Multiple action sites for photosystem II herbicides as revealed by delayed fluorescence

Abstract

DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) at concentrations higher than 10 μM suppresses the second time range delayed fluorescence (DF) of pea chloroplasts, due to inhibition of the oxidizing side of photosystem II (PS II). The inhibition of the reducing side of PS II resulting in the suppression of millisecond DF takes place at much lower (∼0.01 μM) DCMU concentrations. The variation in the herbicide-affinities of the reducing and oxidizing sides of PS II is not the same for DCMU and phenol-type herbicides. The DCMU-affinity of the oxidizing side considerably increases and approximates that of the reducing side upon mild treatment of chloroplasts with oleic acid. Probably this is a result of some changes in the environment of the binding site at the oxidizing side. At DCMU concentrations higher than 1 mM, the chaotropic action of DCMU leads to the generation of millisecond luminescence which is not related to the functioning of the reaction centres.

Evidence for a Novel Quinone-Binding Site in the Photosystem II (PS II) Complex That Regulates the Redox Potential of Cytochrome b559

“…A low-affinity site for atrazine has also been identified on the basis of an antibodybinding analysis (128). Furthermore, it has been suggested that a second site of herbicide action is located on the donor side of the PS II complex (131)(132)(133)(134).…”

Current perceptions of Photosystem II

“…Indeed, radioactively labelled herbicides were originally used to tag the D1 protein and identify it as the location of the QB site (see Biihman et al 1987). It is thought that the inhibitor molecule binds at or close to the QB site in competition with PQ binding, although recent evidence suggests that there may be two herbicide binding sites in the Reaction Core (Hsu et al 1986, Giardi ,et al 1988, Vasil'ev et al 1988). It is thought that the inhibitor molecule binds at or close to the QB site in competition with PQ binding, although recent evidence suggests that there may be two herbicide binding sites in the Reaction Core (Hsu et al 1986, Giardi ,et al 1988, Vasil'ev et al 1988).…”

Marine Colloids Boost Nitrogen Fixation in Trichodesmium erythraeum by Photoelectrophy

“…The contribution of photoelectrons from MC to nitrogen fixation and assimilation was validated by adding silver nitrate (AgNO 3 , an electron scavenger) to cell cultures. 3-(3,4-Dichlorophenyl)-1,1-dimethylurea (DCMU) blocks the plastoquinone binding site of PSII, preventing electron flow from PSII to plastoquinone . In the nitrogenase system, the Fe protein cycle involves a transient association between the reduced, MgATP-bound Fe protein and the MoFe protein and includes electron transfer, ATP hydrolysis, the release of Pi, and the dissociation of the oxidized, MgADP-bound Fe protein from the MoFe protein. , MgATP hydrolysis has been proposed to be absolutely required for electron transfer from the Fe protein to the MoFe protein.…”