Photosynthetic electron transport control

Triazines are selective herbicides used to control a wide spectrum of grass and broadleaf weeds in cereal, oilseed, and horticultural crops. Triazine herbicides kill weeds by interfering with the electron transport chain in photosystem II (PS II). These herbicides bind to the QB protein in the PS II reaction center and block the flow of electrons through the photosynthetic electron transport chain. 

Triazine-resistant weeds have an impaired electron transport system. Reduced electron transport, in turn, reduces photosynthetic activity and fitness. Under controlled conditions, most triazine-resistant biotypes exhibit impaired photosynthesis. Significant fitness costs from resistance (10-50%) have been reported in most studies (Warwick, 1991). This substantial fitness cost or handicap has been important in the management of triazine-resistant weeds (Radosevich et al., 1991 Bergelson and Purrington, 1996). Anderson et al. (1996b) found that the competitive advantage of triazine-susceptible waterhemp over triazine-resistant waterhemp isolated from one field in Nebraska was equal to or less than that for other species or isolates. This indicates that additional factors contributed to the slow and limited distribution of resistance for this waterhemp biotype. 

The triazine herbicides currently used are mostly 4,6-alkylarmno-v-triazine compounds with either a 2-chloro, 2-methylthio, or 2-methoxy substituent (Table 23.1). The /V-alkyl groups may be methyl, ethyl, 1-methylethyl (isopropyl), 1,1-dimethylethyl (tertiary-butyl), 1,2-dimethylpropyl, or 2-methylpropanenitrile. Absorbed by roots or leaves of plants, these herbicides are applied either preemergence or postemergence to control annual broadleaf weeds and annual grasses in a wide variety of crops. The triazine herbicides listed in Table 23.1 have the same mechanism of action in plants, as all are photosynthetic electron transport inhibitors. 

The ability of membranes to compartmentalize reagents and control the permeation of chemical species may also allow the control of electron transfer in a more sophisticated way within the aggregate bilayer [86]. Photosynthetic processes occur specifically in membranes [87] (thylakoid membranes) so there is continuous interest in mimicking these phenomena with synthetic vesicles [86]. Though a large amount of information is available on the components of biological systems that operate electron transport, the actual mechanism of the process is far from being understood in detail. 

The photosynthetic process in plants can be divided into four stages, each localized to a defined area of the chloro-plast (1) absorption of light, (2) electron transport leading to formation of O2 from H2O, reduction of NADP to NADPH, and generation of a proton-motive force, (3) synthesis of ATP, and (4) conversion of CO2 into carbohydrates, commonly referred to as carbon fixation. All four stages of photosynthesis are tightly coupled and controlled so as to produce the amount of carbohydrate required by the plant. All the reactions in stages 1-3 are catalyzed by proteins in the thylakoid membrane. The enzymes that incorporate CO2 into chemical intermediates and then convert them to starch are soluble constituents of the chloroplast stroma. The enzymes that form sucrose from three-carbon intermediates are in the cytosol. 

Foyer CH and Harbinson J (1997) The photosynthetic electron transport system efficiency and control. In Foyer CH, Quick WP (eds), A molecular approach to primary metabolism in higher plants, pp 3-39. Taylor and Francis, London, UK Foyer CH and Lelandais M (1995) Ascorbate transport into protoplasts, chloroplasts and thylakoid membranes of pea leaves. In Mathis P (ed), Photosynthesis From light to Biosphere, Vol V,pp511-514. Kluwer Academic Publishers, Dordrecht... 

Foyer CH and Lelandais M (1996) A comparison ofthe relative rates of transport of ascorbate and glucose across the thylakoid, chloroplast and plasmalemma membranes of pea leaf mesophyll cells. J Plant Physiol 148 391-398 Foyer CH, Rowell J and Walker D (1983) Measurement ofthe ascorbate content of spinach leaf protoplasts and chloroplasts during illumination. Planta 157 239-244 Foyer CH, Furbank R, Harbinson J and Horton P (1990) The mechanisms contributing to photosynthetic control of electron transport by carbon assimilation in leaves. Photosynth Res 25 83-100... 

Mild-Trypsin Digestion. When a preparation of stripped chloroplasts (thylakoids) is incubated with trypsin, the electron transport block imposed by photosynthetic inhibitors is almost completely overcome. Based on responses obtained, inhibitors can be divided into 3 groups (Table IV). Chemicals in Group 1 behave like DCMU, i.e., most of the reducing activity is restored in trypsin-treated chloroplasts relative to untreated controls (i7-phenylureas, s-triazines, i7-acylanilides, i 7-phenylcarbamates, uracils, pyridazinones, and triazinones). loxynil and i-dinoseb... 

TABLE 1. Photosynthetic electron-transport reactions in Cu-sufficient and Cu-deficient pea plants. Values in brackets are % of +Cu control MV, methylviologen asc, ascorbate DCIP, dichlorophenol indo-... 

Photosynthetically grown (red) as well as heterotrophically in the dark developed (white) membranes from R. palustris undergo "respiratory control" (2) by influencing the electron flow rates, when substrate is present in excess, upon the addition of ADP, Pi and Mg2+ (3). This acceptor control by phosphorylating substrates resulted in a retardation of electron transport in chromatophores, whereas the flow rate was stimulated in white membranes from / . palustris (Fig.2). However, even in totally RpFi-depleted chromatophores an acceptor control by ADP, Pi and Mg2+ can be observed (Tab.2) with an acceptor control index (4) of 1.6. 

Plastocyanin (PC) and ferredoxin (FD) are two nuclear-encoded chloroplast proteins which are functional in photosynthetic electron transport. In higher plants the expression of the PC and FD genes is light controlled, most likely by a phytochrome-dependent mechanism (1, 2). In addition, expression of these genes is mostly confined to green tissue. 

LOCALIZED PROTON DOMAINS IN pH-DEPENDENT CONTROL OF PHOTOSYNTHETIC ELECTRON TRANSPORT UNDER THE INFLUENCE OF LIPOPHILIC TERTIARY AMINES... 

Photosynthetic electron transport is under control of the intrathyla-koid proton potential, pHj. (1,2). When pH. increases in the light, with the build-up of a transthylakoid proton gradient (ApH), electron flow is decelerated. Two major sites of pH. -dependent feed-back control have been discussed the reduction of the primary photosystem (PS) II acceptor (2) and the oxidation of plastohydroquinone at the cytochrome... 

However changes in the kinetics of electron flow between the photosystems (IE the degree of photosynthetic control) can be produced by changes in CO2 partial pressure (7, submitted) or photorespiratory activity (5). Therefore there is photosynthetic control of the electron transport chain, but the nature of this control is constant with regard to irradiance under normal conditions. 

Conclusions. The experiments reported here demonstrate the application of extrapolation to infinite pulse intensity in analysis of the control of electron transport by non-photochemical quenching in several species. The results are signiflcantly different from inferences based on direct analysis of fluorescence responses at a single, finite pulse intensity. In particular, a single set of parameters fit equally well to all members of a group of species selected to have diverse photosynthetic properties. 

At high irradiance photosynthesis is limited by CO2 assimilation and more light is absorbed than can be effectively used to drive photosynthesis. Dissipation of surplus excitation energy is essential because excessive excitation leads to the light-induced loss of thylakoid efficiency called photoinhibition. When the rate of carbon assimilation limits the overall rate of photosynthesis the capacity of ATP and NADPH production exceeds that of demand by the Benson-Calvin cycle and photosynthetic control of electron transport would be expected (1, 2). Photosynthetic control... 

Figure 3 Dependence of (normalized) photosynthetic control (PC), high-energy quenching of fluorescence, qE and steady state level of oxidized P700 on the pH and the pH in the inner thylakoid space, respectively. Methyviologogen was an electron acceptor, light was saturating. pH was calculated from 9-aminoacridine fluorescence. Measurements in presence of 0.2 mM ADP and different level of control have been obtained different level of Pi. PS controlled/uncoupled electron transport.

Before and during stress were collected A(230 p ), A(800 Pi) and electron transport rate, J. J was taken as 02 evolution rate x 4 in 8% C02. We extimate a potential assimilation rate from J and NADPH limitation at pi of 800 ubar by modelling a potential photosynthetic flux as Am =J(Pi-r ) / (4pj +8r ) - Rd, where eventually r and Rd can be evaluated by gas exchange on the same apparatus (6). Within these constraints we can compare Am values scaled to the Am of control and this should give the metabolic effect of the stress. The same comparison between C02 gas exchange actual rates includes diffusional heterogeneity and one can quantitate its relative effect. 

Localized Proton Domains in pH-Dependent Control of Photosynthetic Electron Transport under the Influence of Lipophilic Tertiary Amines 211... 

Cyclic voltammetry of spinach plastocyanin portrays an interesting view of how these factors affect its ability to transfer electrons. It was observed [99] that the eonditions required to promote electrochemistry at a PGE electrode were broadly similar to those pertaining to the photosynthetic electron-transport system. For a solution of the protein (oxidized or reduced) at low ionic strength, well-defined diffusion-controlled voltammetric waves were observed upon addition of Mg " or by acidification to pH 4. The peak-current response as a function of these variables is shown in Fig. 12. At pH 7 (3 °C), E was found to be 375 mV, in good agreement with the potentiometric value reported by Katoh and co-workers [156]. The electrode reaction was found to be essentially reversible at pH 4. Whilst this appears at first to be in conflict with the evidence for plastocyanin being inactive at this pH, closer consideration shows that this is a consistent result. The corresponding electrode reaction may be written as in Eqs. (11)-(12)... 

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