Changes in [¹⁴C]Atrazine Binding Associated with the Oxidation-Reduction State of the Secondary Quinone Acceptor of Photosystem II

Abstract

One hypothesis of triazne-type herbicide action in photosynthetic material is that the herbicide molecule competes with a secondary quinone acceptor, B, for a binding site at the reaction center of photosystem II. The binding affinity of B has been suggested to change with its level of reduction, being most strongly bound in its semiquinone form. Among the major classes of herbicides are those that inhibit electron transport in photosynthesis (21). The mode of action of most of these inhibitors is blockage of electron flow associated with PSII. Diuron and atrazine are two herbicides of this type that have been studied extensively and are believed to act near the same site (22,23).It has been hypothesized that diuron acts on the oxidizing side (7, 10, 25) as well as on the reducing side (9; for a review, see 13) of PSII. The effects on the reducing side correlate with binding of high specificity (30) and occur at much lower concentrations than the oxidizing-side effects. In this work, we have studied the inhibition of electron flow on the reducing side of PSII.'Supported by funds from National Science Foundation grant PCM 8004075 to A. S.The modes of action ofdiuron and atrazine are still unknown. One theory is that these herbicides block electron flow between the primary acceptor (Q) and the secondary acceptor (B) by decreasing the redox potential of B, which shifts the equilibrium of Q-B ± QB-to the left (32). Changes in redox potential due to herbicides have yet to be demonstrated (29). A second theory is that B is a quinone free to diffuse in the thylakoid membrane and exchange with the plastoquinone pool (12,26). This vacates a binding site at the PSII reaction center, which can be filled in a competitive manner by a herbicide molecule. With the herbicide bound, the binding site is not available for B, and normal electron flow past Q is not possible (31). This proposal is quite analogous to the suggested mechanism of action of o-phenanthroline in purple photosynthetic-bacteria (35,36). Support for this theory is based on the kinetics of charge transfer at the reaction center, and on how these kinetics are altered when sufficient time is allowed for the hypothesized exchange of herbicide for the quinone B to take place (31,35,36 (1,6,8,15), caused the affinity for atrazine to significantly decrease. MATERIALS AND METHODSBroken chloroplasts (thylakoids) were isolated from leaves of dwarf peas (Pisum sativum) grown in a laboratory growth chamber and harvested 14 to 21 d after germination. The isolation procedure was as previously described (14), and the reaction medium used was 400 mm saccharose, 50 mm Tes, 10 mm NaCl, 5 mM MgCl2 (pH 7.5

Bicarbonate, not CO2, is the species required for the stimulation of Photosystem II electron transport

“…It is now generally accepted that Qs is a bound PQ which, when fully reduced, exchanges for an oxidized PQ from the PQ pool [13,14]. Competitive binding between herbicides and quinones supports this view [15][16][17][18]. Our current hypothesis for the HCO 3 requirement, as noted earlier, is that HCO 3 acts as an allosteric activator for the reduction of bound PQ, and induces a conformational change which permits the efficient exchange between the bound PQH 2 and an oxidized PQ.…”

Temperature and Light Modulation of Herbicide Toxicity on Algal and Cyanobacterial Physiology

“…In contrast, anthropogenic additions of hazardous compounds to waterways can significantly affect photosynthesis of aquatic organisms (DeLorenzo et al, 2001), and agrotoxics (mainly herbicides) heavily used in agricultural and industrial practices are notorious for their effects on algal and cyanobacterial physiology and growth (Bérard et al, 1999;Chalifour and Juneau, 2011). Photosynthesis-targeting herbicides, such as atrazine, simazine and terbuthylazine, act primarily on photosynthetic light reactions and inhibit carbon assimilation (Jursinic and Stemler, 1983). On the other hand, even if photosynthesis and respiration are not the primary target of some herbicides (such as glyphosate), these metabolic pathways may be also affected by these substances (Qiu et al, 2013;.…”

Effects of repeated pulsed herbicide exposures on the growth of aquatic macrophytes

“…32,2013exceeded 100 mg/L; for the 2-d pulse scenario the EC50 was lower, at 97 mg/L (Table 3). Inhibition of L. minor growth rate under pulsed and continuous exposure scenarios is summarized in Figure 2.…”

“…Isoproturon is a urea herbicide that inhibits PSII by blocking the flow of electrons between the primary acceptor (Q B binding site of the D1 protein) and secondary acceptor (plastoquinone) in the PSII complex [32,33]. In plants, recovery from exposure to PSII-inhibiting herbicides is typically rapid [4].…”