Photosynthesis Research (v.123, #2)

Andrew Benson honored on birthday № 97 by Bob B. Buchanan; Roland Douce (115-116).
We present a brief account of the 97th birthday celebration of Andrew A. Benson, a scientific legend who is known, among other contributions, for his pioneering work on the path of carbon in photosynthesis (the Calvin-Benson cycle).
Keywords: Andrew A. Benson award; Brandt iHammer; Calvin-Benson cycle; Plant lectins

C3 carbon fixation has a bad reputation, primarily because it is associated with photorespiration, a biochemical pathway thought to waste a substantial amount of the carbohydrate produced in a plant. This review presents evidence collected over nearly a century that (1) Rubisco when associated with Mn2+ generates additional reductant during photorespiration, (2) this reductant participates in the assimilation of nitrate into protein, and (3) this nitrate assimilation facilitates the use of a nitrogen source that other organisms tend to avoid. This phenomenon explains the continued dominance of C3 plants during the past 23 million years of low CO2 atmospheres as well as the decline in plant protein concentrations as atmospheric CO2 rises.
Keywords: Photorespiration; C3 carbon fixation; Nitrate assimilation; Photosynthesis; Plant evolution; Nitrogen sources

A loop unique to ferredoxin-dependent glutamate synthases is not absolutely essential for ferredoxin-dependent catalytic activity by Jatindra N. Tripathy; Masakazu Hirasawa; R. Bryan Sutton; Afia Dasgupta; Nanditha Vaidyanathan; Masoud Zabet-Moghaddam; Francisco J. Florencio; Anurag P. Srivastava; David B. Knaff (129-139).
It had been proposed that a loop, typically containing 26 or 27 amino acids, which is only present in monomeric, ferredoxin-dependent, “plant-type” glutamate synthases and is absent from the catalytic α-subunits of both NADPH-dependent, heterodimeric glutamate synthases found in non-photosynthetic bacteria and NADH-dependent heterodimeric cyanobacterial glutamate synthases, plays a key role in productive binding of ferredoxin to the plant-type enzymes. Site-directed mutagenesis has been used to delete the entire 27 amino acid-long loop in the ferredoxin-dependent glutamate synthase from the cyanobacterium Synechocystis sp. PCC 6803. The specific activity of the resulting loopless variant of this glutamate synthase, when reduced ferredoxin serves as the electron donor, is actually higher than that of the wild-type enzyme, suggesting that this loop is not absolutely essential for efficient electron transfer from reduced ferredoxin to the enzyme. These results are consistent with the results of an in-silico study that suggests that the loop is unlikely to interact directly with ferredoxin in the energetically most favorable model of a 1:1 complex of ferredoxin with the wild-type enzyme.
Keywords: Glutamate synthase; Ferredoxin; Protein/protein interactions; FMN; Iron–sulfur clusters

Canopy light heterogeneity drives leaf anatomical, eco-physiological, and photosynthetic changes in olive trees grown in a high-density plantation by Ajmi Larbi; Saúl Vázquez; Hamdi El-Jendoubi; Monji Msallem; Javier Abadía; Anunciación Abadía; Fermín Morales (141-155).
In the field, leaves may face very different light intensities within the tree canopy. Leaves usually respond with light-induced morphological and photosynthetic changes, in a phenomenon known as phenotypic plasticity. Canopy light distribution, leaf anatomy, gas exchange, chlorophyll fluorescence, and pigment composition were investigated in an olive (Olea europaea, cvs. Arbequina and Arbosana) orchard planted with a high-density system (1,250 trees ha−1). Sampling was made from three canopy zones: a lower canopy (<1 m), a central one (1–2 m), and an upper one (>2 m). Light interception decreased significantly in the lower canopy when compared to the central and top ones. Leaf angle increased and photosynthetic rates and non-photochemical quenching (NPQ) decreased significantly and progressively from the upper canopy to the central and the lower canopies. The largest leaf areas were found in the lower canopy, especially in the cultivar Arbequina. The palisade and spongy parenchyma were reduced in thickness in the lower canopy when compared to the upper one, in the former due to a decrease in the number of cell layers from three to two (clearly distinguishable in the light and fluorescence microscopy images). In both cultivars, the concentration of violaxanthin-cycle pigments and β-carotene was higher in the upper than in the lower canopy. Furthermore, the de-epoxidized forms zeaxanthin and antheraxanthin increased significantly in those leaves from the upper canopy, in parallel to the NPQ increases. In conclusion, olive leaves react with morphological and photosynthetic changes to within-crown light gradients. These results strengthen the idea of olive trees as “modular organisms” that adjust the modules morphology and physiology in response to light intensity.
Keywords: Canopy light distribution; High-density olive orchards; Leaf anatomy; Leaf phenotypic plasticity; Photosynthesis

Production of ketocarotenoids in tobacco alters the photosynthetic efficiency by reducing photosystem II supercomplex and LHCII trimer stability by Anja Röding; Lars Dietzel; Hagen Schlicke; Bernhard Grimm; Gerhard Sandmann; Claudia Büchel (157-165).
The consequences of ketocarotenoid production in transgenic tobacco (Nicotiana tabacum) plants expressing a Chlamydomonas reinhardtii gene encoding a β-carotene ketolase were examined concerning the functionality of the photosynthetic apparatus. T1 plants produced less photosynthetic pigments per dry weight, but Chl a/Chl b ratios remained unchanged. Almost as much ketocarotenoids as accessory xanthophylls accumulated per Chl a molecule. These ketocarotenoids were found mainly in the thylakoid membranes, but were not functionally bound to light-harvesting complexes, although LHCII is known to be able to bind astaxanthin. On the contrary, high amounts of ketocarotenoids probably changed the properties of the lipid phase of the thylakoids, thereby reducing the stability of photosystem II supercomplexes and LHCII trimers and ultimately decreasing grana formation. In addition, photosystem II function in electron transport was impaired, and plants exhibited less non-photochemical quenching compared to wild-type plants. Thus, in order not to disturb vital functions of the plants, production of astaxanthin and other nutritionally valuable ketocarotenoids apparently requires ways to sequester the additional carotenoids to plastoglobuli.
Keywords: Ketocarotenoids; LHCII; Thylakoid structure; PSII supercomplexes

Quantifying the effects of light intensity on bioproduction and maintenance energy during photosynthetic growth of Rhodobacter sphaeroides by Saheed Imam; Colin M. Fitzgerald; Emily M. Cook; Timothy J. Donohue; Daniel R. Noguera (167-182).
Obtaining a better understanding of the physiology and bioenergetics of photosynthetic microbes is an important step toward optimizing these systems for light energy capture or production of valuable commodities. In this work, we analyzed the effect of light intensity on bioproduction, biomass formation, and maintenance energy during photoheterotrophic growth of Rhodobacter sphaeroides. Using data obtained from steady-state bioreactors operated at varying dilution rates and light intensities, we found that irradiance had a significant impact on biomass yield and composition, with significant changes in photopigment, phospholipid, and biopolymer storage contents. We also observed a linear relationship between incident light intensity and H2 production rate between 3 and 10 W m−2, with saturation observed at 100 W m−2. The light conversion efficiency to H2 was also higher at lower light intensities. Photosynthetic maintenance energy requirements were also significantly affected by light intensity, with links to differences in biomass composition and the need to maintain redox homeostasis. Inclusion of the measured condition-dependent biomass and maintenance energy parameters and the measured photon uptake rate into a genome-scale metabolic model for R. sphaeroides (iRsp1140) significantly improved its predictive performance. We discuss how our analyses provide new insights into the light-dependent changes in bioenergetic requirements and physiology during photosynthetic growth of R. sphaeroides and potentially other photosynthetic organisms.
Keywords: Photosynthesis; Maintenance energy; Rhodobacter sphaeroides ; Metabolic modeling; Bioenergetics; Hydrogen

Temperature response of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) catalytic properties directly determines the CO2 assimilation capacity of photosynthetic organisms as well as their survival in environments with different thermal conditions. Despite unquestionable importance of Rubisco, the comprehensive analysis summarizing temperature responses of Rubisco traits across lineages of carbon-fixing organisms is lacking. Here, we present a review of the temperature responses of Rubisco carboxylase specific activity ( $$k_{ ext{cat}}^{ ext{c}}$$ k cat c ) within and across domains of life. In particular, we consider the variability of temperature responses, and their ecological, physiological, and evolutionary controls. We observed over two-fold differences in the energy of activation (ΔH a) among different groups of photosynthetic organisms, and found significant differences between C3 plants from cool habitats, C3 plants from warm habitats and C4 plants. According to phylogenetically independent contrast analysis, ΔH a was not related to the species optimum growth temperature (T growth), but was positively correlated with Rubisco specificity factor (S c/o) across all organisms. However, when only land plants were analyzed, ΔH a was positively correlated with both T growth and S c/o, indicating different trends for these traits in plants versus unicellular aquatic organisms, such as algae and bacteria. The optimum temperature (T opt) for $$k_{ ext{cat}}^{ ext{c}}$$ k cat c correlated with S c/o for land plants and for all organisms pooled, but the effect of T growth on T opt was driven by species phylogeny. The overall phylogenetic signal was significant for all analyzed parameters, stressing the importance of considering the evolutionary framework and accounting for shared ancestry when deciphering relationships between Rubisco kinetic parameters. We argue that these findings have important implications for improving global photosynthesis models.
Keywords: Activation energy; Adaptation; Carboxylation; Evolution; Photosynthesis; Temperature dependencies

A stable and efficient nuclear transformation system for the diatom Chaetoceros gracilis by Kentaro Ifuku; Dongyi Yan; Mado Miyahara; Natsuko Inoue-Kashino; Yoshiharu Y. Yamamoto; Yasuhiro Kashino (203-211).
Chaetoceros gracilis belongs to the centric diatoms, and has recently been used in basic research on photosynthesis. In addition, it has been commercially used in fisheries and is also attracting interest as a feedstock for biofuels production and biorefinery. In this study, we developed an efficient genetic transformation system for C. gracilis. The diatom cells were transformed via multi-pulse electroporation using plasmids containing various promoters to drive expression of the nourseothricin acetyltransferase gene (nat) as a selectable marker. The transformation efficiency reached ~400 positive transgenic clones per 108 recipient cells, which is the first example of successful transformation with electroporation in a centric diatom species. We further produced two expression vectors: the vector pCgLhcr5p contains the light-dependent promoter of a fucoxanthin chlorophyll a/c binding protein gene and the vector pCgNRp contains the inducible promoter of a nitrate reductase gene to drive the expression of introduced genes. In both vectors, an acetyl-CoA acetyltransferase promoter drives nat gene expression for antibiotic selection. Stable integration and expression of reporter genes, such as the firefly luciferase and green fluorescent protein Azami–Green genes, were observed in transformed C. gracilis cells. This efficient and stable transformation system for C. gracilis will enable both functional analysis of diatom-specific genes and strain improvement for further biotechnological applications.
Keywords: Chaetoceros gracilis ; Diatom; Electroporation; Genetic transformation; Green fluorescent protein; Luciferase

We provide here a News Report on the 2014 Gordon Research Conference on Photosynthesis, with the subtitle “From Evolution of Fundamental Mechanisms to Radical Re-Engineering.” It was held at Mount Snow Resort, West Dover, Vermont, during August 10–15, 2014. After the formal sessions ended, four young scientists (Ute Ambruster of USA; Han Bao of USA; Nicoletta Liguori of the Netherlands; and Anat Shperberg-Avni of Israel) were recognized for their research; they each received a book from one of us (G) in memory of Colin A. Wraight (1945–2014), a brilliant and admired scientist who had been very active in the bioenergetics field in general and in past Gordon Conferences in particular, having chaired the 1988 Gordon Conference on Photosynthesis. (See an article on Wraight by one of us (Govindjee) at http://www.life.illinois.edu/plantbio/Features/ColinWraight/ColinWraight.html .)
Keywords: Ute Ambruster; Han Bao; Gordon research conference; Arthur Grossman; David M. Kramer; Nicoletta Liguori; Photosynthesis; Fabrice Rappaport; Anat Shperberg-Avni; Colin A. Wraight