The multiple roles of light-harvesting chlorophyll a/b-protein complexes define structure and optimize function of Arabidopsis chloroplasts: A study using two chlorophyll b-less mutants

The multiple roles of light-harvesting chlorophyll a/b-protein complexes in the structure and function of Arabidopsis chloroplasts were investigated using two chlorophyll b-less mutants grown under metal halide lamps with a significant far-red component. In ch1-3, all six light-harvesting proteins of photosystem (PS) II were greatly decreased; in ch1-3lhcb5, Lhcb5 was completely absent while the other five proteins were further decreased. The thylakoids of ch1-3 were less negatively-charged than the wild type, and those of ch1-3lhcb5 were even less so. Despite the expected weaker electrostatic repulsion, however, thylakoids in leaves of the mutants were not well stacked, an effect we attribute to lower van der Waals attraction, lower electrostatic attraction between opposite charges, and the absence or instability of PSII supercomplexes and peripheral light-harvesting trimers. The quantum yield of oxygen evolution in leaves decreased from 0.109 (wild type) to 0.087 (ch1-3) and 0.081 (ch1-3lhcb5) O₂ (photon absorbed)^− 1; we attribute this decrease to an excessive spillover from PSII to PSI, a limited PSII antenna, and increased light-independent thermal dissipation in PSII in the mutants. Destabilization of the donor side of PSII, indicated by slower electron donation to the redox-active tyrosine Y_Z in ch1-3, probably enhanced PSII susceptibility to photoinactivation, increased the non-functional PSII complexes in vivo, and further inactivated PSII complexes in vitro. The evolution of chlorophyll b-containing chloroplasts seems to fine-tune oxygenic photosynthesis.     

类囊体膜的垛叠、松散与它的功能关系

菠菜完整叶绿体置于4mM MgCl_2或20 mM KCl低浓度介质中低渗10秒钟后,得到由Mg~(++)或K~+离子诱导的类囊体垛叠膜和松散膜。它们在功能上表现出明显的差异。垛叠膜有较高的毫秒级延迟光发射(ms-DLE),松散膜显著降低DLE的快相,垛叠膜比松散膜的9-AA荧光猝灭快,并保持稳定;而松散膜有H~+渗漏。在非循环或Fd催化的循环光合磷酸化中,垛叠膜比松散膜活力高。但是,若在同样的低渗介质中低渗1分钟以上,Mg~(++)离子诱导的垛叠膜,在显微结构上不同于低渗过10秒钟的垛叠膜,它垛叠较松,而且在磷酸化活力上也与松散膜差别不大。揭示了H~+传递速度受二个光系统、电子载体间的距离及偶联程度的限制。新鲜制备的垛叠或松散膜,在NADP~+还原系统中,具有相同的电子传递放O_2速度,说明电子传递速度在一定范围内不受膜间的距离和偶联程度的影响。但是松散膜不稳定,随着膜的老化而解联,牛血清白蛋白(BSA)能稳定松散膜的电子传递。     

Photoregulation systems for light-oriented chloroplast movement

Movements of the chloroplasts induced by a directional stimulus of light are found to occur in various plant materials ranging from algae to terrestrial angiosperms. Depending on the fluence rate of light, chloroplasts move toward the area of the maximum light absorption and escape from it, which are named as low- and high-fluence-rate responses, respectively. In most materials the effective wavelengths are exclusively found in the blue-UV region of spectrum, (a) flavin pigment(s) being considered as the photoreceptor, while in a few species of plants phytochrome is involved. The arrangement of chloroplasts as a result of the movement depends on the orientation of the electrical vector of light, which reflects the dichroic orientation of the photoreceptor for perception of light direction. Photosystems involved in these responses, however, are not only for perception of light direction, but also for realization of the movement and for holding of the chloroplasts in the reached site. Possible interactions and dual roles of photosystems in regulation of chloroplast movement are discussed.     

Ontogenetic approach to the assessment of plant resistance to prolonged stress using chlorophyll fluorescence induction method

The review deals with objective reasons that limit the use of chlorophyll fluorescence induction kinetics (Chl FIK) method in plant ecology. Based on the ontogenetic approach (analysis and comparison of the dynamics of the Chl fluorescence ratio F_p/F_s and physiological characteristics of plant leaves in ontogeny) possible criteria for the estimation of general plant resistance of photosynthetic apparatus (PSA) to prolonged stress affects are proposed. One of these criteria is the presence or absence of a steady-state phase in the dynamics of Chl fluorescence ratio F_p/F_s (or R_fd) of plant leaves after they stop growing. We also propose to use the duration of the steady-state phase and variability of Chl fluorescence ratios R_fd and F_p/F_s in this period for quantitative assessment of plant PSA resistance to prolonged stress during plant leaf ontogeny. The paper is dedicated to memory of Prof. F.Ya. Sid’ko.     

Composition and Characteristic Differences in Photosynthetic Membranes of Two Ecotypes of Reed (Phragmites communis L.) from Different Habitats

As compared with the swamp reed (SR) ecotype of Phragmites communis growing in the desert region of northwest China, plants of the dune reed (DR) ecotype from the same region possessed lower chlorophyll (Chl) content in leaves, and less thylakoids and grana stacks in chloroplasts. Tube gel electrophoresis without stain showed that the contents of Chl-protein (Pro) components related to photosystem 2 (PS2) were markedly lower in the DR thylakoid membranes than in the SR thylakoid membranes, while the contents of Chl-Pro components associated with PS1 were almost the same in both types. SDS-PAGE analysis indicated that the content of polypeptides of the light-harvesting Chl a/b complex of PS2 (LHC2) was lower in the DR thylakoids. Besides, the conformation of LHC2 within the DR thylakoid membranes was also altered as indicated by circular dichroism spectra. Hence in the DR, reduced energy harvesting by declining the size of LHC2 might be responsible for the down-regulated PS2 activity. Chl fluorescence parameters. F_v/F_m and quantum efficiency of PS2 (_PS2), were lower in the DR leaves than in the SR ones. However, non-photochemical quenching coefficient (q_N) was greater in DR than that in SR, implying other energy dissipation way exists in the DR photosynthetic membranes.     

Chlorophyll fluorescence parameters characterizing the development of two cacao genotypes infected by witches' broom

Greenhouse-grown susceptible 20-d-old seedlings of Theobroma cacao genotypes Catongo and tolerant genotype SCA6xCatongo were inoculated with a mixture of isolates of Crinipellis perniciosa, the causal agent of witches' broom. The characteristics of chlorophyll a fluorescence emission were monitored during leaf ontogeny using a portable system PAM-2000. In both inoculated and non-inoculated genotypes, significant differences were found for the effective quantum yield values of photosystem (PS) 2 (F/Fm') at the B (7 to 14-d-old), D (21 to 30-d-old), and E (>30-d-old) stages of leaf development, and in quantum yield of the non-cyclic photosynthetic electron transport between PS2 and PS1 [qp(Fv/Fm)] and quencher efficiency [(Fm-Ft)/F0] at the B, C (15 to 20-d-old) and D stages. Intergenotypic differences were found only for the [qp(Fv/Fm)] and [(Fm-Ft)/F0] values at the E stage, and for fluorescence quenching (Fm-Ft) at the B and E stages. Highly significant inter- and intragenotype relationships were found between the rate of photosynthetic electron transport to PS2 (Amax) and maximum fluorescence during actinic irradiation (Fm'). Also, each of the highly significant relationships between (Fm-Ft) and Amax, [(Fm-Ft)/F0] and F/Fm', and between [(Fm-Ft)/F0] and Amax were represented by a general model, independent of treatments. Therefore, alterations in energy distribution in the radiant energy collector complex interior of PS2 and reduction in absorption of photosynthetically active radiation were observed in the infected plants, mainly in the hybrid at the C stage. Also, variations were found in the noncyclic photosynthetic electron transport at the B and C stages in the infected Catongo.     

Alterations in Electron Transport Characteristics during Senescence of Cucumis Cotyledonary Leaves. Analysis of the Effects of Inhibitors

Cotyledonary leaves of Cucumis sativus cv. Poinsette exhibited senescence-induced losses in chlorophyll (Chl) and protein contents within three weeks since germination. Chl and protein concentrations in cotyledonary leaves approached maximum on 6th d after germination and they declined to 50 and 41 %, respectively, by the 20th day of growth. Activities of both photosystem (PS) 2 and PS1 decreased by 33 and 31 %, respectively, on the 20th day, compared to the control 6th day. Changes in sensitivity of PS2 to inhibitors like atrazine and dibromothymoquinone and sensitivity of PS1 to KCN accompanied the changes in PS2 and PS1 activities. Hence both the acceptor side of PS2 and the donor side of PS1 are affected by senescence-induced changes in cucumber cotyledonary leaves.     

Regulation of nitrate reduction in wheat leaves

Wheat leaves exposed to 710 nm monochromatic light, when only photosystem 1 operates, reduced small but significant amount of nitrate to nitrite. This could be due to partial inhibition of mitochondrial oxidation of NADH, brought about by cyclic photo-phosphorylation. Under dark aerobic conditions, citric acid cycle intermediates only slightly stimulated nitrate reduction. Under dark anaerobic conditions, when maximum reduction of nitrate occurred, the time course showed a 1:1 stoichiometry between nitrite and CO₂. It is suggested that for maximum reduction of nitrate under physiological conditions, CO₂ fixation and export of ATP via triose phosphate shuttle is essential.     

Inner Structure of Intact Chloroplasts Observed by a Low Temperature Laser Scanning Microscope

Inner structure of isolated intact chloroplasts was observed for the first time by a method of laser scanning microscopy at the temperature of liquid nitrogen at 77 K. The microscope, based on gradient index optics, has a maximum resolution of 440 nm at the wavelength of 650 nm. Chloroplasts were excited into the Q-band of chlorophyll b by a krypton laser line at 647.6 nm and fluorescence was detected using two different interference filters. The 680 nm interference filter detects the regions where photosystem (PS) 2 mainly occurs, the 730 nm interference filter detects domains with predominant location of PS1. Since PS1 occurs mainly in stroma lamellae, whereas PS2 occurs mainly in grana regions we were able to view the structure of thylakoid membrane in isolated intact chloroplast that is the closest to in vivo state.     

Enhanced Thermal Energy Dissipation Depending on Xanthophyll Cycle and D1 Protein Turnover in Iron-Deficient Maize Leaves Under High Irradiance

Pigment contents of chloroplasts and net photosynthetic rate were dramatically reduced in maize leaves suffering from iron deficiency. However, the reduction in photosynthesis was probably not caused by decreased contents of chlorophylls and carotenoids and by photon absorption; the primary limiting factor for photosynthesis may rather be the decrease of electron transport activity in photosystem 1. Iron-deficient leaves suffered serious acceptor-side photoinhibition, and more than 60 % of absorbed photons were dissipated, while less than 40 % was used in photochemical reaction. Thermal energy dissipation depending on xanthophyll cycle and D1 protein turnover was enhanced when acceptor-side photoinhibition occurred in iron-deficient maize leaves.