Herbicide thylakoids

DEVELOPMENT OF A CHEMILUMINESCENCE DETECTION OF HERBICIDES RELATIVE TO THE MEDIATED INHIBITION OF THYLAKOIDS IN A ji-FLUIDIC SYSTEM... 

Photosynthesis in all photosynthetic organisms is blocked by triazines, as well as by other PS II herbicides, when isolated thylakoid systems are tested. However, in intact plants, they express either different inhibitory potency or no inhibition. This shows that the specificity of these photosynthesis herbicides to certain weeds is not related to a difference in the chemistry of their primary target, but rather is attributed to degradative mechanisms, translocation, and translocation mechanisms. 

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. 

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. 

In 1979, the concept of a photosystem II herbicide binding protein with different but overlapping binding sites for the various photosystem II herbicides was simultaneously established by Trebst and Draber ( 5) and Pfister and Arntzen (6). This idea of a herbicide receptor protein proved to be extremely fruitful because the techniques of receptor biochemistry were now applicable. Tischer and Strotmann (7) were the first investigators to study binding of radiolabeled herbicides in isolated thylakoids. 

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.

The excitement generated by these hydropathy index plots, which predict that most of the thylakoid proteins indeed span the membrane, may have led to a view that it is unnecessary to experimentally demonstrate that the proteins have segments exposed at the outer and inner surfaces. Nevertheless, biochemical evidence such as antibody labelling, protease studies and chemical modification is needed to prove all predicted structures. Already it has become evident that caution is needed in the interpretation of the Hydropathy index plots. This need is demonstrated by the hydropathy index plots of the D1 (herbicide-binding) and D2... 

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.

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