Herbicide thylakoid membranes

Trebst, A. (1986). The topology of the plastoquinone and herbicide binding peptides of photosystem II in the thylakoid membrane. Z Naturforsch. Sect. C Biosci., 41 240-245. 

Vermass, W.F.J. and C.J. Arntzen (1984). Synthetic quinones influencing herbicide binding and photosystem II electron transport. The effects of triazine-resistance on quinone binding properties in thylakoid membranes. Biochim. Biophys. Acta., 725 483 -91. 

Shortly after the introduction of the triazine herbicides, it was confirmed that their target site in the photosystem II (PS II) complex was in the thylakoid membranes. Triazines displace plastoquinone at the QB-binding site on the D1 protein, thereby blocking electron flow from QA to QB. This in turn inhibits NADPH2 and ATP synthesis, preventing C02 fixation. 

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. 

It is worthy of special interest to study directly the displacement of a herbicide by plastoquinone or its analogues. In normal thy-lakoids, almost no displacement of DCMU even by a million-fold excess of the short-chain plastoquinone analogue plastoquinone-1 can be observed (28). This may be due to the high endogenous plastoquinone content of the thylakoid membrane. If the thylakoids are depleted of plastoquinone by means of n-hexane extraction, a competitive displacement of DCMU by plastoquinone-1 is observed (28). This result establishes a direct interaction between herbicide and plastoquinone, though not necessarily at an identical binding site. From the displacement experiments, a binding constant for plastoquinone-1 of 51 19 jiM in plastoquinone-depleted thylakoids can be calculated (28). ... 

Figure 8. Schematic drawing of the possible location of the 34 kDa herbicide binding protein within the thylakoid membrane.

In an effort to generate images of the morphology of thylakoid membranes inside plant chloroplasts, isolated chloroplasts resuspended in a buffer solution containing a herbicide, 3-(3,4-dichlorophenyl)-l,l-dimethylurea (DCMU), have often been used [10, 11]. The DCMU inhibits photosynthesis by blocking electron transport at the electron acceptor side of PSII and enhances chlorophyll fluorescence from PSII. 

Fig. 9. Binding of quinones QAand Qb and the nonheme iron atom to four histidines of the D1 and D2 subunits of photosystem II, based on the amino-acid sequence homology and the crystal structure of the bacterial reaction center. See text for discussiori. Figure (C) adapted from Trebst (1986) The topology of the plastoquinone and herbicide binding peptides of photosystem II In the thylaKoid membrane. Z Naturforsch 41C 243.

Piletskaya EV, Piletsky SA, Sergeyeva TA et al. Thylakoid membranes-based test-system for detecting of trace quantities of the photosynthesis-inhibiting herbicides in drinking water. Anal Chim... 

Laberge D, Rouillon R, Carpentier R. Comparative study of thylakoid membranes sensitivity for herbicide detection after physical or chemical immobilization. Enzyme Microb Tech 2000 26 332-336. 

Great attention has been paid to the application of thylakoid membranes and photosynthetic microorganisms in environmental pollution control. The biorecognition system based on the binding of certain herbicides to the photosynthetic reaction center of plants and microorganisms seems to be the most direa and simple method for herbicide detection. These systems used as sensor s recognition elements allow the detection of a broad range of herbicides. Unfortunately, their stability and sensitivity are insufficient in the most cases. From this point of view, the DI protein, which binds specifically... 

The data suggested that the toxic action of herbicides on chloroplasts traditionally interpreted by inhibition of electron flow along the chloroplast membrane also may be the result of the thylakoid membrane depolarization. 

For example, glyphosate inhibits the enzyme, EPSP (5-enolpyruvylshikimate 3-phosphate) synthase, that catalyzes a step in the synthesis of the aromatic amino acids. Similarly, both the imidazolinones and sulfonylureas inhibit acetolactate synthase (ALS), the enzyme that catalyzes the first step in the formation of branched-chain amino acids (11). Triazine herbicides act by binding to a specific protein in the thylakoid membranes of the chloroplasts, preventing the flow of electrons and inhibiting photosynthesis (12). 

Identification of the Receptor Site for Triazine Herbicides in Chloroplast Thylakoid Membranes... 

---