Characterization of high temperature induced stress impairments in thylakoids of rice seedlings.

Exposure of isolated thylakoids or intact plants to elevated temperature is known to inhibit photosynthesis at multiple sites. We have investigated the effect of elevated temperature (40 degrees C) for 24 hr in dark on rice seedlings to characterize the extent of damage by in vivo heat stress on photofunctions of photosystem II (PSII). Chl a fluorescence transient analysis in the intact rice leaves indicated a loss in PSII photochemistry (Fv) and an associated loss in the number of functional PSII units. Thylakoids isolated from rice seedlings exposed to mild heat stress exhibited >50% reduction in PSII catalyzed oxygen evolution activity compared to the corresponding control thylakoids. The ability of thylakoid membranes from heat exposed seedlings to photooxidize artificial PSII electron donor, DPC, subsequent to washing the thylakoids with alkaline Tris or NH2OH was also reduced by approximately 40% compared to control Tris or NH2OH washed thylakoids. This clearly indicated that besides the disruption of oxygen evolving complex (OEC) by 40 degrees C heat exposure for 24 hr, the PSII reaction centers were impaired by in vivo heat stress. The analysis of Mn and manganese stabilizing protein (MSP) contents showed no breakdown of 33 kDa extrinsic MSP and only a marginal loss in Mn. Thus, we suggest that the extent of heat induced loss of OEC must be due to disorganization of the OEC complex by in vivo heat stress. Studies with inhibitors like DCMU and atrazine clearly indicated that in vivo heat stress altered the acceptor side significantly. [14C] Atrazine binding studies clearly demonstrated that there is a significant alteration in the QB binding site on D1 as well as altered QA to QB equilibrium. Thus, our results show that the loss in PSII photochemistry by in vivo heat exposure not only alters the donor side but significantly alters the acceptor side of PSII.

Multiple functions of photosystem II.

The most important function of photosystem II (PSII) is its action as a water-plastoquinone oxido-reductase. At the expense of light energy, water is split, and oxygen and plastoquinol are formed. In addition to this most important activity, PSII has additional functions, especially in the regulation of (light) energy distribution. The downregulation of PSII during photoinhibition is a protection measure. PSII is an anthropogenic target for many herbicides. There is a unique action of bicarbonate on PSII. Decrease in the activity of PSII is the first effect in a plant under stress; this decreased activity can be most easily measured with fluorescence. PSII is a sensor for stress, and induces regulatory processes with different time scales: photochemical quenching, formation of a proton gradient, state transitions, downregulation by photoinhibition and gene expression.

Shortening of life-time of the pair [P680+Pheo-] contributes to general inhibitory effect of dinoseb on electron transfer in PS-II.

It is shown that dinoseb, added to subchloroplast photosystem-II (PS-II) preparations from pea at a concentration higher than 5 microM, along with blocking the electron transfer on the acceptor side of PS-II, induces the following effects revealing its capability to have redox interaction with the components of PS-II reaction center (RC)-pheophytin (Pheo) and chlorophyll P680: (1) acceleration of the dark relaxation of absorbance (delta A) and chlorophyll fluorescence (delta F) changes related to photoreduction of Pheo as a result of the photoreaction [P680Pheo] [symbol: see text] [P680Pheo-] that leads to elimination of the delta A and delta F at a concentration of the inhibitor higher than 50 microM; (2) lowering of the maximum level of fluorescence (F) due to a decrease of delta F under the condition when the electron acceptor, QA, is reduced; (3) loss of the described effects of dinoseb and appearance of its capability to donate electron to RC of PS-II in the presence of dithionite which reduces dinoseb in the dark; (4) inhibition of delta A related to photooxidation of P680; (5) activation of delta A related to photooxidation P700 in photosystem-I (PS-I) preparations (a similar effect is observed upon the addition of 0.2 mM methylviologen). It is suggested that redox interaction with the pair [P680+Pheo-] leading to the shortening of its life-time contributes to the general effect of inhibition of electron transfer in PS-II by dinoseb.

Age dependent alterations in photosystem II acceptor side in Cucumis sativus cotyledonary leaf thylakoids: analysis of binding characteristics of herbicide [14C]-atrazine.

Senescence induced temporal changes in photosystems can be conveniently studied in cotyledonary leaves. We monitored the protein, chlorophyll and electron transport activities in Cucumis sativus cv Poinsette cotyledonary leaves and observed that by 20th day, there was a 50%, 41% and 30-33% decline in the chlorophyll, protein and photosystem II activity respectively when compared to 6th day cotyledonary leaves taken as control. We investigated the changes in photosystem II activity (O2 evolution) as a function of light intensity. The photosystem II functional antenna decreased by 27% and the functional photosystem II units decreased by 30% in 20-day old cotyledonary leaf thylakoids. The herbicide [14C]-atrazine binding assay to monitor specific binding of the herbicide to the acceptor side of photosystem II reaction centre protein, D1, showed an increase in the affinity for atrazine towards D1 protein and decrease in the QB binding sites in 20th day leaf thylakoids when compared to 6th day leaf thylakoids. The western blot analysis also suggested a decrease in steady state levels of D1 protein in 20th day cotyledonary leaf thylakoids as compared to 6th day sample which is in agreement with [14C]-atrazine binding assay and light saturation kinetics.

Effect of hydration and dehydration on initiation and dynamics of some physiological reactions in desiccation tolerant cyanobacterium Scytonema geitleri.

The effect of hydration and dehydration has been studied on extent and recovery of some metabolic reactions in desiccation tolerant terrestrial cyanobacterium Scytonema geitleri. The results show that the energy transducing reactions like photochemical reactions of photosynthesis recover first, followed by increase in ATP pool size. During later phase of hydration, appearance of energy consuming processes such as CO2 fixation and nitrogen fixation have been observed. Sensitivity of reactions during dehydration followed the pattern reverse to recovery processes.

Studies on electron donation to photosystem I sites by exogenous donors in Spirulina thylakoids.

The kinetic parameters of different sites of electron donation to photosystem I (PS I) were evaluated in Spirulina platensis thylakoids. Reduced 2,6-dichlorophenolindophenol (DCIPH2) exhibited two sites of electron donation, with apparent K(m) values of 8 and 40 microM each. The corresponding value for reduced N-tetramethyl-p-phenylenediamine (TMPDH2) and diaminodurene (DADH2) which donate electrons at a single site to PS I were 103 and 48 microM, respectively. The electron donation by these three exogenous donors were differentially inhibited by KCN (70 mM) affecting the apparent K(m) and Rmax values to varying extent. This cyanide inhibition of PS I catalyzed electron transport suggests the presence of plastocyanin in the photosynthetic electron transport chain of Spirulina platensis.