. 10). The hydrolysis product bromoxynil is the active ingredient and has a specific mode of action Bromoxynil is a potent inhibitor of Photosystem II and is also an uncoupler of photophosphorylation (i.e. destroys the electrochemical proton gradient formed in the electron transport... 

Photoaffinity labels are an efficient tool for identification of inhibitor binding proteins in the photosynthetic electron transport chain. [ H]Azido-dinoseb, an azido-deri-vative of the phenolic herbicide dinoseb, was synthesized almost a decade ago and was shown to bind primarily to a 41 kDa protein (1,2). Contrary, labeling with azido-deri-vates of diuron-type herbicides revealed that these herbicides bind to a 32 kDa protein, which has now been recognized as the D-1 protein of the photosystem II reaction center core complex (see references in (3)). Tyrosine residues in positions 237 and 254 of the D-1 sequence were demonstrated to be the primary target of [ CJazido-monuron (3). The phenolic herbicide [ I]azido-ioxynil also labels predominantly the D-1 protein in position of Val249 and only in trace amounts a 41 kDa protein (4). 

Binding of [ H]2-acetoxymethyl-1,4-naphthoquinone is unspecific, i. e. there exists a linear correlation between free and bound inhibitor (Fig. 1). 50 % inhibition of uncoupled photosynthetic NADP-reduction by 2-acetoxymethyl-1,4-naphthoquinone was achieved at 22.5 uM (pI Q-value 4.65) and 50 % inhibition of uncoupled DCPIP-re-duction at photosystem II at 0.18 mM (pI Q-value 3.7). 

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