Effect of manganese on the nuclear magnetic relaxivity of water protons in chloroplast suspensions

Consequences of the reduction of the Photosystem II antenna size on the light acclimation capacity of Arabidopsis thaliana

In several systems, from plant's canopy to algal bioreactors, the decrease of the antenna size has been proposed as a strategy to increase the photosynthetic efficiency. However, still little is known about possible secondary effects of such modifications. This is particularly relevant because the modulation of the antenna size is one of the most important light acclimation responses in photosynthetic organisms. In our study, we used an Arabidopsis thaliana mutant (dLhcb2), which has a 60% decrease of Lhcb1 and Lhcb2, the two main components of the major Photosystem II antenna complex. We show that the mutant maintains the photosynthetic and photoprotective capacity of the Wild Type (WT) and adapts to different light conditions by remodelling its photosynthetic apparatus, but the regulatory mechanism differs from that of the WT. Surprisingly, it does not compensate for the decreased light-harvesting capacity by increasing other pigment-protein complexes. Instead, it lowers the ratio of the cytochrome b₆ f and ATP synthase to the photosystems, regulating linear electron flow and maintaining the photosynthetic control at the level of these complexes as in the WT. We show that targeting the reduction of two specific antenna proteins, Lhcb1 and Lhcb2, represents a viable solution to obtain plants with a truncated antenna size, which still maintain the capacity to acclimate to different light conditions.

Non-cooperative immobilization of residual water bound in lyophilized photosynthetic lamellae

This study applied 1H-NMR in time and in frequency domain measurements to monitor the changes that occur in bound water dynamics at decreased temperature and with increased hydration level in lyophilizates of native wheat photosynthetic lamellae and in photosynthetic lamellae reconstituted from lyophilizate. Proton relaxometry (measured as free induction decay = FID) distinguishes a Gaussian component S within the NMR signal (o). This comes from protons of the solid matrix of the lamellae and consists of (i) an exponentially decaying contribution L1 from mobile membrane protons, presumably from lipids, and from water that is tightly bound to the membrane surface and thus restricted in mobility; and (ii) an exponentially decaying component L2 from more mobile, loosely bound water pool. Both proton relaxometry data and proton spectroscopy show that dry lyophilizate incubated in dry air, i.e., at a relative humidity (p/p0) of 0% reveals a relatively high hydration level. The observed liquid signal most likely originates from mobile membrane protons and a tightly bound water fraction that is sealed in pores of dry lyophilizate and thus restricted in mobility. The estimations suggest that the amount of sealed water does not exceed the value characteristic for the main hydration shell of a phospholipid. Proton spectra collected for dry lyophilizate of photosynthetic lamellae show a continuous decrease in the liquid signal component without a distinct freezing transition when it is cooled down to -60ºC, which is significantly lower than the homogeneous ice nucleation temperature [Bronshteyn, V.L. et al. Biophys. J. 65 (1993) 1853].

Effect of Light Acclimation on the Organization of Photosystem II Super- and Sub-Complexes in Arabidopsis thaliana

To survive under highly variable environmental conditions, higher plants have acquired a large variety of acclimation responses. Different strategies are used to cope with changes in light intensity with the common goal of modulating the functional antenna size of Photosystem II (PSII). Here we use a combination of biochemical and biophysical methods to study these changes in response to acclimation to high light (HL). After 2 h of exposure, a decrease in the amount of the large PSII supercomplexes is observed indicating that plants are already acclimating to HL at this stage. It is also shown that in HL the relative amount of antenna proteins decreases but this decrease is far less than the observed decrease of the functional antenna size, suggesting that part of the antenna present in the membranes in HL does not transfer energy efficiently to the reaction center. Finally, we observed LHCII monomers in all conditions. As the solubilization conditions used do not lead to monomerization of purified LHCII trimers, we should conclude that a population of LHCII monomers exists in the membrane. The relative amount of LHCII monomers strongly increases in plants acclimated to HL, while no changes in the trimer to monomer ratio are observed upon short exposure to stress.

ROS-mediated enhanced transcription of CYP38 promotes the plant tolerance to high light stress by suppressing GTPase activation of PsbO2

As a member of the Immunophilin family, cyclophilin38 (CYP38) is discovered to be localized in the thylakoid lumen, and is reported to be a participant in the function regulation of thylakoid membrane protein. However, the molecule mechanisms remain unclear. We found that, CYP38 plays an important role in the process of regulating and protecting the plant to resist high light (HL) stress. Under HL condition, the gene expression of CYP38 is enhanced, and if CYP38 gene is deficient, photochemistry efficiency, and chlorophyll content falls distinctly, and excessive reactive oxygen species synthesis occurs in the chloroplast. Western blot results showed that the D1 degradation rate of cyp38 mutant plants is faster than that of wide type plants. Interestingly, both gene expression and activity of PsbO2 were drastically enhanced in cyp38 mutant plants and less changed when the deleted gene of CYP38 was restored under HL treatment. This indicates that CYP38 may impose a negative regulation effect on PsbO2, which exerts a positive regulation effect in facilitating the dephosphorylation and subsequent degradation of D1. It is also found that, under HL condition, the cytoplasmic calcium ([Ca(2+)]cyt) concentration and the gene expression level of calmodulin 3 (CaM3) arose markedly, which occurs upstream of CYP38 gene expression. In conclusion, our results indicate that CYP38 plays an important role in plant strengthening HL resistibility, which provides a new insight in the research of mechanisms of CYP38 protein in plants.

Electron nuclear double resonance evidence supporting a monomeric nature for P700 in spinach chloroplasts

Proton electron nuclear double resonance (ENDOR) spectra of P700(+) in spinach chloroplasts and in photosystem I particles have been obtained and compared with the corresponding ENDOR spectrum of monomeric chlorophyl a(+) (Chla(+)) cation radical. The hyperfine couplings for P700(+) can be interpreted in terms of those expected for a monomer Chla(+) radical. The reduction in alpha-carbon spin densities observed for the in vivo species when compared to the in vitro radical is attributed to differences in the composition of the ground-state orbital for the two systems. For P700(+), a mixture of 75% D(0)/25% D(1), in which D(0) and D(1) represent the ground-and first excited-state orbitals calculated by Petke et al. for Chla(+) [Petke, J. D., Maggiora, G. M., Shipman, L. L. & Christoffersen, R. E. (1980) Photochem. Photobiol. 31, 243-257], gives good agreement between calculated and experimental spin-density reduction factors. Interaction of the pigment ion with its protein environment such as through ligation of the central Mg atom, hydrogen bonding to the 9-keto-carbonyl group, and electrostatic interactions with charged amino acid residues are proposed as factors responsible for the lowering in energy of the D(1) level in vivo. Combined with similar previous proposals for P680(+) of photosystem II, the data suggest that both primary donor cation radicals of green plant photosynthesis can be viewed as monomeric Chla(+) species in which the D(1) orbital makes a significant contribution to the spin-density distribution.

Stoichiometry, inhibitor sensitivity, and organization of manganese associated with photosynthetic oxygen evolution

Chloroplast thylakoid membranes isolated in the presence of EDTA retain high rates of O(2) evolution (>/=340 mumol.h(-1).mg chlorophyll(-1)) but contain no Mn(2+) that is detectable by electron paramagnetic resonance (EPR) at room temperature. The total Mn(2+) content of these preparations is 4.6 per 400 chlorophylls; 0.6 Mn(2+) can be released by addition of Ca(2+), a treatment that does not affect O(2) evolution. The remaining Mn(2+) (4 per 400 chlorophylls) appears to be functionally associated with O(2) evolution activity. Inhibition by Tris, NH(2)OH, or heat will release a small fraction of Mn(2+) from these membranes ( approximately 25% with Tris, for example). Addition of Ca(2+) further enhances Mn(2+) release so that for Tris and for NH(2)OH, 2 and 3, respectively, Mn(2+) per 400 chlorophylls are extracted from the O(2)-evolving complex. Based on the microwave power-saturation properties of the EPR signal IIf, which arises from an intermediate electron carrier in the water splitting process, it appears that one of the four Mn(2+) associated with photosystem II is uniquely sensitive to Tris. A new model is proposed for the organization and inhibitor sensitivity of manganese in the O(2)-evolving complex.

Effect of bicarbonate on the water-oxidizing complex of photosystem II in the super-reduced S-states

It is shown that the hydrazine-induced transition of the water-oxidizing complex (WOC) to super-reduced S-states depends on the presence of bicarbonate in the medium so that after a 20 min treatment of isolated spinach thylakoids with 3 mM NH(2)NH(2) at 20 degrees C in the CO(2)/HCO(3)(-)-depleted buffer the S-state populations are: 42% of S(-3), 42% of S(-2), 16% of S(-1) and even formal S(-4) state is reached, while in the presence of 2 mM NaHCO(3), the same treatment produces 30% of S(-3), 38% of S(-2), and 32% of S(-1) and there is no indication of the S(-4) state. Bicarbonate requirement for the oxygen-evolving activity, very low in untreated thylakoids, considerably increases upon the transition of the WOC to the super-reduced S-states, and the requirement becomes low again when the WOC returns back to the normal S-states using pre-illumination. The results are discussed as a possible indication of ligation of bicarbonate to manganese ions within the WOC.

Topological studies of spinach 22 kDa protein of Photosystem II

An intrinsic 22 kDa polypeptide is associated with the O2-evolving Photosystem II core complex in a variety of green plants, although it does not appear to be required for O2 evolution. Digestion of thylakoid membranes and isolated Photosystem II preparations with trypsin, followed by immunoblotting using spinach anti-22 kDa antibodies, leads to two observations: (1) the domain between the 2nd and 3rd transmembrane helices of the 22 kDa protein is stromally exposed, and (2) only in a reaction center complex preparation, lacking the chlorophyll a/b-light harvesting complex II, is there extensive proteolytic cleavage of the 22 kDa protein. We also found that after, but not prior to, selective extraction of the 22 and 10 kDa proteins from Photosystem II membranes, the chlorophyll a/b-light harvesting complex II can be separated from the Photosystem II reaction center core by precipitation with MgCl2. This result suggests that the 22 kDa polypeptide is located between the Photosystem II reaction center polypeptides and light-harvesting complex II; it is possible that the protein serves as a link between the two protein complexes. The presence of the 22 kDa protein in several species was also examined by immunoblotting with polyclonal spinach anti-22 kDa antibodies.

NMR paramagnetic relaxation enhancements due to manganese in the S0 and S 2 states of Photosystem II-enriched membrane fragments and in the detergent-solubilized Photosystem II complex

The NMR paramagnetic relaxation enhancement (NMR-PRE) produced in the solvent proton resonance by manganese in the S0 and S2 states of the oxygen evolving center (OEC) has been recorded for three Photosystem II (PS II)-enriched preparations: (1) PS II-enriched thylakoid membrane fragments (TMF-2 particles); (2) salt-washed (2M NaCl) TMF-2 particles; and (3) the octylglucopyranoside (OGP)-solubilized PS II complex. The second and third preparations, but not the first, are depleted of the peripheral 17 and 23 kD polypeptides associated with the OEC. It has been proposed that depletion of these polypeptides increases the exposure of OEC manganese to the aqueous phase. The NMR-PRE response measures the quantity (T1m+τm)(-1), where T1m is the spin relaxation time and τm is the mean residence time with respect to chemical exchange reactions of solvent protons in the manganese coordination sphere, and, thus, the NMR-PRE provides a direct measure of the solvent proton chemical exchange rate constant τm (-1). This study tested whether the 17 and 23 kD polypeptides shield the OEC from the solvent phase and whether their depletion enhances the S2 and S0 NMR-PRE signals by removing a kinetic barrier to the solvent proton chemical exchange reaction. The amplitude of the S2 NMR-PRE signal, measured in its chemical exchange-limited regime (τm>T1m), is slightly decreased, rather than increased, in preparations (2) and (3) relative to (1), indicating that removal of the 17 and 23 kD polypeptides slightly slows, rather than accelerates, the rate-limiting steps of the solvent proton chemical exchange reactions. In addition, the lifetime of the S2 state was shortened several-fold in the solubilized PS II complex and in salt-washed TMF-2 membranes relative to untreated TMF-2 control samples. The S0 NMR-PRE signal, which is present in TMF-2 suspensions, was not detected in suspensions of the solubilized PS II complex, even though these samples contained high concentrations of active manganese centers (approximately double those of the TMF-2 control) and exhibited an S2 NMR-PRE signal of comparable amplitude to that of the TMF-2 preparation. These results suggest that the 17 and 23 kD extrinsic polypeptides do not shield the NMR-visible water binding site in the OEC from the aqueous phase, although their removal substantially alters the proton relaxation efficiency by shortening T1m.

Factors influencing hydroxylamine inactivation of photosynthetic water oxidation

The kinetics of Mn release during NH2OH inactivation of the water oxidizing reaction is largely insensitive to the S-state present during addition of NH2OH. This appears to reflect reduction by NH2OH of higher S-states to a common more reduced state (S0 or S-1) which alone is susceptible to NH2OH inactivation. Sequences of saturating flashes with dark intervals in the range 0.2--5 S-1 effectively prevent NH2OH inactivation and the associated liberation of manganese. This light-induced protection disappears rapidly when the dark interval is longer than about 5 S. Under continuous illumination, protection against NH2OH inactivation is maximally effective at intensities in the range 10(3)--10(4) erg . cm-2 . S-1. This behavior differs from that of NH2OH-induced Mn release, which is strongly inhibited at all intensities greater than 10(3) erg . cm-2 . S-1. This indicates that two distinct processes are responsible for inactivation of water oxidation at high and low intensities. Higher S-states appear to be immune to the reaction by which NH2OH liberates manganese, although the overall process of water oxidation is inactivated by NH2OH in the presence of intense light. The light-induced protection phenomenon is abolished by 50 microM DCMU, but not by high concentrations of carbonyl cyanide m-chlorophenylhydrazone, which accelerates inactivation reactions of the water-splitting enzyme, Y (an ADRY reagent). The latter compound accelerates both inactivation of water oxidation and manganese extraction in the dark.

NMR relaxivity changes in chloroplast suspensions. Effects of NH2OH and of treatments altering the redox state of the photosynthetic electron transport chain

Treatments (illumination, chemical oxidation or reduction) which are potentially capable of producing paramagnetic centers in chloroplast thylakoid membranes do not produce enhancements of the proton magnetic relaxivities of these preparations. However, exposure of thylakoid membranes to varying concentrations of hydroxylamine induces a time-dependent increase in relaxivity for which the steady-state magnitude is dependent on hydroxylamine concentration. The appearance of relaxivity is correlated kinetically with inactivation of oxygen-evolving centers; in addition both processes show a threshold effect with respect to hydroxylamine concentration. Kinetic analyses of these hydroxylamine-induced effects suggest that at low (less than or equal to 100 microM) and at intermediate (200--500 microM) concentrations, hydroxylamine extraction is partially counteracted by a reverse process that reactivates oxygen-evolving centers in the dark.

Field-dispersion profiles of the proton spin-lattice relaxation rate in chloroplast suspensions. Effect of manganese extraction by EDTA, Tris, and hydroxylamine

Proton spin-lattice relaxation rates (R1) have been measured in a variety of dark-adapted chloroplast suspensions over a range of field stengths between 1 and 15 kG (4-65 MHz). When the effects of EDTA or Tris washing on chloroplast relaxivities are compared, the pool of Mn associated with oxygen evolution is seen not to contribute significantly to relaxivity. Instead, nearly all of the observed relaxivity, which is characterized by a paramagnetic maximum near 20.7 MHz in the field dispersion profile of R1, appears to arise from contaminating non-functional Mn(II) that can be removed by EDTA during the isolation procedure. These observations, which contradict previous reports ascribing chloroplast relaxivity to the water-oxidizing system, require a reevaluation of proposed models, derived from NMR studies, of the state of Mn in the water-splitting reaction. Chloroplasts from which loosely bound non-functional Mn has been removed by EDTA washing do show an enhancement of relaxivity when exposed to NH2OH at concentrations known to inactivate water oxidation. This NH2OH-induced relaxivity is comprised of Mn(II) in two distinct paramagnetic sites. One site is chelatable by EDTA, whereas the other site is not. This finding suggests that some Mn(II) tightly bound to thylakoid membranes can contribute to relaxivity after inactivation of the oxygen-evolving reaction.