Photosynthesis Research (v.105, #2)

Co-regulation of dark and light reactions in three biochemical subtypes of C4 species by Olavi Kiirats; David M. Kramer; Gerald E. Edwards (89-99).
Regulation of light harvesting in response to changes in light intensity, CO2 and O2 concentration was studied in C4 species representing three different metabolic subtypes: Sorghum bicolor (NADP-malic enzyme), Amaranthus edulis (NAD-malic enzyme), and Panicum texanum (PEP-carboxykinase). Several photosynthetic parameters were measured on the intact leaf level including CO2 assimilation rates, O2 evolution, photosystem II activities, thylakoid proton circuit and dissipation of excitation energy. Gross rates of O2 evolution ( $$ J_{{{ ext{O}}_{2} }} $$ , measured by analysis of chlorophyll fluorescence), net rates of O2 evolution and CO2 assimilation responded in parallel to changes in light and CO2 levels. The C4 subtypes had similar energy requirements for photosynthesis since there were no significant differences in maximal quantum efficiencies for gross rates of O2 evolution (average value = 0.072 O2/quanta absorbed, ~14 quanta per O2 evolved). At saturating actinic light intensities, when photosynthesis was suppressed by decreasing CO2, ATP synthase proton conductivity (g H +) responded strongly to changes in electron flow, decreasing linearly with $$ J_{{{ ext{O}}_{2} }} $$ , which was previously observed in C3 plants. It is proposed that g H + is controlled at the substrate level by inorganic phosphate availability. The results suggest development of nonphotochemical quenching in C4 plants is controlled by a decrease in g H +, which causes an increase in proton motive force by restricting proton efflux from the lumen, rather than by cyclic or pseudocyclic electron flow.
Keywords: C4 plants; NAD-ME subtype; NADP-ME subtype; PEP-CK subtype; Photosystem I; Photosystem II

A new method of the chlorophyll (Chl) a fluorescence quenching analysis is described, which allows the calculation of values of (at least) three components of the non-photochemical quenching of the variable Chl a fluorescence (q N) using a non-linear regression of a multi-exponential function within experimental data. Formulae for coefficients of the “energy”-dependent (ΔpH-dependent) quenching (q E), the state-transition quenching (q T) and the photo/inhibitory quenching (q I) of Chl a fluorescence were found on the basis of three assumptions: (i) the dark relaxation kinetics of q N, as well as of all its components, is of an exponential nature, (ii) the superposition principle is valid for individual Chl a fluorescence quenching processes and (iii) the same reference fluorescence level (namely the maximum variable Chl a fluorescence yield in the dark-adapted state, F V) is used to define both q N and its components. All definitions as well as the algorithms for analytical recognition of the q N components are theoretically clarified and experimentally tested. The described theory results in a rather simple equation allowing to compute values for all q N components (q E, q T, q I) as well as the half-times of relaxation (τ1/2) of corresponding quenching processes. It is demonstrated that under the above assumptions it holds: q N = q E + q T + q I. The theoretically derived equations are tested, and the results obtained are discussed for non-stressed and stressed photosynthetically active samples. Semi-empirical formulae for a fast estimation of values of the q N components from experimental data are also given.
Keywords: Dark relaxation; ΔpH-dependent quenching; Fluorescence induction kinetics; Multi-exponential regression; Non-photochemical quenching; Photoinhibitory quenching; Photosystem II; State-transition quenching

The authors present a study of the fluorescence and absorbance transients occurring in whole cells of purple nonsulfur bacterium Rhodobacter sphaeroides on the millisecond timescale under pulsed actinic illumination. The fluorescence induction curve is interpreted in terms of combination of effects of redox changes in the reaction center and the membrane potential. The results of this study support the view that the membrane potential act predominantly to increase the fluorescence yield. Advantages of the pulsed actinic illumination for study of the operation of the electron transport chain in vivo are discussed.
Keywords: Photosynthesis; Charge separation; Membrane potential; Reaction center

Moderate heat stress of Arabidopsis thaliana leaves causes chloroplast swelling and plastoglobule formation by Ru Zhang; Robert R. Wise; Kimberly R. Struck; Thomas D. Sharkey (123-134).
Photosynthesis is inhibited by heat stress. This inhibition is rapidly reversible when heat stress is moderate but irreversible at higher temperature. Absorbance changes can be used to detect a variety of biophysical parameters in intact leaves. We found that moderate heat stress caused a large reduction of the apparent absorbance of green light in light-adapted, intact Arabidopsis thaliana leaves. Three mechanisms that can affect green light absorbance of leaves, namely, zeaxanthin accumulation (absorbance peak at 505 nm), the electrochromic shift (ECS) of carotenoid absorption spectra (peak at 518 nm), and light scattering (peak at 535 nm) were investigated. The change of green light absorbance caused by heat treatment was not caused by changes of zeaxanthin content nor by the ECS. The formation of non-photochemical quenching (NPQ), chloroplast movements, and chloroplast swelling and shrinkage can all affect light scattering inside leaves. The formation of NPQ under high temperature was not well correlated with the heat-induced absorbance change, and light microscopy revealed no appreciable changes of chloroplast location because of heat treatment. Transmission electron microscopy results showed swollen chloroplasts and increased number of plastoglobules in heat-treated leaves, indicating that the structural changes of chloroplasts and thylakoids are significant results of moderate heat stress and may explain the reduced apparent absorbance of green light under moderately high temperature.
Keywords: Photosynthesis; Moderate heat stress; Light scattering; Plastoglobules; Chloroplast swelling

Biosynthesis of fluorescent cyanobacterial allophycocyanin trimer in Escherichia coli by Shaofang Liu; Yingjie Chen; Yandu Lu; Huaxin Chen; Fuchao Li; Song Qin (135-142).
Allophycocyanin (APC), a cyanobacterial photosynthetic phycobiliprotein, functions in energy transfer as a light-harvesting protein. One of the prominent spectroscopic characteristics of APC is a strong red-shift in the absorption and emission maxima when monomers are assembled into a trimer. Previously, holo-APC α and β subunits (holo-ApcA and ApcB) were successfully synthesized in Escherichia coli. In this study, both holo-subunits from Synechocystis sp. PCC 6803 were co-expressed in E. coli, and found to self-assemble into trimers. The recombinant APC trimer was purified by metal affinity and size-exclusion chromatography, and had a native structure identical to native APC, as determined by characteristic spectroscopic measurements, fluorescence quantum yield, tryptic digestion analysis, and molecular weight measurements. Combined with results from a study in which only the monomer was formed, our results indicate that bilin synthesis and the subsequent attachment to apo-subunits are important for the successful assembly of APC trimers. This is the first study to report on the assembly of recombinant ApcA and ApcB into a trimer with native structure. Our study provides a promising method for producing better fluorescent tags, as well as a method to facilitate the genetic analysis of APC trimer assembly and biological function.
Keywords: Allophycocyanin; Trimer assembly; Recombinant protein; Allophycocyanin monomer; Fluorescence; Escherichia coli

Chloroplast development is usually regarded as proceeding from proplastids. However, direct or indirect conversion pathways have been described in the literature, the latter involving the etioplast or the etio-chloroplast stages. Etioplasts are characterized by the absence of chlorophylls (Chl-s) and the presence of a unique inner membrane network, the prolamellar body (PLB), whereas etio-chloroplasts contain Chl-s and small PLBs interconnected with chloroplast thylakoids. As etioplast development requires growth in darkness for several days, this stage is generally regarded as a nonnatural pathway of chloroplast development occurring only under laboratory conditions. In this article, we have reviewed the data in favor of the involvement of etioplasts and etio-chloroplasts as intermediary stage(s) in chloroplast formation under natural conditions, the molecular aspects of PLB formation and we propose a dynamic model for its regulation.
Keywords: Chlorophyll biosynthesis; Chloroplast differentiation; Etioplast; Etioplast–chloroplast transition; Prolamellar body; Protochlorophyllide

Photosynthesis online by Larry Orr; Govindjee (167-200).
Online access to the Internet and the World Wide Web have become important for public awareness and for educating the world’s population, including its political leaders, students, researchers, teachers, and ordinary citizens seeking information. Relevant information on photosynthesis-related Web sites and other online locations is grouped into several categories: (1) group sites, (2) sites by subject, (3) individual researcher’s sites, (4) sites for educators and students, and (5) other useful sites.
Keywords: Bioenergy; Climate change; Facebook; Internet; K-12 education; PDF; Solar fuel; Twitter; World Wide Web; YouTube