Herbicides Interacting with Photosystem

Although it has been known for over 30 years that electron transport inhibitor herbicides interact with PSII, our concept of the structural organization of this photosystem, and the exact site of interaction, has advanced greatly during the last few years. 

Cyclic photophosphorylation is also a highly energetic reaction. The bipyridyliums, paraquat and diquat (Figure 2.2), divert the electron flow of cyclic photophosphorylation (photosystem I). The capture of an electron from the chlorophyll reduces the herbicide and the reduced herbicide reacts with oxygen to form superoxide. Superoxide produces hydrogen peroxide within the chloroplast and these two compounds interact to form hydroxyl radicals in the presence of an iron catalyst. Hydroxyl radicals are very damaging and lead to the destruction of the cellular components leading to rapid plant death. 

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

Herbicides that inhibit photosynthetic electron flow prevent reduction of plastoquinone by the photosystem II acceptor complex. The properties of the photosystem II herbicide receptor proteins have been investigated by binding and displacement studies with radiolabeled herbicides. The herbicide receptor proteins have been identified with herbicide-derived photoaffinity labels. Herbicides, similar in their mode of action to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) bind to a 34 kDa protein, whereas phenolic herbicides bind to the 43-51 kDa photosystem II reaction center proteins. At these receptor proteins, plastoquinone/herbicide interactions and plastoquinone binding sites have been studied, the latter by means of a plastoquinone-deriv-ed photoaffinity label. For the 34 kDa herbicide binding protein, whose amino acid sequence is known, herbicide and plastoquinone binding are discussed at the molecular level. 

The phenolic photoaffinity label azidodinoseb (Figure 4) binds less specifically than either azidoatrazine or azidotriazinone (14). In addition to other proteins, it labels predominantly the photosystem II reaction center proteins (spinach 43 and 47 kDa Chlamydomo-nas 47 and 51 kDa) (17). Because of the unspecific binding of azidodinoseb, this can best be seen in photosystem II preparations (17). Thus, the phenolic herbicides bind predominantly to the photosystem II reaction center, which might explain many of the differences observed between "DCMU-type" and phenolic herbicides (9). The photosystem II reaction center proteins and the 34 kDa herbicide binding protein must be located closely to and interact with each other in order to explain the mutual displacement of both types of herbicides (8,12,21). Furthermore, it should be noted that for phenolic herbicides, some effects at the donor side of photosystem II (22) and on carotenoid oxidation in the photosystem II reaction center have been found (23). 

Due to the fact that the arylutea type herbicides, such as diuron and monuron, did not inhibit the wild type bacterial reaction centers, the predictions have been based mainly upon mutations of the Qp-binding domain, which was affected by interaction with diuron. For example, the characterization of the herbicide-resistant mutants from Bps. viridis has revealed that one of the mutants, T4 (Tyr L222 to Phe) was sensitive to the urea type inhibitors similar to the D1 protein of PSII reaction centre. The semiquinone-iron electron paramagnetic resonance (EPR) signal of Qp in viridisTA mutants was also similar to that reported for photosystem II. 

Approximately half of all commercial herbicides act by inhibiting photosynthesis by interacting with specific sites along the photosynthetic electron transport chain. A number of diverse chemicals including the ureas, amides, triazines, triazinones, uracils, pyridazinones, quinazolines, thiadiazoles, and certain phenols are thought to act specifically at a common inhibitory site at the reducing side of photosystem II (PS II) (U ). 

The data presented here indicates that LY181977 inhibits photosynthetic electron transport through photosystem II. LY181977 has some interaction with that portion of the herbicide binding site which confers atrazine resistance in mutant DCMU4. 

Figure 4. Proposed plastoquinine (QB) and herbicide binding site on the 32 kDalton D-1 polypeptide of photosystem II. The quinone is bound through an iron-complexed histidine residue (his 215) and hydrogen bonding to ser 264. Further interactions occur with arg 269 and phe 255 lying above and below the binding site. Amino acid substitutions in herbicide-tolerant mutants have been identified at the residues numbered 219. 255, 264 and 275. Reproduced with permission from Ref. 57. Copyright 1986 Verlag der Zeitschrift fur Naturforschung.

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