Acclimation of barley to changes in light intensity: photosynthetic electron transport activity and components

Barley seedlings (Hordeum vulgare L. Boone) were grown at 20°C with 16 h/8 h light/dark cycle of either high (H) intensity (500 mole m^-2 s^-1) or low (L) intensity (55 mole m^-2 s^-1) white light. Plants were transferred from high to low (H L) and low to high (L H) light intensity at various times from 4 to 8 d after leaf emergence from the soil. Primary leaves were harvested at the beginning of the photoperiod. Thylakoid membranes were isolated from 3 cm apical segments and assayed for photosynthetic electron transport, Photosystem II (PS II) atrazine-binding sites (Q_B), cytochrome f(Cytf) and the P-700 reaction center of Photosystem I (PS I). Whole chain, PS I and PS II electron transport activities were higher in H than in L controls. Q_B and Cytf were elevated in H plants compared with L plants. The acclimation of H L plants to low light occurred slowly over a period of 7 days and resulted in decreased whole chain and PS II electron transport with variable effects on PS I activity. The decrease in electron transport of H L plants was associated with a decrease in both Q_B and Cytf. In L H plants, acclimation to high light occurred slowly over a period of 7 days with increased whole chain, PS I and PS II activities. The increase in L H electron transport was associated with increased levels of Q_B and Cytf. In contrast to the light intensity effects on Q_B levels, the P-700 content was similar in both control and transferred plants. Therefore, PS II/PS I ratios were dependent on light environment.Abbreviations Asc ascorbate - BQ 2,5-dimethyl-p-benzoquinone - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCIP 2,6-dichlorophenolindophenol - H control plants grown under high light intensity - H L plants transferred from high to low light intensity - L low control plants grown under low light intensity - L H plants transferred from low to high light intensity - MV methyl viologen - P-700 photoreaction center of Photosystem I - Q_B atrazine binding site - TMPD N,N,N,N-tetramethyl-p-phenylenediamine Cooperative investigations of the United States Department of Agriculture, Agricultural Research Service, and the North Carolina Agricultural Research Service, Raleigh, NC. Paper No. 11990 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7643, USA.     

Analysis of chlorophyll a fluoresence changes in weak light in heat treated Amaranthus chloroplasts

After preheating of Amaranthus chloroplasts at elevated temperatures (up to 45°C), the chlorophyll a fluorescence level under low excitation light rises as compared to control (unheated) as observed earlier in other chloroplasts (Schreiber U and Armond PA (1978) Biochim Biophys Acta 502: 138–151). This elevation of heat induced fluorescence yield is quenched by addition of 0.1 mM potassium ferricyanide, suggesting that with mild heat stress the primary electron acceptor of photosystem II is more easily reduced than the unheated samples. Furthermore, the level of fluorescence attained after illumination of dithionite-treated samples is independent of preheating (up to 45°C). Thus, these experiments indicate that the heat induced rise of fluorescence level at low light can not be due to changes in the elevation in the true constant F₀ level, that must by definition, be independent of the concentration of Q_A. It is supposed that the increase in the fluorescence level by weak modulated light is either partly associated with dark reduction of Q_A due to exposure of chloroplasts to elevated temperature or due to temperature induced fluorescence rise in the so called inactive photosystem II centre where Q_A are not connected to plastoquinone pool. In the presence of dichlorophenyldimethylurea the fluorescence level triggered by weak modulated light increases at alkaline pH, both in control and heat stressed chloroplasts. This result suggests that the alkaline pH accelerates electron donation from secondary electron donor of photosystem II to Q_A both in control and heat stressed samples. Thus the increase in fluorescence level probed by weak modulated light due to preheating is not solely linked to increase in true F₀ level, but largely associated with the shift in the redox state of Q_A, the primary stable electron acceptor of photosystem II.Abbreviations ADRY Acceleration of Deactivation of Reaction of Enzyme Y - CCCP Carbonyl cyanide 4-(trifluoromethoxy)-phenylhydrazone - Chl Chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - FeCN potassium ferricyanide - HEPES 4-(2-hydroxy ethyl)-1-piperazine ethane sulfonic acid - LHCP Light harvesting chlorophyll protein - MES (4-morpholine ethane sulfonic acid) - PS photosystem - Q_A and Q_B first and second consecutive electron acceptors of photosystem II - TES (2-[tris(hydroxymethyl)-methylamino]-1-ethanesulfonic acid) sulfonic acid - TRICINE N-[tris(hydroxymethyl)methyl] glycine     

The distribution of NADPH-protochlorophyllide oxidoreductase in relation to chlorophyll accumulation along the barley leaf gradient

The possible regulatory role of NADPH-protochlorophyllide oxidoreductase for chlorophyll accumulation has been investigated in barley plants. Within the primary leaf of etiolated plants the different maturation stages of etioplasts are found in a linear series with the youngest in cells near the base and the oldest in cells near the tip. This distribution of different plastid forms is paralleled by drastic differences in the NADPH-protochlorophyllide-oxidoreductase content of the plastids and their capacity to accumulate chlorophyll during illumination. The amount of enzyme and the rate of chlorophyll accumulation are highest in the mature etioplast in the tip of the leaf and both decline rapidly with decreasing age of the leaf tissue, being almost undetectable in the leaf base. The translatable mRNA coding for the enzyme shows a different distribution pattern within the leaf. The highest concentration is found in the middle part of the leaf while in the top part only traces of this mRNA are detectable. It is concluded that during leaf development the enzyme is synthesized rapidly only during a limited time period and that it is stored subsequently in the mature etioplast as a stable protein. The close correlation between the distribution of the enzyme within the barley leaf and that of the potential to accumulate chlorophyll during illumination would favour a control of chlorophyll accumulation by the amount of NADPH-protochlorophyllide oxidoreductase. Dark-grown plants which were exposed to far-red light were used to test this possibility. The far-red-absorbing form of phytochrome (P_fr) has an inverse effect on the kinetics of chlorophyll accumulation and the enzyme concentration. Our results indicate that the rate of chlorophyll accumulation in barley is not determined by the level of NADPH-protochlorophyllide oxidoreductase present in the leaves.     

Acclimation of the diatom Stephanodiscus neoastraea and the cyanobacterium Planktothrix agardhii to simulated natural light fluctuations

Functional and structural characteristics of the photosynthetic apparatus were studied in the diatom Stephanodiscus neoastraea and the cyanobacterium Planktothrix agardhii which were grown semi-continuously under constant irradiance or under simulated natural light fluctuations. The light fluctuations consisted of 24 oscillations of exponentially increasing and decreasing irradiance over a 12-h light period. Maximum irradiance was 1100 μmol photons m⁻² s⁻¹ with the ratio of maximum to minimum intensities being 100, simulating Langmuir circulations with a ratio of euphotic to mixing depth of 1. S. neoastraea acclimated to the light fluctuations by doubling the number and halving the size of photosynthetic units (PS II) while the amount of chlorophylls and carotenoids remained unchanged. The chlorophyll-specific maximum photosynthetic rate was enhanced while the slope of the photosynthesis versus irradiance curves was not influenced by the light fluctuations. Acclimation of P. agardhii was mainly characterized by an increase in chlorophyll content. Both photosystems showed only little changes in number and size. Maximum photosynthetic rate, saturating irradiance and initial slope of the photosynthesis versus irradiance curves did not vary. Although both high and low light were contained in the fluctuating light, an analogy to low or high light acclimation was not found for the diatom nor for the cyanobacterium acclimated to light fluctuations. We suggest that the acclimation to fluctuating light is a response type outside the known scheme of low and high light acclimation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Evidence for a light-harvesting chlorophyll a-protein complex in a chlorophyll b-less barley mutant

Chloroplasts of a chlorophyll (Chl) b-less barley mutant were solubilized with digitonin and fractionated by polyacrylamide gel electrophoresis with sodium deoxycholate in the running buffer. By this procedure, in contrast to using sodium dodecylsulfate (SDS) for solubilization, a Chl a-protein analogous to the major light-harvesting Chl a-b protein complex from wildtype chloroplasts was recovered. This mutant Chl a-protein comprises about fifty percent of the total Chl a, and is very similar in carotenoid, amino acid, protein and polypeptide composition to the major wildtype antenna Chl a-b protein. The only major differences we have found is its instability in the presence of SDS and sensitivity to protease action. Even with deoxycholate, the mutant Chl a complex often dissociates during electrophoresis into two green bands. The lack of Chl b appears to affect the normal organization of Chl a and protein in such a way as to render the complex more unstable.CIW-DPB No. 917.     

Light-induced fluorescence quenching in the light-harvesting chlorophyll a/b protein complex

Summary Irradiation of the principal photosystem II light-harvesting chlorophyll-protein antenna complex, LHC II, with high light intensities brings about a pronounced quenching of the chlorophyll fluorescence. Illumination of isolated thylakoids with high light intensities generates the formation of quenching centres within LHC II in vivo, as demonstrated by fluorescence excitation spectroscopy. In the isolated complex it is demonstrated that the light-induced fluorescence quenching: a) shows a partial, biphasic reversibility in the dark; b) is approximately proportional to the light intensity; c) is almost independent of temperature in the range 0–30°C; d) is substantially insensitive to protein modifying reagents and treatments; e) occurs in the absence of oxygen. A possible physiological importance of the phenomenon is discussed in terms of a mechanism capable of dissipating excess excitation energy within the photosystem II antenna.Abbreviations chla chlorophyll a - chlb chlorophyll b - F₀ fluorescence yield with reaction centers open - F_m fluorescence yield with reaction centres closed - F_i fluorescence at the plateau level of the fast induction phase - LHC II light-harvesting chlorophyll a/b protein complex II - PS II photosystem II - PSI photosystem I - Tricine N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine     

Photosystem II chlorophyll a fluorescence lifetimes and intensity are independent of the antenna size differences between barley wild-type and chlorina mutants: Photochemical quenching and xanthophyll cycle-dependent nonphotochemical quenching of fluorescence

Photosystem II (PS II) chlorophyll (Chl) a fluorescence lifetimes were measured in thylakoids and leaves of barley wild-type and chlorina f104 and f2 mutants to determine the effects of the PS II Chl a+b antenna size on the deexcitation of absorbed light energy. These barley chlorina mutants have drastically reduced levels of PS II light-harvesting Chls and pigment-proteins when compared to wild-type plants. However, the mutant and wild-type PS II Chl a fluorescence lifetimes and intensity parameters were remarkably similar and thus independent of the PS II light-harvesting antenna size for both maximal (at minimum Chl fluorescence level, F_o) and minimal rates of PS II photochemistry (at maximum Chl fluorescence level, F_m). Further, the fluorescence lifetimes and intensity parameters, as affected by the trans-thylakoid membrane pH gradient (pH) and the carotenoid pigments of the xanthophyll cycle, were also similar and independent of the antenna size differences. In the presence of a pH, the xanthophyll cycle-dependent processes increased the fractional intensity of a Chl a fluorescence lifetime distribution centered around 0.4–0.5 ns, at the expense of a 1.6 ns lifetime distribution (see Gilmore et al. (1995) Proc Natl Acad Sci USA 92: 2273–2277). When the zeaxanthin and antheraxanthin concentrations were measured relative to the number of PS II reaction center units, the ratios of fluorescence quenching to [xanthophyll] were similar between the wild-type and chlorina f104. However, the chlorina f104, compared to the wild-type, required around 2.5 times higher concentrations of these xanthophylls relative to Chl a+b to obtain the same levels of xanthophyll cycle-dependent fluorescence quenching. We thus suggest that, at a constant pH, the fraction of the short lifetime distribution is determined by the concentration and thus binding frequency of the xanthophylls in the PS II inner antenna. The pH also affected both the widths and centers of the lifetime distributions independent of the xanthophyll cycle. We suggest that the combined effects of the xanthophyll cycle and pH cause major conformational changes in the pigment-protein complexes of the PS II inner or core antennae that switch a normal PS II unit to an increased rate constant of heat dissipation. We discuss a model of the PS II photochemical apparatus where PS II photochemistry and xanthophyll cycle-dependent energy dissipation are independent of the Peripheral antenna size.Abbreviations Ax antheraxanthin - BSA bovine serum albumin - c_x lifetime center of fluorescence decay component x - CP chlorophyll binding protein of PS II inner antenna - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DTT dithiothreitol - f_x fractional intensity of fluorescence lifetime component x - F_m, F_m maximal PS II Chl a fluorescence intensity with all Q_A reduced in the absence, presence of thylakoid membrane energization - F_o minimal PS II Chl a fluorescence intensity with all Q_A oxidized - F_v=F_m–F_o variable level of PS II Chl a fluorescence - HPLC high performance liquid chromatography - k_A rate constant of all combined energy dissipation pathways in PS II except photochemistry and fluorescence - k_F rate constant of PS II Chl a fluorescence - LHCIIb main light harvesting pigment-protein complex (of PS II) - N_pig mols Chl a+b per PS II - NPQ=(F_m/F_m–1) nonphotochemical quenching of PS II Chl a fluorescence - PAM pulse-amplitude modulation fluorometer - PFD photon-flux density, mols photons m^–2 s^–1 - PS II Photosystem II - P680 special-pair Chls of PS II reaction center - Q_A primary quinone electron acceptor of PS II - V_x violaxanthin - w_x width at half maximum of Lorentzian fluorescence lifetime distribution x - Zx zeaxanthin - pH trans-thylakoid proton gradient - % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqef0uAJj3BZ9Mz0bYu% H52CGmvzYLMzaerbd9wDYLwzYbItLDharqqr1ngBPrgifHhDYfgasa% acOqpw0xe9v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8Wq% Ffea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8frFve9Fve9Ff0dme% GabaqaaiGacaGaamqadaabaeaafiaakeaacqGH8aapcqaHepaDcqGH% +aGpdaWgaaWcbaGaamOraiaad2gaaeqaaaaa!4989!\[< \tau > _{Fm}\],% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqef0uAJj3BZ9Mz0bYu% H52CGmvzYLMzaerbd9wDYLwzYbItLDharqqr1ngBPrgifHhDYfgasa% acOqpw0xe9v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8Wq% Ffea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8frFve9Fve9Ff0dme% GabaqaaiGacaGaamqadaabaeaafiaakeaacqGH8aapcqaHepaDcqGH% +aGpdaWgaaWcbaGaamOraiaad+gaaeqaaOGaeyypa0Zaaabqaeaaca% WGMbWaaSbaaSqaaiaadIhaaeqaaOGaam4yamaaBaaaleaacaWG4baa% beaaaeqabeqdcqGHris5aaaa!50D3!\[< \tau > _{Fo} = \sum {f_x c_x }\] average lifetime of Chl a fluorescence calculated from a multi-exponential model under F_m, F_o conditions     

The early genetic response to light in the green unicellular alga Chlamydomonas eugametos grown under light/dark cycles involves genes that represent direct responses to light and photosynthesis

In the green unicellular alga Chlamydomonas eugametos, cellular division is readily synchronized by light/dark cycles. Under these conditions, light initiates photosynthetic growth in daughter cells and begins the G1 phase. Genes whose expression is regulated upon illumination are likely to be important mechanisms controlling cell proliferation. To identify some of those genes, two cDNA libraries were prepared with poly(A)⁺ extracted from cells either stimulated with light for 1 h or held in darkness (quiescent cells) during the same period. To restrict our analysis to those genes that are part of the primary response, cells were incubated in presence of cycloheximide. Differential screening of approximately 40 000 clones in each library revealed 44 clones which hybridize preferentially with a [³²P] cDNA probe derived from RNA of light-stimulated cells and 15 clones which react selectively with a [³²P] cDNA probe synthesized from poly(A)⁺ RNA of quiescent cells. Cross-hybridization of these clones identified 4 independent sequences in the light-induced (LI) collection and 2 in the uninduced (LR) library. Four of these cDNAs correspond to mRNAs that are positively or negatively regulated upon activation of photosynthesis. One clone represents a mRNA that accumulates transitorily at both transitions. Finally, LI818 cDNA identifies a new chlorophyll a/b-binding (cab) gene family whose mRNA accumulation is controlled by light and a circadian oscillator. The endogenous timing system controls LI818 mRNA accumulation so that it precedes the onset of illumination by a few hours. On the other hand, light affects LI818 mRNA levels independently of active photosynthesis.     

Acclimation of Arabidopsis thaliana to the light environment: changes in photosynthetic function

Arabidopsis thaliana (L.) Heynh. cv. Landsberg erecta was grown under light regimes of differing spectral qualities, which results in differences in the stoichiometries of the two photosynthetic reaction centres. The acclimative value of these changes was investigated by assessing photosynthetic function in these plants when exposed to two spectrally distinct actinic lights. Plants grown in an environment enriched in far-red light were better able to make efficient use of non-saturating levels of actinic light enriched in long-wavelength red light. Simultaneous measurements of chlorophyll fluorescence and absorption changes at 820 nm indicated that differences between plants grown under alternative light regimes can be ascribed to imbalances in excitation of photosystems I and II (PSI, PSII). Measurements of chlorophyll fluorescence emission and excitation spectra at 77 K provided strong evidence that there was little or no difference in the composition or function of PSI or PSII between the two sets of plants, implying that changes in photosynthetic stoichiometry are primarily responsible for the observed differences in photosynthetic function.Abbreviations Chl chlorophyll - FR far-red light - HF highirradiance FR-enriched light (400 mol·m^–2·s^–1, RFR = 0.72) - HW high-irradiance white light (400 mol·m^–2 1·1 s^–1RFR = 1.40) - LHCI, LHCII light-harvesting complex of PSI, PSII - qO quenching of dark-level chlorophyll fluorescence - qN non-photochemical quenching of variable chlorophyll fluorescence - qP photochemical quenching of variable chlorophyll fluorescence - R red light - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase We thank Dr. Sasha Ruban for assistance with the 77 K fluorescence measurements and for helpful discussions. This work was supported by Natural Environment Research Council Grant GR3/7571A.     

Cytokinins modulate the expression of genes encoding the protein of the light-harvesting chlorophyll a/b complex

Summary Tobacco cell suspension cultures responded to cytokinins (for instance kinetin) by full chloroplast differentiation. The hormone had the effect of stimulating the appearance of a few prominent plastid proteins. Synthesis of the light-harvesting chlorophyl a/b-binding protein (LHCP) in response to kinetin was noteworthy (Axelos M. et al.: Plant Sci Lett 33:201–212, 1984).Poly(A)⁺RNAs were prepared from cells grown in the presence of or without added kinetin. Poly(A)⁺RNA recovery and translation activity were not quantitatively altered by the hormone treatment. In vitro translation of polyadenylated mRNA into precursor polypeptides of LHCP (pLHCP) was quantified by immunoprecipitation and SDS-PAGE fractionation of pLHCP immunoprecipitates: pLHCP-mRNA translating activity was found to be stimulated in parallel to mature LHCP accumulation by kinetin-induced cells.Dot-blot and northern-blot hybridizations of poly(A)⁺RNA were carried out, using as a probe a pea LHCP-cDNA clone (Broglie R. et al.: Proc Natl Acad Sci USA 78: 7304–7308, 1981). A ten-fold increase of the level of pLHCP-encoding sequences was observed in poly(A)⁺RNA prepared from 9-d kinetin-stimulated cells, compared to control cells. Oligo(dT)-cellulose-excluded RNA fractions exhibited very low hybridization levels, in the same ratios as those obtained with poly(A)⁺RNA.Thus, the expression of LHCP-gene activity, in response to kinetin addition to tobacco cell suspension cultures, is regulated by the level of pLHCP-encoding mRNA rather than by translational or post-translational controls. re]19850218 rv]19850605 ac]19850613     

Diurnal changes in leaf gas exchange and chlorophyll fluorescence in tropical tree species with contrasting light requirements

Interspecific ecophysiological differences in response to different light environments are important to consider in regeneration behavior and forest dynamics. The diurnal changes in leaf gas exchange and chlorophyll fluorescence of two dipterocarps, Shorea leprosula (a high light-requiring) and Neobalanocarpus heimii (a low light-requiring), and a pioneer tree species (Macaranga gigantea) growing in open and gap sites were examined. In the open site, the maximum net photosynthetic rate (P_n), photosystem II (PSII) quantum yield (; F/Fm), and relative electron transport rate (r-ETR) through PSII at a given photosynthetic photon flux density (PPFD) was higher in S. leprosula and M. gigantea than in N. heimii, while non-photochemical quenching (NPQ) at a given PPFD was higher in N. heimii. The maximum values of net photosynthetic rate (P_n) in M. gigantea and S. leprosula was higher in the open site (8–11 mol m^–2 s^–1) than in the gap site (5 mol m^–2 s^–1), whereas that in N. heimii was lower in the open site (2 mol m^–2 s^–1) than in the gap site (4 mol m^–2 s^–1), indicating that N. heimii was less favorable to the open site. These data provide evidence to support the hypothesis that ecophysiological characteristics link with plants regeneration behavior and successional status. Although P_n and stomatal conductance decreased at midday in M. gigantea and S. leprosula in the open site, both r-ETR and leaf temperature remained unchanged. This indicates that stomatal closure rather than reduced photochemical capacity limited P_n in the daytime. Conversely, there was reduced r-ETR under high PPFD conditions in N. heimii in the open site, indicating reduced photochemical capacity. In the gap site, P_n increased in all leaves in the morning before exposure to direct sunlight, suggesting a relatively high use of diffuse light in the morning.     

Cloning and expression of a gene coding for the major light-harvesting chlorophyll a/b protein of photosystem II in the green alga <Emphasis Type="Italic">Dunaliella salina</Emphasis>

Genes encoding proteins of the major light-harvesting complex of photosystem II (LHCII) in higher plants are well studied. However, little is known about the corresponding genes in the green alga Dunaliella salina, although this knowledge might provide valuable information about the respective roles of each LHCII protein at the molecular level under extreme environmental conditions. Here, we describe an additional LhcII gene from D. salina. An LhcII cDNA cloned by screening a D. salina cDNA library contains an open reading frame encoding a protein of 261 amino acids with a calculated molecular mass of 27.8 kDa. The deduced amino acid sequence shows high homology with other LHCII proteins. Genomic DNA—obtained by PCR using a specific primer set corresponding to the 5′ and 3′ untranslated regions—was used to determine the intron-exon structure. Short-term changes in mRNA levels after a shift from low-light to high-light or dark conditions were analyzed by real-time quantitative PCR, and indicated that this gene expresses different mRNA levels under different light conditions.     

The effect of light on the biosynthesis of the light-harvesting chlorophyll a/b protein

The effect of light on the biosynthesis of the light-harvesting chlorophyll a/b protein (LHCP) is investigated in wild-type barley (Hordeum vulgare L.) and in the chlorophyll b-less mutant chlorina f2. In dark-grown plants a short red light pulse triggers the appearance of mRNA activity for the LHCP. While the accumulation of this mRNA is controlled by phytochrome (Apel (1979) Eur. J. Biochem. 97, 183–188), the red light treatment is not sufficient to induce the appearance of the LHCP within the membrane. Thus, at least one of the subsequent steps in the biosynthetic pathway leading to the assembly of the LHCP is controlled by light. The red light-induced mRNA is taken up into the polysomes during the subsequent dark period and is translated in vitro in a cell-free protein synthesizing system. However, an accumulation of the freshly synthesized polypeptide within the plant is not observed. The apparent instability of the polypeptide might be explained by the deficiency of chlorophyll in the red light-treated plants. In the chlorophyll b-less barley mutant chlorina f2 an accumulation of the freshly synthesized apoprotein of the LHCP can be observed in the light. Thus, chlorophyll a formation seems to be a light-dependent step which is required for the stabilization of the LHCP.Abbreviations mRNA messenger RNA - EDTA ethylenediaminetetraacetic acid - SDS sodium dodecylsulfate - LHCP light-harvesting chlorophyll a/b protein     

Preferential accumulation of apoproteins of the light-harvesting chlorophyll a/b-protein complex in greening barley leaves treated with 5-aminolevulinic acid

In order to study the coordinate accumulation of chlorophyll (Chl) and apoproteins of Chl-protein complexes (CPs) during chloroplast development, we examined changes in the accumulation of the apoproteins in barley (Hordeum vulgare L.) leaves when the rate of Chl synthesis was altered by feeding 5-aminolevulinic acid (ALA), a precursor of Chl biosynthesis. Pretreatment with ALA increased the accumulation of Chl a and Chl b 1.5- and 2.3-fold, respectively, after 12 cycles of intermittent light (2 min light followed by 28 min darkness). Apoproteins of the light-harvesting Chl a/b-protein complex of photosystem II (LHCII) were increased 2.4-fold with ALA treatment. However, apoproteins of the P700-Chl a-protein complex (CP1) and the 43-kDa apoprotein of a Chl a-protein complex of photosystem II (CPa) were not increased by ALA application. With respect to CPs themselves, LHCII was increased when Chl synthesis was raised by ALA feeding, whereas CP1 exhibited no remarkable increase. These results indicate that LHCII serves a role in maintaining the stoichiometry of Chl to apoproteins by acting as a temporary pool for Chl molecules.Abbreviations ALA 5-aminolevulinic acid - Chl chlorophyll - CP chlorophyll-protein complex - CPa chlorophyll a-protein complex of PSII - CP1 P700-chlorophyll a-protein complex - LDS lithium dodecyl sulfate - LHCII light-harvesting chlorophyll a/b-protein complex of PSII This work was supported by the Grants-in-Aid for Scientific Research (04304004) from the Ministry of Education, Science and Culture, Japan.     

Acclimation of Arabidopsis thaliana to the light environment: regulation of chloroplast composition

The regulation by light of the composition of the photosynthetic apparatus was investigated in Arabidopsis thaliana (L.) Heynh. cv. Landsberg erecta. When grown in high- and low-irradiance white light, wild-type plants and photomorphogenic mutants showed large differences in their maximum photosynthetic rate and chlorophyll a/b ratios; such changes were abolished by growth in red light. Photosystem I (PSI) and PSII levels were measured in wild-type plants grown under a range of light environments; the results indicate that regulation of photosystem stoichiometry involves the specific detection of blue light. Supplementing red growth lights with low levels of blue light led to large increases in PSII content, while further increases in blue irradiance had the opposite effect; this latter response was abolished by the hy4 mutation, which affects certain events controlled by a blue-light receptor. Mutants defective in the phytochrome photoreceptors retained regulation of photosystem stoichiometry. We discuss the results in terms of two separate responses controlled by blue-light receptors: a blue-high-fluence response which controls photosystem stoichiometry; and a blue-low-fluence response necessary for activation of such control. Variation in the irradiance of the red growth light revealed that the blue-high-fluence response is attenuated by red light; this may be evidence that photosystem stoichiometry is controlled not only by photoreceptors, but also by photosynthetic metabolism.Abbreviations BHF blue-high-fluence - BLF blue-low-fluence - Chl chlorophyll - FR far-red light - LHCII light-harvesting complex of PSII - P_max maximum photosynthetic rate - R red light - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase This work was supported by Natural Environment Research Council Grant No. GR3/7571A. We would like to thank H. Smith (Botany Department, University of Leicester) and E. Murchie (INRA, Versailles) for helpful discussions.