The decreased PG content of pgp1 inhibits PSI photochemistry and limits reaction center and light-harvesting polypeptide accumulation in response to cold acclimation.

MAIN CONCLUSION: Decreased PG constrains PSI activity due to inhibition of transcript and polypeptide abundance of light-harvesting and reaction center polypeptides generating a reversible, yellow phenotype during cold acclimation of pgp1. Cold acclimation of the Arabidopsis pgp1 mutant at 5 °C resulted in a pale-yellow phenotype with abnormal chloroplast ultrastructure compared to its green phenotype upon growth at 20 °C despite a normal cold-acclimation response at the transcript level. In contrast, wild type maintained its normal green phenotype and chloroplast ultrastructure irrespective of growth temperature. In contrast to cold acclimation of WT, growth of pgp1 at 5 °C limited the accumulation of Lhcbs and Lhcas assessed by immunoblotting. However, a novel 43 kD polypeptide of Lhcb1 as well as a 29 kD polypeptide of Lhcb3 accumulated in the soluble fraction which was absent in the thylakoid membrane fraction of cold-acclimated pgp1 which was not observed in WT. Cold acclimation of pgp1 destabilized the Chl-protein complexes associated with PSI and predisposed energy distribution in favor of PSII rather than PSI compared to the WT. Functionally, in vivo PSI versus PSII photochemistry was inhibited in cold-acclimated pgp1 to a greater extent than in WT relative to controls. Greening of the pale-yellow pgp1 was induced when cold-acclimated pgp1 was shifted from 5 to 20 °C which resulted in a marked decrease in excitation pressure to a level comparable to WT. Concomitantly, Lhcbs and Lhcas accumulated with a simultaneous decrease in the novel 43 and 29kD polypeptides. We conclude that the reduced levels of phosphatidyldiacylglycerol in the pgp1 limit the capacity of the mutant to maintain the structure and function of its photosynthetic apparatus during cold acclimation. Thus, maintenance of normal thylakoid phosphatidyldiacylglycerol levels is essential to stabilize the photosynthetic apparatus during cold acclimation.

Chlorella vulgaris integrates photoperiod and chloroplast redox signals in response to growth at high light.

MAIN CONCLUSION: Photoacclimation to variable light and photoperiod regimes in C. vulgaris represents a complex interplay between "biogenic" phytochrome-mediated sensing and "operational" redox sensing signaling pathways. Chlorella vulgaris Beijerinck UTEX 265 exhibits a yellow-green phenotype when grown under high light (HL) in contrast to a dark green phenotype when grown at low light (LL). The redox state of the photosynthetic electron transport chain (PETC) as estimated by excitation pressure has been proposed to govern this phenotypic response. We hypothesized that if the redox state of the PETC was the sole regulator of the HL phenotype, C. vulgaris should photoacclimate in response to the steady-state excitation pressure during the light period regardless of the length of the photoperiod. As expected, LL-grown cells exhibited a dark green phenotype, low excitation pressure (1 - qP = 0.22 ± 0.02), high chlorophyll (Chl) content (375 ± 77 fg Chl/cell), low Chl a/b ratio (2.97 ± 0.18) as well as high photosynthetic efficiency and photosynthetic capacity regardless of the photoperiod. In contrast, C. vulgaris grown under continuous HL developed a yellow-green phenotype characterized by high excitation pressure (1 - qP = 0.68 ± 0.01), a relatively low Chl content (180 ± 53 fg Chl/cell), high Chl a/b ratio (6.36 ± 0.54) with concomitantly reduced light-harvesting polypeptide abundance, as well as low photosynthetic capacity and efficiency measured on a per cell basis. Although cells grown under HL and an 18 h photoperiod developed a typical yellow-green phenotype, cells grown at HL but a 12 h photoperiod exhibited a dark green phenotype comparable to LL-grown cells despite exhibiting growth under high excitation pressure (1 - qP = 0.80 ± 0.04). The apparent uncoupling of excitation pressure and phenotype in HL-grown cells and a 12 h photoperiod indicates that chloroplast redox status cannot be the sole regulator of photoacclimation in C. vulgaris. We conclude that photoacclimation in C. vulgaris to HL is dependent upon growth history and reflects a complex interaction of endogenous systems that sense changes in photoperiod as well as photosynthetic redox balance.

Two Hymenophyllaceae species from contrasting natural environments exhibit a homoiochlorophyllous strategy in response to desiccation stress.

Hymenophyllaceae is a desiccation tolerant family of Pteridophytes which are poikilohydric epiphytes. Their fronds are composed by a single layer of cells and lack true mesophyll cells and stomata. Although they are associated with humid and shady environments, their vertical distribution varies along the trunk of the host plant with some species inhabiting the drier sides with a higher irradiance. The aim of this work was to compare the structure and function of the photosynthetic apparatus during desiccation and rehydration in two species, Hymenophyllum dentatum and Hymenoglossum cruentum, isolated from a contrasting vertical distribution along the trunk of their hosts. Both species were subjected to desiccation and rehydration kinetics to analyze frond phenotypic plasticity, as well as the structure, composition and function of the photosynthetic apparatus. Minimal differences in photosynthetic pigments were observed upon dehydration. Measurements of ϕPSII (effective quantum yield of PSII), ϕNPQ (quantum yield of the regulated energy dissipation of PSII), ϕNO (quantum yield of non-regulated energy dissipation of PSII), and TL (thermoluminescence) indicate that both species convert a functional photochemical apparatus into a structure which exhibits maximum quenching capacity in the dehydrated state with minimal changes in photosynthetic pigments and polypeptide compositions. This dehydration-induced conversion in the photosynthetic apparatus is completely reversible upon rehydration. We conclude that H. dentatum and H. cruentum are homoiochlorophyllous with respect to desiccation stress and exhibited no correlation between inherent desiccation tolerance and the vertical distribution along the host tree trunk.

The Antarctic Psychrophile Chlamydomonas sp. UWO 241 Preferentially Phosphorylates a Photosystem I-Cytochrome b6/f Supercomplex.

Chlamydomonas sp. UWO 241 (UWO 241) is a psychrophilic green alga isolated from Antarctica. A unique characteristic of this algal strain is its inability to undergo state transitions coupled with the absence of photosystem II (PSII) light-harvesting complex protein phosphorylation. We show that UWO 241 preferentially phosphorylates specific polypeptides associated with an approximately 1,000-kD pigment-protein supercomplex that contains components of both photosystem I (PSI) and the cytochrome b6/f (Cyt b6/f) complex. Liquid chromatography nano-tandem mass spectrometry was used to identify three major phosphorylated proteins associated with this PSI-Cyt b6/f supercomplex, two 17-kD PSII subunit P-like proteins and a 70-kD ATP-dependent zinc metalloprotease, FtsH. The PSII subunit P-like protein sequence exhibited 70.6% similarity to the authentic PSII subunit P protein associated with the oxygen-evolving complex of PSII in Chlamydomonas reinhardtii. Tyrosine-146 was identified as a unique phosphorylation site on the UWO 241 PSII subunit P-like polypeptide. Assessment of PSI cyclic electron transport by in vivo P700 photooxidation and the dark relaxation kinetics of P700(+) indicated that UWO 241 exhibited PSI cyclic electron transport rates that were 3 times faster and more sensitive to antimycin A than the mesophile control, Chlamydomonas raudensis SAG 49.72. The stability of the PSI-Cyt b6/f supercomplex was dependent upon the phosphorylation status of the PsbP-like protein and the zinc metalloprotease FtsH as well as the presence of high salt. We suggest that adaptation of UWO 241 to its unique low-temperature and high-salt environment favors the phosphorylation of a PSI-Cyt b6/f supercomplex to regulate PSI cyclic electron transport rather than the regulation of state transitions through the phosphorylation of PSII light-harvesting complex proteins.

Stress-related hormones and glycinebetaine interplay in protection of photosynthesis under abiotic stress conditions.

Plants subjected to abiotic stresses such as extreme high and low temperatures, drought or salinity, often exhibit decreased vegetative growth and reduced reproductive capabilities. This is often associated with decreased photosynthesis via an increase in photoinhibition, and accompanied by rapid changes in endogenous levels of stress-related hormones such as abscisic acid (ABA), salicylic acid (SA) and ethylene. However, certain plant species and/or genotypes exhibit greater tolerance to abiotic stress because they are capable of accumulating endogenous levels of the zwitterionic osmolyte-glycinebetaine (GB). The accumulation of GB via natural production, exogenous application or genetic engineering, enhances plant osmoregulation and thus increases abiotic stress tolerance. The final steps of GB biosynthesis occur in chloroplasts where GB has been shown to play a key role in increasing the protection of soluble stromal and lumenal enzymes, lipids and proteins, of the photosynthetic apparatus. In addition, we suggest that the stress-induced GB biosynthesis pathway may well serve as an additional or alternative biochemical sink, one which consumes excess photosynthesis-generated electrons, thus protecting photosynthetic apparatus from overreduction. Glycinebetaine biosynthesis in chloroplasts is up-regulated by increases in endogenous ABA or SA levels. In this review, we propose and discuss a model describing the close interaction and synergistic physiological effects of GB and ABA in the process of cold acclimation of higher plants.

Preferential damaging effects of limited magnesium bioavailability on photosystem I in Sulla carnosa plants.

MAIN CONCLUSION: Magnesium deficiency preferentially inhibits photosystem I rather than photosystem II in Sulla carnosa plants. The effects of magnesium (Mg(2+)) deficiency on growth, photosynthetic performance, pigment and polypeptide composition of chloroplast membranes were studied in the halophyte Sulla carnosa (Desf.), an annual legume endemic to Tunisia and Algeria. The results demonstrate a gradual decrease in biomass production with decreasing Mg(2+) availability in the growth medium. The increase of Mg(2+) deficiency was also associated with a decline of the net CO2 assimilation (Pn) in fully expanded leaves, a decrease in the amount of photosynthetic pigments, and an increase in the lipid peroxidation in plants exposed to decreased Mg(2+) concentrations. Interestingly, while CO2 assimilation already was affected at Mg(2+) concentrations below 1.5 mM, the photochemical efficiency of photosystem II (PSII) declined only in the absence of Mg(2+). In contrast, plants of S. carnosa grown in Mg(2+)-deficient conditions exhibited a significant decrease in photosystem I (PSI) photochemistry in vivo at much higher Mg(2+) levels compared to PSII photochemical activity. The inhibitory effect of Mg(2+) deficiency on PSI photochemistry strongly correlated with significantly lower relative abundance of PSI-related chlorophyll-protein complexes and lower amounts of PSI-associated polypeptides, PsaA, PsaB, and Lhca proteins within the same range of Mg(2+) concentrations. These observations were associated with a higher intersystem electron pool size, restricted linear electron transport and a lower rate of reduction of P700(+) in the dark indicating restricted capacity for PSI cyclic electron transfer in plants exposed to Mg(2+)-deficient conditions compared to controls. These results clearly indicate that PSI, rather than PSII is preferentially targeted and damaged under Mg(2+)-deficiency conditions.

Absence of the major light-harvesting antenna proteins alters the redox properties of photosystem II reaction centres in the chlorina F2 mutant of barley.

Although the chlorina F2 mutant of barley specifically exhibits reduced levels of the major light-harvesting polypeptides associated with photosystem II (PSII), thermoluminescence measurements of photosystem reaction centre photochemistry revealed that S2/S3QB- charge recombinations were shifted to lower temperatures, while the characteristic peak of S2QA- charge recombinations was shifted to higher temperatures compared with wild-type (WT) barley. Thus, we show that the absence of the major light-harvesting polypeptides affects the redox properties of PSII reaction centres. Radiolabeling studies in vivo and in vitro with [32P]orthophosphate or [gamma-32P]ATP, respectively, demonstrated that the D1 PSII reaction centre polypeptide is phosphorylated in both the WT and the F2 mutant. In contrast with the radiolabeling results, phosphorylation of D1 and other PSII proteins, although detected in WT barley, was ambiguous in the F2 mutant when the phosphothreonine antibody method of detection was used. Thus, caution must be exercised in the use of commercially available phosphothreonine antibodies to estimate thylakoid polypeptide phosphorylation. Furthermore, in membrano, the D1 polypeptide of the F2 mutant was less susceptible to trypsin treatment than that of WT barley. The role of the light-harvesting complex in modulating the structure and function of the D1 polypeptide of PSII reaction centers is discussed.

The induction of CP43' by iron-stress in Synechococcus sp. PCC 7942 is associated with carotenoid accumulation and enhanced fatty acid unsaturation.

Comparative lipid analysis demonstrated reduced amount of PG (50%) and lower ratio of MGDG/DGDG in iron-stressed Synechococcus sp. PCC 7942 cells compared to cells grown under iron sufficient conditions. In parallel, the monoenoic (C:1) fatty acids in MGDG, DGDG and PG increased from 46.8%, 43.7% and 45.6%, respectively in control cells to 51.6%, 48.8% and 48.7%, respectively in iron-stressed cells. This suggests increased membrane dynamics, which may facilitate the diffusion of PQ and keep the PQ pool in relatively more oxidized state in iron-stressed compared to control cells. This was confirmed by chlorophyll fluorescence and thermoluminescence measurements. Analysis of carotenoid composition demonstrated that the induction of isiA (CP43') protein in response to iron stress is accompanied by significant increase of the relative abundance of all carotenoids. The quantity of carotenoids calculated on a Chl basis increased differentially with nostoxanthin, cryptoxanthin, zeaxanthin and beta-carotene showing 2.6-, 3.1-, 1.9- and 1.9-fold increases, respectively, while the relative amount of caloxanthin was increased only by 30%. HPLC analyses of the pigment composition of Chl-protein complexes separated by non-denaturating SDS-PAGE demonstrated even higher relative carotenoids content, especially of cryptoxanthin, in trimer and monomer PSI Chl-protein complexes co-migrating with CP43' from iron-stressed cells than in PSI complexes from control cells where CP43' is not present. This implies a carotenoid-binding role for the CP43' protein which supports our previous suggestion for effective energy quenching and photoprotective role of CP43' protein in cyanobacteria under iron stress.

Psychrophily is associated with differential energy partitioning, photosystem stoichiometry and polypeptide phosphorylation in Chlamydomonas raudensis.

Chlamydomonas raudensis UWO 241 and SAG 49.72 represent the psychrophilic and mesophilic strains of this green algal species. This novel discovery was exploited to assess the role of psychrophily in photoacclimation to growth temperature and growth irradiance. At their optimal growth temperatures of 8 degrees C and 28 degrees C respectively, UWO 241 and SAG 49.72 maintained comparable photostasis, that is energy balance, as measured by PSII excitation pressure. Although UWO 241 exhibited higher excitation pressure, measured as 1-qL, at all growth light intensities, the relative changes in 1-qL were similar to that of SAG 49.72 in response to growth light. In response to suboptimal temperatures and increased growth irradiance, SAG 49.72 favoured energy partitioning of excess excitation energy through inducible, down regulatory processes (Phi(NPQ)) associated with the xanthophyll cycle and antenna quenching, while UWO 241 favoured xanthophyll cycle-independent energy partitioning through constitutive processes involved in energy dissipation (Phi(NO)). In contrast to SAG 49.72, an elevation in growth temperature induced an increase in PSI/PSII stoichiometry in UWO 241. Furthermore, SAG 49.72 showed typical threonine-phosphorylation of LHCII, whereas UWO 241 exhibited phosphorylation of polypeptides of comparable molecular mass to PSI reaction centres but the absence of LHCII phosphorylation. Thus, although both strains maintain an energy balance irrespective of their differences in optimal growth temperatures, the mechanisms used to maintain photostasis were distinct. We conclude that psychrophily in C. raudensis is complex and appears to involve differential energy partitioning, photosystem stoichiometry and polypeptide phosphorylation.

IMMUTANS does not act as a stress-induced safety valve in the protection of the photosynthetic apparatus of Arabidopsis during steady-state photosynthesis.

IMMUTANS (IM) encodes a thylakoid membrane protein that has been hypothesized to act as a terminal oxidase that couples the reduction of O(2) to the oxidation of the plastoquinone (PQ) pool of the photosynthetic electron transport chain. Because IM shares sequence similarity to the stress-induced mitochondrial alternative oxidase (AOX), it has been suggested that the protein encoded by IM acts as a safety valve during the generation of excess photosynthetically generated electrons. We combined in vivo chlorophyll fluorescence quenching analyses with measurements of the redox state of P(700) to assess the capacity of IM to compete with photosystem I for intersystem electrons during steady-state photosynthesis in Arabidopsis (Arabidopsis thaliana). Comparisons were made between wild-type plants, im mutant plants, as well as transgenics in which IM protein levels had been overexpressed six (OE-6 x) and 16 (OE-16 x) times. Immunoblots indicated that IM abundance was the only major variant that we could detect between these genotypes. Overexpression of IM did not result in increased capacity to keep the PQ pool oxidized compared to either the wild type or im grown under control conditions (25 degrees C and photosynthetic photon flux density of 150 micromol photons m(-2) s(-1)). Similar results were observed either after 3-d cold stress at 5 degrees C or after full-leaf expansion at 5 degrees C and photosynthetic photon flux density of 150 micromol photons m(-2) s(-1). Furthermore, IM abundance did not enhance protection of either photosystem II or photosystem I from photoinhibition at either 25 degrees C or 5 degrees C. Our in vivo data indicate that modulation of IM expression and polypeptide accumulation does not alter the flux of intersystem electrons to P(700)(+) during steady-state photosynthesis and does not provide any significant photoprotection. In contrast to AOX1a, meta-analyses of published Arabidopsis microarray data indicated that IM expression exhibited minimal modulation in response to myriad abiotic stresses, which is consistent with our functional data. However, IM exhibited significant modulation in response to development in concert with changes in AOX1a expression. Thus, neither our functional analyses of the IM knockout and overexpression lines nor meta-analyses of gene expression support the model that IM acts as a safety valve to regulate the redox state of the PQ pool during stress and acclimation. Rather, IM appears to be strongly regulated by developmental stage of Arabidopsis.

Digalactosyl-diacylglycerol deficiency impairs the capacity for photosynthetic intersystem electron transport and state transitions in Arabidopsis thaliana due to photosystem I acceptor-side limitations.

Compared with wild type, the dgd1 mutant of Arabidopsis thaliana exhibited a lower amount of PSI-related Chl-protein complexes and lower abundance of the PSI-associated polypeptides, PsaA, PsaB, PsaC, PsaL and PsaH, with no changes in the levels of Lhca1-4. Functionally, the dgd1 mutant exhibited a significantly lower light-dependent, steady-state oxidation level of P700 (P700(+)) in vivo, a higher intersystem electron pool size, restricted linear electron transport and a higher rate of reduction of P700(+) in the dark, indicating an increased capacity for PSI cyclic electron transfer compared with the wild type. Concomitantly, the dgd1 mutant exhibited a higher sensitivity to and incomplete recovery of photoinhibition of PSI. Furthermore, dgd1 exhibited a lower capacity to undergo state transitions compared with the wild type, which was associated with a higher reduction state of the plastoquinone (PQ) pool. We conclude that digalactosyl-diacylglycerol (DGDG) deficiency results in PSI acceptor-side limitations that alter the flux of electrons through the photosynthetic electron chain and impair the regulation of distribution of excitation energy between the photosystems. These results are discussed in terms of thylakoid membrane domain reorganization in response to DGDG deficiency in A. thaliana.

Iron deficiency in cyanobacteria causes monomerization of photosystem I trimers and reduces the capacity for state transitions and the effective absorption cross section of photosystem I in vivo.

The induction of the isiA (CP43') protein in iron-stressed cyanobacteria is accompanied by the formation of a ring of 18 CP43' proteins around the photosystem I (PSI) trimer and is thought to increase the absorption cross section of PSI within the CP43'-PSI supercomplex. In contrast to these in vitro studies, our in vivo measurements failed to demonstrate any increase of the PSI absorption cross section in two strains (Synechococcus sp. PCC 7942 and Synechocystis sp. PCC 6803) of iron-stressed cells. We report that iron-stressed cells exhibited a reduced capacity for state transitions and limited dark reduction of the plastoquinone pool, which accounts for the increase in PSII-related 685 nm chlorophyll fluorescence under iron deficiency. This was accompanied by lower abundance of the NADP-dehydrogenase complex and the PSI-associated subunit PsaL, as well as a reduced amount of phosphatidylglycerol. Nondenaturating polyacrylamide gel electrophoresis separation of the chlorophyll-protein complexes indicated that the monomeric form of PSI is favored over the trimeric form of PSI under iron stress. Thus, we demonstrate that the induction of CP43' does not increase the PSI functional absorption cross section of whole cells in vivo, but rather, induces monomerization of PSI trimers and reduces the capacity for state transitions. We discuss the role of CP43' as an effective energy quencher to photoprotect PSII and PSI under unfavorable environmental conditions in cyanobacteria in vivo.

Characterization of the photosynthetic apparatus in cortical bark chlorenchyma of Scots pine.

Winter-induced inhibition of photosynthesis in Scots pine (Pinus sylvestris L.) needles is accompanied by a 65% reduction of the maximum photochemical efficiency of photosystem II (PSII), measured as Fv/Fm, but relatively stable photosystem I (PSI) activity. In contrast, the photochemical efficiency of PSII in bark chlorenchyma of Scots pine twigs was shown to be well preserved, while PSI capacity was severely decreased. Low-temperature (77 K) chlorophyll fluorescence measurements also revealed lower relative fluorescence intensity emitted from PSI in bark chlorenchyma compared to needles regardless of the growing season. Nondenaturating SDS-PAGE analysis of the chlorophyll-protein complexes also revealed much lower abundance of LHCI and the CPI band related to light harvesting and the core complex of PSI, respectively, in bark chlorenchyma. These changes were associated with a 38% reduction in the total amount of chlorophyll in the bark chlorenchyma relative to winter needles, but the Chl a/b ratio and carotenoid composition were similar in the two tissues. As distinct from winter pine needles exhibiting ATP/ADP ratio of 11.3, the total adenylate content in winter bark chlorenchyma was 2.5-fold higher and the estimated ATP/ADP ratio was 20.7. The photochemical efficiency of PSII in needles attached to the twig recovered significantly faster (28-30 h) then in detached needles. Fluorescence quenching analysis revealed a high reduction state of Q(A) and the PQ-pool in the green bark tissue. The role of bark chlorenchyma and its photochemical performance during the recovery of photosynthesis from winter stress in Scots pine is discussed.

Glycinebetaine protects the D1/D2/Cytb559 complex of photosystem II against photo-induced and heat-induced inactivation.

The presence of 1.0 mol/L glycinebetaine during isolation of D1/D2/Cytb559 reaction centre (RC) complexes from photosystem II (PSII) membrane fragments preserved the photochemical activity, monitored as the light-induced reduction of pheophytin and electron transport from diphenylcarbazide to 2.6-dichlorophenol-indophenol.-Glycinebetaine also protected the D1/D2/Cytb559 complexes against strong light-induced damage to the photochemical reactions and the irreversible bleaching of beta-carotene and chlorophyll. The presence of glycinebetaine also enhanced thermotolerance of the D1/D2/Cytb559 complexes isolated in the presence of 1.0 mol/L betaine with an increase in the temperature for 50% inactivation from 29 degrees C to 35 degrees C. The results indicate an increased supramolecular structural stability in the presence of glycinebetaine.

Stoichiometry of the photosynthetic apparatus and phycobilisome structure of the cyanobacterium Plectonema boryanum UTEX 485 are regulated by both light and temperature.

The role of growth temperature and growth irradiance on the regulation of the stoichiometry and function of the photosynthetic apparatus was examined in the cyanobacterium Plectonema boryanum UTEX 485 by comparing mid-log phase cultures grown at either 29 degrees C/150 micromol m(-2) s(-1), 29 degrees C/750 micromol m(-2) s(-1), 15 degrees C/150 micromol m(-2) s(-1), or 15 degrees C/10 micromol m(-2) s(-1). Cultures grown at 29 degrees C/750 micromol m(-2) s(-1) were structurally and functionally similar to those grown at 15 degrees C/150 micromol m(-2) s(-1), whereas cultures grown at 29 degrees C/150 micromol m(-2) s(-1) were structurally and functionally similar to those grown at 15 degrees C/10 micromol m(-2) s(-1). The stoichiometry of specific components of the photosynthetic apparatus, such as the ratio of photosystem (PS) I to PSII, phycobilisome size and the relative abundance of the cytochrome b(6)/f complex, the plastoquinone pool size, and the NAD(P)H dehydrogenase complex were regulated by both growth temperature and growth irradiance in a similar manner. This indicates that temperature and irradiance may share a common sensing/signaling pathway to regulate the stoichiometry and function of the photosynthetic apparatus in P. boryanum. In contrast, the accumulation of neither the D1 polypeptide of PSII, the large subunit of Rubisco, nor the CF(1) alpha-subunit appeared to be regulated by the same mechanism. Measurements of P700 photooxidation in vivo in the presence and absence of inhibitors of photosynthetic electron transport coupled with immunoblots of the NAD(P)H dehydrogenase complex in cells grown at either 29 degrees C/750 micromol m(-2) s(-1) or 15 degrees C/150 micromol m(-2) s(-1) are consistent with an increased flow of respiratory electrons into the photosynthetic intersystem electron transport chain maintaining P700 in a reduced state relative to cells grown at either 29 degrees C/150 micromol m(-2) s(-1) or 15 degrees C/10 micromol m(-2) s(-1). These results are discussed in terms of acclimation to excitation pressure imposed by either low growth temperature or high growth irradiance.

Iron stress responses in the cyanobacterium Synechococcus sp. PCC7942.

In the present study, we describe the sequential events by which the cyanobacterium Synechococcus sp. PCC 7942 adapts to iron deficiency. In doing so, we have tried to elucidate both short and long-term acclimation to low iron stress in order to understand how the photosynthetic apparatus adjusts to low iron conditions. Our results show that after an initial step, where CP43' is induced and where ferredoxin is partly replaced by flavodoxin, the photosynthetic unit starts to undergo major rearrangements. All measured components of Photosystem I (PSI), PSII and cytochrome (Cyt) f decrease relative to chlorophyll (Chl) a. The photochemical efficiencies of the two photosystems also decline during this phase of acclimation. The well-known drop in phycobilisome content measured as phycocyanin (PC)/Chl was not due to an increased degradation, but rather to a decreased rate of synthesis. The largest effects of iron deficiency were observed on PSI, the most iron-rich structure of the photosynthetic apparatus. In the light of the recent discovery of an iron deficiency induced CP43' ring around PSI a possible dual function of this protein as both an antenna and a quencher is discussed. We also describe the time course of a blue shift in the low temperature Chl emission peak around 715 nm, which originates in PSI. The shift might reflect the disassembly and/or degradation of PSI during iron deficiency and, as a consequence, PSI might under these conditions be found predominantly in a monomeric form. We suggest that the observed functional and compositional alterations represent cellular acclimation enabling growth and development under iron deficiency, and that growth ceases when the acclimation capacity is exhausted. However, the cells remain viable even after growth has ceased, since they resumed growth once iron was added back to the culture.

The Antarctic psychrophile, Chlamydomonas subcaudata, is deficient in state I-state II transitions.

State I-State II transitions were monitored in vivo and in vitro in the Antarctic, psychrophillic, green alga, Chlamydomonas subcaudata, as changes in the low-temperature (77 K) chlorophyll fluorescence emission maxima at 722 nm (F722) relative to 699 nm (F699). As expected, the control mesophillic species, Chlamydomonas reinhardtii, was able to modulate the light energy distribution between photosystem II and photosystem I in response to exposure to four different conditions: (i) dark/anaerobic conditions, (ii) a change in Mg2+ concentration, (iii) red light, and (iv) increased incubation temperature. This was correlated with the ability to phosphorylate both of its major light-harvesting polypeptides. In contrast, exposure of C. subcaudata to the same four conditions induced minimum alterations in the 77 K fluorescence emission spectra, which was correlated with the ability to phosphorylate only one of its major light-harvesting polypeptides. Thus, C. subcaudata appears to be deficient in the ability to undergo a State I-State II transition. Functionally, this is associated with alterations in the apparent redox status of the intersystem electron transport chain and with higher rates of photosystem I cyclic electron transport in the psychrophile than in the mesophile, based on in vivo P700 measurements. Structurally, this deficiency is associated with reduced levels of Psa A/B relative to D1, the absence of specific photosystem I light-harvesting polypeptides [R.M. Morgan et al. (1998) Photosynth Res 56:303-314] and a cytochrome b6/f complex that exhibits a form of cytochrome f that is approximately 7 kDa smaller than that observed in C. reinhardtii. We conclude that the Antarctic psychrophile, C. subcaudata, is an example of a natural variant deficient in State I-State II transitions.