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PubMed1991

The PQ/PQH2 ratio and occupancy of photosystem II-QB site by plastoquinone control the degradation of D1 protein during photoinhibition in vivo.

Photoinactivation of photosystem II (PSII) and light-dependent degradation of the reaction center II (RCII) protein D1 have been investigated in Chlamydomonas reinhardtii mutants D6, AC208, and B4 deficient in cytochrome b6/f, plastocyanin, and photosystem I (PSI) activity, respectively. These mutants possess active PSII and reduce plastoquinone (PQ) but cannot oxidize plastoquinol (PQH2) via light-dependent reduction of NADP. In light-exposed cells a high ratio PQH2/PQ and a low turnover of PQ/PQH2 at the RCII-QB site are maintained. In all mutants photoinactivation of RCII was slower as compared to the wild-type (wt) cells, and D1 degradation was drastically decreased. The degradation of D1 was also lower in the wt cells under anaerobic conditions and presence of ascorbate, while raising the concentration of dissolved oxygen increased the degradation of the D1 protein in the AC208 mutant. Photoinactivation and light-dependent degradation of the D1 protein were drastically increased in the Scenedesmus obliquus LF-1 mutant cells altered in its PSII manganese binding and thus unable to reduce PQ using water as an electron donor. Diuron inhibited the light-dependent degradation of D1 protein in both the LF-1 mutant and wt cells. Based on these results we propose that availability of PQ at the QB site is required for (i) the photoinactivation process of the RCII acceptor side followed by inactivation of the donor side leading to the generation of harmful cation radicals (Z+, P680+, chlz+) which damage the D1 protein, and (ii) the accessibility of the cleavage site of the damaged D1 protein to proteolytic degradation.

Animals↗

PubMed1989

Antenna structure and excitation dynamics in photosystem I. II. Studies with mutants of Chlamydomonas reinhardtii lacking photosystem II.

Using time-resolved single photon counting, fluorescence decay in photosystem I (PS I) was analyzed in mutant strains of Chlamydomonas reinhardtii that lack photosystem II. Two strains are compared: one with a wild-type PS I core antenna (120 chlorophyll a/P700) and a second showing an apparent reduction in core antenna size (60 chlorophyll a/P700). These data were calculated from the lifetimes of core antenna excited states (75 and 45 ps, respectively) and from pigment stoichiometries. Fluorescence decay in wild type PS I is composed of two components: a fast 75-ps decay that represents the photochemically limited lifetime of excited states in the core antenna, and a minor (less than 10%) 300-800 ps component that has spectral characteristics of both peripheral and core antenna pigments. Temporal and spectral properties of the fast PS I decay indicate that (a) excitations are nearly equilibrated among the range of spectral forms present in the PS I core antenna, (b) an average excitation visits a representative distribution of core antenna spectral forms on all pigment-binding subunits regardless of the origin of the excitation, (c) reduction in core antenna size does not alter the range of antenna spectral forms present, and (d) transfer from peripheral antennae to the PS I core complex is rapid (less than 5 ps).

Chlamydomonas↗

PubMed2003

Light stress-induced one-helix protein of the chlorophyll a/b-binding family associated with photosystem I.

The superfamily of light-harvesting chlorophyll a/b-binding (Lhc) proteins in higher plants and green algae is composed of more than 20 different antenna proteins associated either with photosystem I (PSI) or photosystem II (PSII). Several distant relatives of this family with conserved chlorophyll-binding residues and proposed photoprotective functions are induced transiently under various stress conditions. Whereas "classical" Lhc proteins contain three-transmembrane alpha-helices, their distant relatives span the membrane with between one and four transmembrane segments. Here, we report the identification and isolation of a novel member of the Lhc family from Arabidopsis with one predicted transmembrane alpha-helix closely related to helix I of Lhc protein from PSI (Lhca4) that we named Ohp2 (for a second one-helix protein of Lhc family described from higher plants). We showed that the Ohp2 gene expression is triggered by light stress and that the Ohp2 transcript and protein accumulated in a light intensity-dependent manner. Other stress conditions did not up-regulate the expression of the Ohp2 gene. Localization studies revealed that Ohp2 is associated with PSI under low- or high-light conditions. Because all stress-induced Lhc relatives reported so far were found in PSII, we propose that the accumulation of Ohp2 might represent a novel photoprotective strategy induced within PSI in response to light stress.

Amino Acid Sequence↗

PubMed2002

Polyphenolic allelochemicals from the aquatic angiosperm Myriophyllum spicatum inhibit photosystem II.

Myriophyllum spicatum (Haloragaceae) is a highly competitive freshwater macrophyte that produces and releases algicidal and cyanobactericidal polyphenols. Among them, beta-1,2,3-tri-O-galloyl-4,6-(S)-hexahydroxydiphenoyl-D-glucose (tellimagrandin II) is the major active substance and is an effective inhibitor of microalgal exoenzymes. However, this mode of action does not fully explain the strong allelopathic activity observed in bioassays. Lipophilic extracts of M. spicatum inhibit photosynthetic oxygen evolution of intact cyanobacteria and other photoautotrophs. Fractionation of the extract provided evidence for tellimagrandin II as the active compound. Separate measurements of photosystem I and II activity with spinach (Spinacia oleracea) thylakoid membranes indicated that the site of inhibition is located at photosystem II (PSII). In thermoluminescence measurements with thylakoid membranes and PSII-enriched membrane fragments M. spicatum extracts shifted the maximum temperature of the B-band (S(2)Q(B)(-) recombination) to higher temperatures. Purified tellimagrandin II in concentrations as low as 3 microM caused a comparable shift of the B-band. This demonstrates that the target site of this inhibitor is different from the Q(B)-binding site, a common target of commercial herbicides like 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Measurements with electron paramagnetic resonance spectroscopy suggest a higher redox midpoint potential for the non-heme iron, located between the primary and the secondary quinone electron acceptors, Q(A) and Q(B). Thus, tellimagrandin II has at least two modes of action, inhibition of exoenzymes and inhibition of PSII. Multiple target sites are a common characteristic of many potent allelochemicals.

Cyanobacteria↗

PubMed2001

Three-dimensional model and characterization of the iron stress-induced CP43'-photosystem I supercomplex isolated from the cyanobacterium Synechocystis PCC 6803.

The cyanobacterium Synechocystis PCC 6803 has been subjected to growth under iron-deficient conditions. As a consequence, the isiA gene is expressed, and its product, the chlorophyll a-binding protein CP43', accumulates in the cell. Recently, we have shown for the first time that 18 copies of this photosystem II (PSII)-like chlorophyll a-binding protein forms a ring around the trimeric photosystem I (PSI) reaction center (Bibby, T. S., Nield, J., and Barber, J. (2001) Nature, 412, 743-745). Here we further characterize the biochemical and structural properties of this novel CP43'-PSI supercomplex confirming that it is a functional unit of approximately 1900 kDa where the antenna size of PSI is increased by 70% or more. Using electron microscopy and single particle analysis, we have constructed a preliminary three-dimensional model of the CP43'-PSI supercomplex and used it as a framework to incorporate higher resolution structures of PSI and CP43 recently derived from x-ray crystallography. Not only does this work emphasize the flexibility of cyanobacterial light-harvesting systems in response to the lowering of phycobilisome and PSI levels under iron-deficient conditions, but it also has implications for understanding the organization of the related chlorophyll a/b-binding Pcb proteins of oxychlorobacteria, formerly known as prochlorophytes.

Bacterial Proteins↗

PubMed2005

Localization of the N-terminal domain in light-harvesting chlorophyll a/b protein by EPR measurements.

The conformational distribution of the N-terminal domain of the major light-harvesting chlorophyll a/b protein (LHCIIb) has been characterized by electron-electron double resonance yielding distances between spin labels placed in various domains of the protein. Distance distributions involving residue 3 near the N terminus turned out to be bimodal, revealing that this domain, which is involved in regulatory functions such as balancing the energy flow through photosystems (PS) I and II, exists in at least two conformational states. Models of the conformational sub-ensembles were generated on the basis of experimental distance restraints from measurements on LHCIIb monomers and then checked for consistency with the experimental distance distribution between residues 3 in trimers. Only models where residue 3 is located above the core of the protein and extends into the aqueous phase on the stromal side fit the trimer data. In the other state, which consequently is populated only in monomers, the N-terminal domain extends sideways from the protein core. The two conformational states may correspond to two functional states of LHCIIb, namely trimeric LHCIIb associated with PSII in stacked thylakoid membranes and presumably monomeric LHCIIb associated with PSI in nonstacked thylakoids. The switch between these two is known to be triggered by phosphorylation of Thr-6. A similar phosphorylation-induced conformational change of the N-terminal domain has been observed by others in bovine annexin IV which, due to the conformational switch, also loses its membrane-aggregating property.

Animals↗

PubMed1994

Role of plastoquinol oxidoreduction in regulation of photochemical reaction center IID1 protein turnover in vivo.

The light-induced turnover of the D1 protein subunit of reaction center II (RCII) was investigated in Chlamydomonas reinhardtii y-1 (control) and D6, AC208, and B4 mutants lacking cytochrome b6/f, plastocyanin or photosystem I activity, respectively, and, thus, impaired in light-dependent plastoquinol (PQH2) oxidation. Charge recombination assayed by thermoluminescence measurements indicated similar RCII properties in control and mutant cells. The D1 protein is not degraded in the mutants during photoinactivation; however, RCII-D1 is irreversibly altered, and the protein is degraded when the cells are incubated in low light permitting slow reoxidation of the PQH2 pool. Photoinactivation precedes D1 degradation also in the control cells. Thus, in vivo under physiological conditions photoinactivation and "tagging" of RCII-D1 are resolved from the degradation process. RCII activity in photoinactivated cells may be recovered only following D1 degradation and replacement. Recovery may occur either in the light or dark in the absence of de novo chlorophyll synthesis. The degradation of the photoinactivated RCII-D1 protein is a prerequisite for the synthesis and stable integration of new D1 indicating that tagged D1 is still assembled in the inactive reaction centers. The physiological implication of these results is that oxidation of the PQH2 pool in photoinactivated cells affects RCII-D1 protein degradation and replacement, and, thus, D1 turnover in vivo is regulated by the turnover of PQ at the binding site of the secondary stable electron acceptor quinone of RCII.

Animals↗

PubMed2004

Intact mass measurements for unequivocal identification of hydrophobic photosynthetic photosystems I and II antenna proteins.

The purpose of this study was to determine the optimal experimental conditions necessary to allow rapid and accurate identification of highly hydrophobic proteins, such as the antenna proteins from photosystems I and II. The antenna proteins were derived from two different species, tomato and Arabidopsis, whose photosynthetic genome is well known. The separation techniques included sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) followed by immunoblotting, microamino acid sequencing, reversed-phase high-performance liquid chromatography (RP-HPLC), intact mass measurements, and peptide mass fingerprinting by mass spectrometry. Immunoblotting was time-consuming, and success was limited due to cross-reactivity between these highly conserved sequences. The best results by far were achieved by separating intact proteins through hyphenation of reversed-phase liquid chromatography on-line to an electrospray ionization mass spectrometer (ESI-MS) and consequently identifying individual proteins from their intact mass measurements (IMMs), whereas peptide mass fingerprinting was hampered by the highly hydrophobic nature of these proteins. RP-HPLC-ESI-MS was superior in the quality of separation. Moreover, the high quality of mass spectra recorded during the RP-HPLC-ESI-MS analysis meant that the relative deviations of the molecular masses determined with a quadrupole ion trap mass analyzer ranged between 100 and 300 ppm. Thus, the correspondence between the intact mass values measured with those deduced from the DNA sequences allowed the different types of antenna proteins to be identified and assigned to their corresponding gene families. By utilizing this correlation, it was possible to spot gene products of previously cloned genes.

Arabidopsis↗

PubMed2000

Chromatographic separation of a small subunit (PsbW/PsaY) and its assignment to Photosystem I reaction center.

By using a hydroxyapatite column, the five major Photosystem I (PSI) subunits (PsaA,-B,-C,-D,-E) solubilized by sodium dodecyl sulfate (SDS) were fractionated from a spinach PSI reaction center preparation. Another small (5-6 kDa) polypeptide was also separated, and purified to homogeneity. Mass spectroscopy yielded its molecular weight to be 5942 +/- 10. This polypeptide had an N-terminal sequence homologous to those of previously reported 5-kDa subunits from spinach and wheat and a 6.1-kDa subunit of Chlamydomonas, which had all been assigned to Photosystem II (PSII) and designated as PsbW. However, we found similar 5-kDa polypeptides with highly conserved N-terminal sequences ubiquitously in PSI particles from other plants including Daikon (Raphanus sativus, Japanese radish), Chingensai (Brassica parachinensis, Chinese cabbage), parsley and Shungiku (Chrysanthemum coronarium, Garland chrysanthemum) as well. Preparations of spinach PSI particles prepared by using a mild detergent (digitonin) had this 5-kDa subunit, while PSII particles did not. Moreover, a bare-bone PSI reaction center preparation consisting of PsaA/B alone had a more than stoichiometric amount of this 5-kDa polypeptide. A mechanically (without detergent) fractionated stroma thylakoid preparation from Phytolacca americana, which lacked other PSII subunits, also contained this 5-kDa subunit. Thus, we propose that this 5-kDa polypeptide, previously designated as a PSII subunit (PsbW), is an integral subunit of PSI as well.

Amino Acid Sequence↗

PubMed2001

Cl- channel inhibitors of the arylaminobenzoate type act as photosystem II herbicides: a functional and structural study.

The Cl- channel blocker NPPB (5-nitro-2-(3-phenylpropylamino) benzoic acid) inhibited photosynthetic oxygen evolution of isolated thylakoid membranes in a pH-dependent manner with a K(i) of about 2 microM at pH 6. Applying different electron acceptors, taking electrons either directly from photosystem II (PS II) or photosystem I (PS I), the site of inhibition was localized within PS II. Measurements of fluorescence induction kinetics and thermoluminescence suggest that the binding of NPPB to the QB binding site of PS II is similar to the herbicide DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea). The effects of different arylaminobenzoate derivatives and other Cl- channel inhibitors on photosynthetic electron transport were investigated. The structure--activity relationship of the inhibitory effect on PS II shows interesting parallels to the one observed for the arylaminobenzoate block of mammalian Cl- channels. A molecular modeling approach was used to fit NPPB into the QB binding site and to identify possible molecular interactions between NPPB and the amino acid residues of the binding site in PS II. Taken together, these data give a detailed molecular picture of the mechanism of NPPB binding.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗

PubMed1996

The Chlamydomonas reinhardtii LI818 gene represents a distant relative of the cabI/II genes that is regulated during the cell cycle and in response to illumination.

In the green unicellular alga Chlamydomonas reinhardtii, as in higher plants, the expression of the genes encoding the chlorophyll a/b-binding (CAB) polypeptides associated with photosystem I (PSI) and photosystem II (PSII) is regulated by endogenous (circadian clock) and exogenous signals (light and temperature). The circadian clock ensures that the oscillation in the levels of the different cab mRNAs is continuously kept in phase with light/dark (LD) cycles and is maximal by the middle of the day. On the other hand, light controls the amplitude of the oscillations. We report here the cloning and characterization of the C. reinhardtii LI818 gene, which identifies a CAB-related polypeptide and whose expression is regulated quite differently from the cab I/II genes. We show: (1) that in LD synchronized Chlamydomonas cells LI818 mRNA accumulation is subject to dual regulation that involves separable regulation by light and an endogenous oscillator; (2) that LI818 mRNA is fully expressed several hours before the cab I/II mRNAs and that the latter accumulate concomitantly; (3) that blocking the electron flow through PSII using DCMU prevents cells from accumulating cab I/II mRNAs but not LI818 mRNA and (4) that the accumulation of LI818 mRNA is abolished by blocking cytoplasmic protein synthesis, suggesting that these regulatory mechanisms are mediated by labile proteins.

Algal Proteins↗

PubMed1994

The psaA operon pre-mRNA of the Euglena gracilis chloroplast is processed into photosystem I and II mRNAs that accumulate differentially depending on the conditions of cell growth.

The chloroplast genome of Euglena gracilis contains a psaA operon which encodes a lysine tRNA gene, trnK; psaA and psaB photosystem I genes, and psbE, psbF, psbL and psbJ photosystem II genes. The pre-mRNA of the psaA operon undergoes a complex processing pathway of 5' and 3' tRNA processing, splicing of 11 group II introns and one group II twintron, plus three intercistronic RNA cleavage events. The accumulated transcripts of the psaA operon have been characterized by Northern hybridization, S1 nuclease analysis and primer extension RNA sequencing. The mature 5' end of the psaA-psaB-psbE-psbF-psbL-psbJ hexacistronic transcript lies 8 nt downstream of the trnK gene, and is the result of intercistronic trnK-psaA cleavage. Other intercistronic processing events occur between the psaA and psaB genes and the psaB and psbE genes. Processing at the latter site produces a dicistronic mRNA of PSI genes and a tetracistronic mRNA of PSII genes. The PSI dicistronic transcript is further processed to monocistronic psaA and psaB mRNAs. Secondary structural motifs within the intercistronic regions may be recognition sites for processing. The steady-state levels of psaA operon mRNAs from Euglena grown under several different conditions have been determined. Accumulated transcripts from all growth conditions are spliced, and a proportion are also processed at the intercistronic sites. The products of intercistronic processing increase from heterotrophic dark- to heterotrophic light-grown Euglena, and from heterotrophic light- to photoautotrophic light-grown Euglena. The differential accumulation of psaA operon mRNAs may be a means of chloroplast gene regulation or, alternatively, a consequence of gene expression during chloroplast development.

Animals↗

PubMedDate not supplied

Isolation and structural analysis of two-dimensional crystals of photosystem II from Hordeum vulgare viridis zb63.

Photosystem II (PS II), found in the photosynthetic membranes of plants, has the unique ability to split water, evolving atmospheric oxygen as a byproduct. In the photosystem I deficient barley mutant, viridis zb63, PS II is found in 2D crystals but has normal activity [D. Simpson, (1983) Eur. J. Cell Biol. 31, 305-314]. We have isolated these PS II crystals from the mutants and obtained a projection map at 2.0 nm resolution. This map was compared to a projection map of PS II in crystals derived from spinach. The unit cell for the barley crystal was 16.1 x 24.1 nm; for spinach, the unit cell was 11.9 x 16.6 nm. In both cases, there was p2 symmetry and each half of the unit cell included five subareas. After isolation, the barley crystals were unstable, suggesting that, in this case, interactions across membranes within the grana are required to retain ordering of PS II. A comparison of the estimated masses within the PS II dimer from each species indicated that the two crystals probably did not contain the same complement of polypeptides, suggesting that PS II is labile. Nevertheless, the projection maps contained similar structural features, suggesting that PS II lability is restricted and that there is an underlying stable structure.

Crystallization↗

PubMed2000

Inhibition of photosynthetic electron transport by UV-A radiation targets the photosystem II complex.

We have studied the inhibition of photosynthetic electron transport by UV-A (320-400 nm) radiation in isolated spinach thylakoids. Measurements of Photosystem II (PSII) and Photosystem I activity by Clark-type oxygen electrode demonstrated that electron flow is impaired primarily in PSII. The site and mechanism of UV-A induced damage within PSII was assessed by flash-induced oxygen and thermoluminescence (TL) measurements. The flash pattern of oxygen evolution showed an increased amount of the S0 state in the dark, which indicate a direct effect of UV-A in the water-oxidizing complex. TL measurements revealed the UV-A induced loss of PSII centers in which charge recombination between the S2 state of the water oxidizing complex and the semireduced Q(A)- and Q(B)- quinone electron acceptors occur. Flash-induced oscillation of the B TL band, originating from the S2Q(B)- recombination, showed a decreased amplitude after the second flash relative to that after the first one, which is consistent with a decrease in the amount of Q(B)- relative to Q(B) in dark adapted samples. The efficiency of UV-A light in inhibiting PSII electron transport exceeds that of visible light 45-fold on the basis of equal energy and 60-fold on the basis of equal photon number, respectively. In conclusion, our data show that UV-A radiation is highly damaging for PSII, whose electron transport is affected both at the water oxidizing complex, and the binding site of the Q(B) quinone electron acceptor in a similar way to that caused by UV-B radiation.

Electron Transport↗

PubMed2000

The water-water cycle as alternative photon and electron sinks.

The water-water cycle in chloroplasts is the photoreduction of dioxygen to water in photosystem I (PS I) by the electrons generated in photosystem II (PS II) from water. In the water-water cycle, the rate of photoreduction of dioxygen in PS I is several orders of magnitude lower than those of the disproportionation of superoxide catalysed by superoxide dismutase, the reduction of hydrogen peroxide to water catalysed by ascorbate peroxidase, and the reduction of the resulting oxidized forms of ascorbate by reduced ferredoxin or catalysed by either dehydroascorbate reductase or monodehydroascorbate reductase. The water-water cycle therefore effectively shortens the lifetimes of photoproduced superoxide and hydrogen peroxide to suppress the production of hydroxyl radicals, their interactions with the target molecules in chloroplasts, and resulting photoinhibition. When leaves are exposed to photon intensities of sunlight in excess of that required to support the fixation of CO2, the intersystem electron carriers are over-reduced, resulting in photoinhibition. Under such conditions, the water-water cycle not only scavenges active oxygens, but also safely dissipates excess photon energy and electrons, in addition to downregulation of PS II and photorespiration. The dual functions of the water-water cycle for protection from photoinhibition under photon excess stress are discussed, along with its functional evolution.

Chloroplasts↗

PubMedDate not supplied

Comparative immunological detection of lipids and carotenoids on peptides of photosystem I from higher plants and cyanobacteria.

Photosystem I preparations were obtained from wild-type tobacco Nicotiana tabacum var. JWB, three chlorophyll-deficient tobacco mutants: Su/su, Su/su var. Aurea and yellow-green leaf patches of the variegated mutant NC 95, Spinacia oleracea and furthermore from the mesophilic cyanobacterium Synechococcus PCC 7942 and the thermophilic cyanobacterium Synechococcus sp.. Peptides from these preparations were analyzed by SDS polyacrylamide gel electrophoresis and transferred for detection of bound lipids and carotenoids according to the Western blot procedure to nitrocellulose membranes. The PS I preparations from the Nicotiana tabacum species and spinach consist of the core complex and the LHCP I complex, the latter containing, however, traces of the LHCP II polypeptides. The core complex consists of the two core peptides with the apparent molecular mass of 66 kDa each and peptides with molecular masses of 22, 20, 19, 17, 16, 10 and 9 kDa. The LHCP I complex contains 4 subunits with molecular masses of 28, 26, 25 and 24 kDa. The PS I preparations of the two mutants Su/su and Su/su var. Aurea contain as impurities traces of the core peptides (D1/D2) and the two chlorophyll-binding peptides (CP43/CP47) of photosystem II. The PS I preparation from the mesophilic and thermophilic cyanobacterium consists of the two core peptides with the apparent molecular mass of 66 kDa and peptides with molecular masses of 16, 14 and 10 kDa. The peptides of the PS I preparations were characterized by specific PS I, CP I and LHCP I antisera. The antiserum to the PS I complex reacts in the Western blot with the homologous peptides of PS I from higher plants, but only with the CP I complex from the two cyanobacteria. In comparative studies with PS II from higher plants the PS I antiserum reacts with the LHCP II complex as expected. The antiserum to the CP I complex reacts only with the 66 kDa peptides of PS I from all objects. There is no cross reaction with the 66 kDa peptides (heterodimer of the D1/D2 peptides) of PS II. The antiserum to the LHCP I complex reacts only with the four LHCP I peptides of PS I from higher plants and as expected with the LHCP II of PS II: Because cyanobacteria do not have LHCP complexes, there is no reaction with the LHCP I antiserum. By means of polyclonal monospecific antisera to lipids it was shown by Western blot procedure that only two lipid species are bound to PS I peptides. The galactolipid monogalactosyldiglyceride is bound to the CP I complex of the Nicotiana tabacum species, spinach and the two cyanobacteria as well as to the LHCP I complex of the higher plants. The phospholipid phosphatidylglycerol is only associated with the CP I complex of the analyzed higher plants and cyanobacteria. With polyclonal monospecific antisera to carotenoids it was demonstrated that beta-carotene, lutein, neoxanthin and zeaxanthin are associated with the CP I complex of the higher plants and the cyanobacteria analyzed. Violaxanthin is also bound to the CP I complex of the two cyanobacteria, whereas it is bound together with neoxanthin to the LHCP I complex of the higher plants.

Animals↗

PubMed2003

Role of chloroplast photosystems II and I in apoptosis of pea guard cells.

Received Revision received We investigated the CN--induced apoptosis of guard cells in epidermal peels isolated from pea (Pisum sativum L.) leaves. This process was considerably stimulated by illumination and suppressed by the herbicides DCMU (an inhibitor of the electron transfer between quinones Q(A) and Q(B) in PS II) and methyl viologen (an electron acceptor from PS I). These data favor the conclusion drawn by us earlier that chloroplasts are involved in the apoptosis of guard cells. Pea mutants with impaired PS I (Chl-5), PS II (Chl-I), and PS II + PS I (Xa-17) were tested. Their lesions were confirmed by the ESR spectra of Signal I (oxidized PS I reaction centers) and Signal II (oxidized tyrosine residue Y(D) in PS II). Destruction of nuclei (a symptom of apoptosis) and their consecutive disappearance in guard cells were brought about by CN- in all the three mutants and in the normal pea plants. These results indicate that the light-induced enhancement of apoptosis of guard cells and its removal by DCMU are associated with PS II function. The effect of methyl viologen preventing CN--induced apoptosis in wild-type plants was removed or considerably decreased upon the impairment of the PS II and/or PS I activity.

Apoptosis↗

PubMed2005

PsbS genotype in relation to coordinated function of PS II and PS I in Arabidopsis leaves.

Application of multiple probes to systems that carry specific mutations provides a powerful means for studying how known regulators of light utilization interact in vivo. Two lines of Arabidopsis thaliana were studied, each carrying a unique lesion in the nuclear psbS gene encoding a 22-kDa pigment-binding protein (PS II-S) essential for full expression of photoprotective, rapid-phase, nonphotochemical quenching of chlorophyll fluorescence (NPQ). The PS II-S protein is absent in line npq4-1 due to deletion of psbS. Line npq4-9 expresses normal levels of PS II-S but carries a single amino acid substitution that lowers NPQ capacity by about 50%. A prior report [Peterson RB and Havir EA (2001) Planta 214: 142-152] described an altered pattern of redox states of the acceptor side of Photosystem II (PS II) and donor side of Photosystem I (PS I) for npq4-9 suggesting that interphotosystem electron transport may be restricted by a higher transthylakoid DeltapH in this line. In vivo steady state fluorescence and absorbance measurements (820 nm) confirmed these earlier observations for line npq4-9 but not for npq4-1. Thus, the prior results cannot be correlated simply to a loss of NPQ capacity. Likewise, the kinetics of the 820-nm absorbance change did not indicate a substantial effect of psbS genotype on electron flow from plastoquinol to PS I. A simple model is proposed to relate linear electron transport rate (measured gasometrically) to a parameter (based on fluorescence) that provides a relative measure of the density of excitation available for photochemistry in PS II. Surprisingly, analyses using this model suggested that the in vivo midpoint potential of the primary quinone acceptor in PS II (Q(A)) is lowered in both psbS mutant lines. This heretofore-unsuspected role for PS II-S is discussed with regard to: (1) numerous prior reports indicating plasticity of the redox potential of Q(A) and (2) the basis for the contrasting regulation of quantum yields of PS I and II in npq4-1 and npq4-9.

Amino Acid Substitution↗