Photosystem inhibition

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... 

A method of detecting herbicides is proposed the photosynthetic herbicides act by binding to Photosystem II (PS II), a multiunit chlorophyll-protein complex which plays a vital role in photosynthesis. The inhibition of PS II causes a reduced photoinduced production of hydrogen peroxide, which can be measured by a chemiluminescence reaction with luminol and the enzyme horseradish peroxidase (HRP). The sensing device proposed combines the production and detection of hydrogen peroxide in a single flow assay by combining all the individual steps in a compact, portable device that utilises micro-fluidic components. 

Atrazine enters plants primarily by way of the roots and secondarily by way of the foliage, passively translocated in the xylem with the transpiration stream, and accumulates in the apical meristems and leaves (Hull 1967 Forney 1980 Reed 1982 Wolf and Jackson 1982). The main phytotoxic effect is the inhibition of photosynthesis by blocking the electron transport during Hill reaction of photosystem II. This blockage leads to inhibitory effects on the synthesis of carbohydrate, a reduction in the carbon pool, and a buildup of carbon dioxide within the leaf, which subsequently causes closure of the stomates, thus inhibiting transpiration (Stevenson et al. 1982 Jachetta et al. 1986 Shabana 1987). 

In the search for more effective post-emergent herbicides, many laboratories have measured the inhibition of photosystem II in chloroplasts i.e., the Hill reaction. In a continuing investigation of this system, ( ) Corwin Hansch s group at Pomona College, in cooperation with BASF in Germany, analyzed two sets of phenyl substituted ureas 17 1,1-dimethyl-3-phenyl, and 38... 

C, Inhibition of photosynthesis at photosystem II 1,3,5-Triazines Triazinones Uracils Pyridazinone Phenyl carbamates 5... 

When the primary electron donation pathway in photosystem II is inhibited, chlorophyll and p-carotene are alternate electron donors and EPR signals for Chl+ and Car+ radicals are observed.102 At 130 GHz the signals from the two species are sufficiently resolved to permit relaxation time measurements to be performed individually. Samples were Mn-depleted to remove the relaxation effects of the Mn cluster. Echo-detected saturation-recovery experiments were performed with pump pulses up to 10 ms long to suppress contributions from cross relaxation and spin or spectral diffusion. The difference between relaxation curves in the absence of cyanide, where the Fe(II) is S = 0, and in the presence of cyanide, where the Fe(II) is S = 2, demonstrated that the relaxation enhancement is due to the Fe(II). The known distance of 37 A between Fe(ll) and Tyrz and the decrease of the relaxation enhancement in the order Tyrz > Car+ > Chl+ led to the proposal of 38 A and > 40A for the Fe(II)-Car+ and Fe(II)-Chl+ distances, respectively. Based on these distances, locations of the Car+ and Chl+ were proposed. 

The reagent DCMU specifically inhibits electron transfer to plastoquinone in photosystem II. Discuss how the administration of this compound to a suspension of illuminated chloroplasts will affect the production of oxygen, ATP, and NADPH. 

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. 

Kakkis, E., V.C. Palmire, C.D. Strong. W. Bertsch, C. Hansch, and U. Schirmer (1984). Quantitative structure-activity relationships in the inhibition of photosystem II in chloroplasts by phenylureas. J. Agric. Food Chem. 32 133-144. 

Gronwald, J.W. (1994). Resistance to photosystem II inhibiting herbicides, pp. 27-60. In Powles, S.B. and J.A.M. Holtum, eds., Herbicide Resistance in Plants Biology and Biochemistry. Boca Raton, FL CRC Press. 

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. 

Leu E., Liszkay A.K., Goussias C., Gross E.M. Polyphenolic allelochemicals from the aquatic angiosperm myriophyllum spicatum inhibit photosystem II. Plant Physiol 2002 130 2011-2018. 

The combination of surface-associated reactants with surface-bound H-atoms, occasionally leads to poor photoinduced hydrogenation of the reactant and parallelly to inhibition of H2-evolution. For such systems, tailored bifunctional heterogeneous catalysts have been developed [141], where cooperative catalytic effects are observed in the photohydrogenation reactions. Substitution of ethylene by acetylene, C2H2, in the photosystem composed of Ru(bpy) +/MV2+/Na2EDTA and the Pt colloid results in inefficient hydrogenation of acetylene to ethylene,

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Some responses, such as mortality, are irreversible. However, many sublethal responses may be reversible, such as the impact of the photosynthesis-inhibiting herbicide linuron on macrophytes (Snel et al. 1998). Linuron inhibits photosynthesis by disturbing electron transport in photosystem II. Table 6.2 presents the kinetics of photosynthesis inhibition when shoots of macrophytes are placed in water with 50 pg/L linuron, and subsequent recovery when placed in uncontaminated water. The EC50 values are remarkably similar between macrophytes, and half-life estimates for inhibition and recovery are less than 2 hours (Table 6.2). Except for Potamogeton... 

Note Kinetics of the inhibition of photosystem II electron flow in these macrophytes by 50 pg/L linuron and the subsequent recovery of inhibition by washing with uncontaminated well water are expressed as half-life times (f1/2). 

Snel JFH, Vos JH, Gylstra R, Brock TCM. 1998. Inhibition of photosystem II (PSII) electron transport as a convenient endpoint to assess stress of the herbicide linuron on freshwater plants. Aquat Ecol 32 113-123. 

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). 

Due to the anthraquinone moiety, we tested all compounds for photosystem II (PS II) inhibition using both spinach and com thylakoids. By using both monocot and dicot thylakoids, we accounted for any differences in activity. There was no effect on PS II activity for series 2 analogues (-CH3) (data not shown). However, for series 1 analogues (-OH), there was a 50% decrease in PS II activity at 0.1 pM in thylakoids isolated from spinach (Fig. 1.9A). This was similar for thylakoids isolated from corn (Fig. 1.9B). 

Electron Transport Inhibitors. Electron transport is inhibited when one or more of the intermediate electron transport carriers are removed or inactivated. The site of action of most herbicidal electron transport inhibitors is considered to be associated closely with photosystem II. Consequently, reactions coupled to photosystem II are inhibited, such as basal electron... 

Partial reactions not dependent on photosystem II, such as cyclic phosphorylation or the photoreduction of NADP with an electron donor that circumvents photosystem II (ascorbate + DPIP), are either not inhibited or inhibited only weakly. These herbicides also do not inhibit mitochondrial oxidative phosphorylation. 

Inhibitory Uncouplers. Inhibitory uncouplers inhibit the reactions affected by both electron transport inhibitors and uncouplers. Hence, they inhibit basal, methylamine-uncoupled, and coupled electron transport with ferricyanide as electron acceptor and water as the electron donor, much like electron transport inhibitors. Coupled noncyclic photophosphorylation is inhibited and the phosphorylation reaction is slightly more sensitive than the reduction of ferricyanide. Cyclic photophosphorylation is also inhibited. NADP reduction, when photosystem II is circumvented with ascorbate + DPIP, is not inhibited however, the associated phosphorylation is inhibited. Inhibitory uncouplers act at both sites 1 and 2 (Figure 2). 

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