Photosynthesis solutions to enhance productivity

The concept that photosynthesis is a highly inefficient process in terms of conversion of light energy into biomass is embedded in the literature. It is only in the past decade that the processes limiting photosynthetic efficiency have been understood to an extent that allows a step change in our ability to manipulate light energy assimilation into carbon gain. We can therefore envisage that future increases in the grain yield potential of our major crops may depend largely on increasing the efficiency of photosynthesis. The papers in this issue provide new insights into the nature of current limitations on photosynthesis and identify new targets that can be used for crop improvement, together with information on the impacts of a changing environment on the productivity of photosynthesis on land and in our oceans. This article is part of the themed issue ‘Enhancing photosynthesis in crop plants: targets for improvement’

Early emergence of the FtsH proteases involved in Photosystem II repair

Efficient degradation of damaged D1 during the repair of PSII is carried out by a set of dedicated FtsH proteases in the thylakoid membrane. Here we investigated whether the evolution of FtsH could hold clues to the origin of oxygenic photosynthesis. A phylogenetic analysis of over 6000 FtsH protease sequences revealed that there are three major groups of FtsH proteases originating from gene duplication events in the last common ancestor of bacteria, and that the FtsH proteases involved in PSII repair make a distinct clade branching out before the divergence of FtsH proteases found in all groups of anoxygenic phototrophic bacteria. Furthermore, we showed that the phylogenetic tree of FtsH proteases in phototrophic bacteria is similar to that for Type I and Type II reaction centre proteins. We conclude that the phylogeny of FtsH proteases is consistent with an early origin of water oxidation chemistry

Enhancing photosynthesis in plants: the light reactions

In this review, we highlight recent research and current ideas on how to improve the efficiency of the light reactions of photosynthesis in crops. We note that the efficiency of photosynthesis is a balance between how much energy is used for growth and the energy wasted or spent protecting the photosynthetic machinery from photodamage. There are reasons to be optimistic about enhancing photosynthetic efficiency, but many appealing ideas are still on the drawing board. It is envisioned that the crops of the future will be extensively genetically modified to tailor them to specific natural or artificial environmental conditions

The photosynthetic electron transport chain of oxygenic photosynthesis

Oxygenic photosynthesis, performed by plants, algae, and cyanobacteria, is the major route by which solar energy is converted into chemical energy on earth. This process provides the essentials (e.g., food and fuels) for humans to survive and is responsible for oxygenating the earth's atmosphere, which allowed the evolution of multicellular life. Photon energy is harvested during the so-called “light reactions” and used to extract electrons from water, which are then transported through an electron transport chain—in a type of bioelectric current—that is coupled to the movement of protons across the thylakoid membrane, storing energy in the form of a proton electrochemical gradient. The net result of the light reactions is the synthesis of adenosine triphosphate and reduced nicotinamide adenine dinucleotide phosphate, which are used by the “dark” or “light-independent” reactions to convert carbon dioxide into carbohydrates in the Calvin–Benson cycle. In this study, we summarize the structure and function of the main redox-active proteins involved in electron transfer and highlight some recent developments aiming to enhance the efficiency and robustness of the light reactions

Accessibility controls selective degradation of photosystem II subunits by FtsH protease

The oxygen-evolving photosystem II (PSII) complex located in chloroplasts and cyanobacteria is sensitive to light-induced damage1 that unless repaired causes reduction in photosynthetic capacity and growth. Although a potential target for crop improvement, the mechanism of PSII repair remains unclear. The D1 reaction center protein is the main target for photodamage2, with repair involving the selective degradation of the damaged protein by FtsH protease3. How a single damaged PSII subunit is recognized for replacement is unknown. Here, we have tested the dark stability of PSII subunits in strains of the cyanobacterium Synechocystis PCC 6803 blocked at specific stages of assembly. We have found that when D1, which is normally shielded by the CP43 subunit, becomes exposed in a photochemically active PSII complex lacking CP43, it is selectively degraded by FtsH even in the dark. Removal of the CP47 subunit, which increases accessibility of FtsH to the D2 subunit, induced dark degradation of D2 at a faster rate than that of D1. In contrast, CP47 and CP43 are resistant to degradation in the dark. Our results indicate that protease accessibility induced by PSII disassembly is an important determinant in the selection of the D1 and D2 subunits to be degraded by FtsH

Newly discovered Synechococcus sp. PCC 11901 is a robust cyanobacterial strain for high biomass production.

Cyanobacteria, which use solar energy to convert carbon dioxide into biomass, are potential solar biorefineries for the sustainable production of chemicals and biofuels. However, yields obtained with current strains are still uncompetitive compared to existing heterotrophic production systems. Here we report the discovery and characterization of a new cyanobacterial strain, Synechococcus sp. PCC 11901, with promising features for green biotechnology. It is naturally transformable, has a short doubling time of ≈2 hours, grows at high light intensities and in a wide range of salinities and accumulates up to ≈33 g dry cell weight per litre when cultured in a shake-flask system using a modified growth medium - 1.7 to 3 times more than other strains tested under similar conditions. As a proof of principle, PCC 11901 engineered to produce free fatty acids yielded over 6 mM (1.5 g L-1), an amount comparable to that achieved by similarly engineered heterotrophic organisms

Crystal structure of geranylgeranyl pyrophosphate synthase (CrtE) involved in cyanobacterial terpenoid biosynthesis

Cyanobacteria are photosynthetic prokaryotes that perform oxygenic photosynthesis. Due to their ability to use the photon energy of sunlight to fix carbon dioxide into biomass, cyanobacteria are promising hosts for the sustainable production of terpenoids, also known as isoprenoids, a diverse class of natural products with potential as advanced biofuels and high-value chemicals. However, the cyanobacterial enzymes involved in the biosynthesis of the terpene precursors needed to make more complicated terpenoids are poorly characterized. Here we show that the predicted type II prenyltransferase CrtE encoded by the model cyanobacterium Synechococcus sp. PCC 7002 is homodimeric and able to synthesize C20-geranylgeranyl pyrophosphate (GGPP) from C5-isopentenyl pyrophosphate (IPP) and C5-dimethylallyl pyrophosphate (DMAPP). The crystal structure of CrtE solved to a resolution of 2.7 Å revealed a strong structural similarity to the large subunit of the heterodimeric geranylgeranyl pyrophosphate synthase 1 from Arabidopsis thaliana with each subunit containing 14 helices. Using mutagenesis, we confirmed that the fourth and fifth amino acids (Met-87 and Ser-88) before the first conserved aspartate-rich motif (FARM) play important roles in controlling chain elongation. While the WT enzyme specifically produced GGPP, variants M87F and S88Y could only generate C15-farnesyl pyrophosphate (FPP), indicating that residues with large side chains obstruct product elongation. In contrast, replacement of M87 with the smaller Ala residue allowed the formation of the longer C25-geranylfarnesyl pyrophosphate (GFPP) product. Overall, our results provide new structural and functional information on the cyanobacterial CrtE enzyme that could lead to the development of improved cyanobacterial platforms for terpenoid production

Recent advances in understanding the structural and functional evolution of FtsH proteases

The FtsH family of proteases are membrane-anchored, ATP-dependent, zinc metalloproteases. They are universally present in prokaryotes and the mitochondria and chloroplasts of eukaryotic cells. Most bacteria bear a single ftsH gene that produces hexameric homocomplexes with diverse house-keeping roles. However, in mitochondria, chloroplasts and cyanobacteria, multiple FtsH homologues form homo and heterocomplexes with specialised functions in maintaining photosynthesis and respiration. The diversification of FtsH homologues combined with selective pairing of FtsH isomers is a versatile strategy to enable functional adaptation. In this article we summarise recent progress in understanding the evolution, structure and function of FtsH proteases with a focus on the role of FtsH in photosynthesis and respiration

Functional Roles of D2-Lys317 and the Interacting Chloride Ion in the Water Oxidation Reaction of Photosystem II As Revealed by Fourier Transform Infrared Analysis

[Image: see text] Photosynthetic water oxidation in plants and cyanobacteria is catalyzed by a Mn(4)CaO(5) cluster within the photosystem II (PSII) protein complex. Two Cl(–) ions bound near the Mn(4)CaO(5) cluster act as indispensable cofactors, but their functional roles remain to be clarified. We have investigated the role of the Cl(–) ion interacting with D2-K317 (designated Cl-1) by Fourier transform infrared spectroscopy (FTIR) analysis of the D2-K317R mutant of Synechocystis sp. PCC 6803 in combination with Cl(–)/NO(3)(–) replacement. The D2-K317R mutation perturbed the bands in the regions of the COO(–) stretching and backbone amide vibrations in the FTIR difference spectrum upon the S(1) → S(2) transition. In addition, this mutation altered the (15)N isotope-edited NO(3)(–) bands in the spectrum of NO(3)(–)-treated PSII. These results provide the first experimental evidence that the Cl-1 site is coupled with the Mn(4)CaO(5) cluster and its interaction is affected by the S(1) → S(2) transition. It was also shown that a negative band at 1748 cm(–1) arising from COOH group(s) was altered to a positive intensity by the D2-K317R mutation as well as by NO(3)(–) treatment, suggesting that the Cl-1 site affects the pK(a) of COOH/COO(–) group(s) near the Mn(4)CaO(5) cluster in a common hydrogen bond network. Together with the observation that the efficiency of the S(3) → S(0) transition significantly decreased in the core complexes of D2-K317R upon moderate dehydration, it is suggested that D2-K317 and Cl-1 are involved in a proton transfer pathway from the Mn(4)CaO(5) cluster to the lumen, which functions in the S(3) → S(0) transition