Lipid Composition of Chloroplast Membranes from Weed Biotypes Differentially Sensitive to Triazine Herbicides

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

Section: Discussionsupporting

confidence: 60%

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

Jursinic¹,

Stemler²

1983

“…This pleiotropic cascade includes both structural (25) and functional changes (2). Many of these changes in R are similar to those found in shade-adapted leaves.…”

mentioning

confidence: 73%

Pleiotropy in Triazine-Resistant Brassica napus

Dekker¹,

Burmester²

1992

Studies were conducted that supported the hypothesis that the mutation to the psbA plastid gene that confers S-triazine resistance (R) in Brassica napus also results in an altered diurnal pattern of photosynthetic carbon assimilation (A) relative to that of the susceptible (S) wild type, and that these patterns change over the ontogeny of a plant. Photosynthetic photon flux density, under closely controlled environmental conditions, was incrementally increased and decreased on either side of the midday maxima of 1150 to 1300 ;tmol quanta m-2 s-'. In all experiments, A approximately tracked the increasing and decreasing diurnal light levels. Younger (3-to 4-leaf) R plants had greater photosynthetic rates early and late in the diurnal light period, whereas those of S plants were greater during midday as well as during the photoperiod as a whole. These relative photosynthetic characteristics of R and S plants changed in several ways with ontogeny. As the plants aged during the vegetative phase of development, S plants gradually assimilated more carbon in the early, and then in the late, part of the day. At the end of the vegetative phase of development, R plant carbon assimilation was less relative to S plants at most times of the day, and was never greater. This relationship between the two biotypes dramatically changed with the onset of the reproductive phase (81/2 to 91/2 leaf) of plant development: R plants assimilated more carbon than S plants during all periods of the diurnal light period with the exception of the late part of the day. In addition to these differences in A, R plant stomatal function differed from that in S plants. R plant leaves were always cooler than S plant leaves under the same environmental and diurnal conditions. Correlated with this difference in leaf temperature were equal or greater total conductances to water vapor and intercellular CO2 partial pressures in R compared to S leaves in most instances. These studies indicate a more complex pattern of photosynthetic carbon assimilation than previously observed. The photosynthetic superiority of one biotype relative to the other was a function of the time of day and the age of the plant. These thus, the codon 264 change in the psbA gene caused a change in its product, the Dl protein, a key functional element in PSII electron transport (28). R2 plants have a decreased quantum efficiency of CO2 assimilation compared to S plants (15). This decreased efficiency is due to the altered redox state of PSII quinone acceptors and a shift in the equilibrium constant between QA-and QB in favor of QA-(2). What is less clear in the literature is whether this change in Dl structure and electron transport function directly modifies whole-leaf photosynthesis and plant productivity or only indirectly influences these functions (14).The genetic change in R plants leads to a profound reorganization of functional units in the chloroplast. This pleiotropic cascade includes both structural (25) and functional changes (2). Many of these changes in R ar...

“…However, because recombination cannot occur between strictly maternally inherited traits, the possibility of independent mutational origin in two different plastid genes cannot be formally excluded. This problem also arises in interpreting the experiments of Pillai and St. John (24) …”

Section: Resultsmentioning

confidence: 99%

Non-Mendelian Inheritance of 3-(3,4-Dichlorophenyl)-1,1-dimethylurea-Resistant Thylakoid Membrane Properties in Chlamydomonas

Galloway

Mets²

1982

A uniparentally inherited 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-resistant mutant of Chiamydomonas reinhardii, Dr2, which has a resistance mechanism of the type defined as 'primary,' has been isolated. In vitro Hill reactions catalyzed by isolated thylakoid membranes reveal a reduced apparent affinity of the thylakoids for DCMU. These changes in membrane properties quantitatively account for the resistance of mutant Dr2 to herbicide inhibition of growth. There are several chemically distinct classes of herbicides which inhibit photosynthesis by blocking electron transport away from the reduced PSII acceptor, Q (20). DCMU, or diuron, is a potent member of the urea class, and atrazine is a potent and widely used triazine. In addition to having nearly identical physiological effects, these various herbicides show competitive binding to the same site on the thylakoid membrane (29). All of the herbicides share the common structural feature of a sp2 hybrid orbital carbon bonded to a nitrogen atom (9). At the same time, studies of substituent effects on the binding of different chemical classes show that one class may have important contacts with the binding site which are in a different spatial domain from some of the contacts which are important to a different class. Thus, it seems that each chemical class of these herbicides shares a portion of its binding site with all of the others, but that a portion is specific for that class (2).The derivative of atrazine which retains competitive binding to this site becomes covalently bonded to a 32,000 D thylakoid membrane polypeptide when the complex is UV irradiated (6). The reaction of this protein is dependent upon specific binding at the site, since it cannot be labeled in thylakoids from an atrazine-resistant biotype of Amaranthus which do not bind atrazine (23). Thus, this 32,000 D polypeptide is either part of or close to the binding site, but it is not known whether this part is shared with other herbicides or whether it interacts uniquely with triazines. Mattoo et al. (16) have shown that a 32,000 D polypeptide of Spirodela thylakoids, which is ordinarily quite sensitive to trypsin treatment of the membranes, is rendered resistant to digestion upon DCMU binding. If this protein is homologous with the azido-atrazine labeled protein, then it is apparent that it has important interactions with both triazines and ureas. Oettmeier et al. (21) found that an azido derivative of dinoseb (a dinitrophenol) acted as a photoaffinity label for a spinach thylakoid membrane polypeptide in the 40,000 D range. If this protein is not homologous to the atrazine-labeled protein, then one must conclude that the binding sites for the different classes of herbicides reach into the region of at least two proteins. It will be of interest to determine if herbicide resistance can arise from mutations in more than one thylakoid protein gene.We have initiated a study of resistance to DCMU in Chlamydomonas reinhardii with two main objectives (a) By using genetic variants, we hop...