Enhancing photosynthesis in crop plants: targets for improvement

09:05-09:30 Understanding energy limitations in oxygenic photosynthesis

Professor A William Rutherford FRS, Imperial College London, UK

Abstract

Aerobic biological electron transfer systems avoid side-reactions with O₂ by redox tuning. Two physico-chemical numbers are key to these processes: i) -160mV, the redox midpoint potential of the O₂/O₂^-• couple, determining the favourability of superoxide formation; and ii) ~1eV, the amount of energy needed to convert ³O₂, molecular oxygen, to its highly reactive and damaging singlet form, ¹O₂. The occurrence of anomalously energy-inefficient reactions in aerobic bioenergetics can be understood in these terms. Protective and regulatory mechanisms can also be rationalised within this model (including an unexpected new sink-to-source regulatory mechanism in photosynthesis, which I will present). From this perspective Photosystem II (PSII) is “energy squeezed”, having insufficient energy in the 680nm photon to be able to achieve i) high quantum yield charge separation, ii) PSII chemistry (reduction of quinone and oxidation of water), and iii) the appropriate driving forces (over-potentials), while at the same time avoiding ¹O₂-mediated photodamage from back-reactions. PSII is particularly susceptible to photodamage since it must interface 1-photon-per-electron photochemistry with its multi-electron catalytic chemistry, inevitably generating intermediate states that back-react (i.e. S₂, S₃, Q_A- and Q_B-). This “energy squeeze” may set the “red-limit” for oxygenic photosynthesis. The existence of long-wavelength Chld-containing PSII in Acaryochloris appears to contradict this suggestion. We have suggested however that this improvement in efficiency is only possible due a decrease in the “energy headroom”, that part of the over-potential needed to deal with fluctuations in the environment (light intensity): i.e. efficiency gains are paid for by loss of resilience. If so, extending the spectrum of oxygenic photosynthesis to longer wavelengths will encounter problems. We are currently studying long-wavelength species to test these predictions.

• Listen to audio (mp3)

09:30-10:00 Why is photosynthesis in nature so inefficient?

Professor Paul Falkowski, Rutgers University, USA

Abstract

Over the past two decades, together with several colleagues, I have used remotely sensed observations of oceanic chlorophyll and analogous observations of terrestrial plants to derive global production of the planet.  Simultaneously, we build and deployed dozens of high resolution fluorometers that measured the quantum efficiency of photosystem II across the world oceans from changes in the amplitude of variable fluorescence.  Over the past ca. 5 years we attempted to close the budget on the absorbed solar radiation absorbed by phytoplankton.  To this end, we built and deployed picosecond lifetime based instruments.  From tens of thousands of paired measurements of variable fluorescence amplitudes and lifetimes, we conclude that in the real world oceans only ~35% of absorbed solar photons are used for photochemistry, ~7% are dissipated as fluorescence, and the remainding ~58% are dissipated as heat.  The exceptionally high loss of absorbed solar radiation that is dissipated as heat appears to be a consequence of nutrient limitation of photosynthetic processes.  Two nutrients are especially identified: dissolved inorganic nitrogen and soluble iron.  The data suggest that these nutrients, which can be episodically supplied by turbulent mixing events (e.g., storms) allow for very rapid repair of the PSII reaction centers.  I will discuss how these processes in  phytoplankton potentially allow for pathways to enhance photosynthesis in crop plants.

• Listen to audio (mp3)

11:00-11:30 Functions of chloroplast proteins in stress signalling

Professor Christine Foyer, Centre of Plant Sciences, University of Leeds, UK

Abstract

Signalling between the chloroplasts and the nucleus is required to maintain photosynthesis and other functions. The regulated expression of nuclear genes by chloroplast signals is known as retrograde signalling. Redox signals arising in the chloroplast are considered to be an important in retrograde signalling but the mechanisms involved remain poorly defined.  Proteins such as WHIRLY1, which is a ssDNA-binding protein localised in the chloroplasts and nuclei, have the potential to fulfil redox signalling functions. WHIRLY1 is required for plastid genome stability and plastid gene transcription. We have characterised WHIRLY1 functions in barley using RNAi-knockdown lines (W1-1, W1-7 and W1-9) that have very low levels of HvWHIRLY1 transcripts. Leaves of the WHIRLY1-deficient plants establish photosynthesis more slowly than the wild type, but otherwise are similar to the wild type, Photosynthesis rates were similar in all lines but W1-1, W1-7 and W1-9 leaves had significantly more chlorophyll and less sucrose than the wild type. Transcripts encoding specific sub-sets of chloroplast-localised proteins such as ribosomal proteins, subunits of the RNA polymerase and the thylakoid NADH and cytochrome b6/f complexes were much more abundant in the W1-7 leaves than the wild type. The roles of WHIRLY1 in the acclimation of photosynthesis to stress will be discussed, together with possible retrograde signalling mechanisms.

• Listen to audio (mp3)

11:30-12:00 Strategies and tools to improve crop productivity by targeting photosynthesis

Dr Michael Nuccio, Syngenta Crop Protection, LLC, USA

Abstract

Crop productivity needs to substantially increase to meet global food and feed demand of a rapidly growing world population. Agricultural technology developers are pursuing a variety of approaches based on both traditional technologies like genetic improvement, pest control and mechanization as well as new technologies like genomics, gene manipulation and environmental modeling to develop crops that are capable of meeting growing demand. Photosynthesis is a key biochemical process that many suggest is not yet optimized for industrial agriculture or the modern global environment. We are interested in identifying control points in maize photoassimilation that are amenable to gene manipulation to improve overall productivity. Our approach encompasses: developing and using novel gene discovery techniques, translating our discoveries into traits, and evaluating each trait in a stepwise manner that reflects a modern production environment. Our aim is to provide step change advancement in overall crop productivity and deliver this new technology into the hands of growers.

• Listen to audio (mp3)

13:30-14:00 The quantum design of photosynthesis

Professor Dr Rienk van Grondelle, VU University, Amsterdam, Netherlands

Abstract

In photosynthesis absorbed sun light produces collective excitations (excitons) that form a coherent superposition of electronic and vibrational states of the individual pigments. Two-dimensional (2D) electronic spectroscopy allows a visualization of how these coherences are involved in the primary processes of energy and charge transfer. Based on quantitative modeling we identify the exciton-vibrational coherences observed in 2D photon echo of the photosystem II reaction center (PSII-RC). We find that the vibrations resonant with the exciton splittings can modify the delocalization of the exciton states and produce additional states, thus promoting directed energy transfer and allowing a switch between the two charge separation pathways. We conclude that the coincidence of the frequencies of the most intense vibrations with the splittings within the manifold of exciton and charge-transfer states in the PSII-RC is not occurring by chance, but reflects a fundamental principle of how energy conversion in photosynthesis was optimized.

• Listen to audio (mp3)

14:00-14:30 Quantifying photoprotection

Professor Alexander V. Ruban, Queen Mary University London, UK

Abstract

Non-photochemical chlorophyll fluorescence quenching (NPQ) is broadly considered to reflect the major mechanism of rapid regulation of light harvesting which defends the photosystem II reaction center (RCII) against photodamage that leads to the photoinhibition of photosynthesis. Whilst there is a great deal of the knowledge concerning the elements that trigger, tune and actually cause the quenching, little is known about its protective efficiency, the critical light intensity that is ‘safe’ for the photosynthetic organism to live given that it has a capacity for a certain level of NPQ. A newly-developed methodology aimed at radically changing our understanding of the effectiveness of the NPQ process, by quantifying its photoprotective potential, will be presented. The technique is essential if we are to fully understand the trade-offs between the metabolic costs of photoinhibition and the reduction in quantum yield caused by engaging NPQ.  In this approach the value of photochemical quenching (qP) measured in the dark following illumination enables monitoring of the state of active PSII reaction centres in order to detect the early signs of photoinhibition. The method allows for the determination of the amplitude of photoprotective NPQ (pNPQ) and its potential to protect against photoinhibition. It also allows accurate quantification of the relationship between the protective component of pNPQ and actinic light intensity. This in turn makes possible the estimation of the maximum light intensity tolerated by the PSII reaction centers in a plant population, the photoptotective effectiveness of NPQ in plants with different levels of PsbS protein or zeaxanthin, and the fraction of captured energy that may be unnecessarily, or ‘wastefully’, dissipated.

• Listen to audio (mp3)

15:30-16:00 The photosystem II repair cycle as a target for enhancing photosynthesis

Professor Peter J Nixon, Imperial College London, UK

Abstract

Exposure of plants to excessive light intensities, particularly in combination with other abiotic stresses, such as elevated temperature and lack of water, causes a decline in the maximum rate of photosynthesis leading to a state of chronic photoinhibition. It is now well established that the oxygen-evolving photosystem II (PSII) complex of the thylakoid membrane is a primary target of light damage and that the D1 reaction centre polypeptide is most prone to irreversible damage. In vivo damaged PSII complexes can be efficiently repaired via the so-called PSII repair cycle which involves partial disassembly of the inactivated complex, proteolytic degradation of the damaged subunit and specific replacement by a newly synthesized copy followed by reassembly of the active complex. However, when the rate of PSII repair is unable to keep up with the rate of damage, there is a net loss of oxygen-evolving PSII complexes and decline of photosynthetic activity. Many of the proteases and accessory factors involved in the PSII repair cycle have now been identified and substantial progress has been made in understanding their function at the molecular level. In this talk, I will summarise the current view of PSII repair in cyanobacteria and chloroplasts and discuss the feasibility of improving the robustness of photosynthesis in crop plants at the level of the PSII repair cycle.

• Listen to audio (mp3)

16:00-16:30 Biochemical characterisation and physiological role of the plastid terminal oxidase PTOX

Dr Anja Krieger-Liszkay, CEA Saclay, France

Abstract

Protein levels of the plastid terminal oxidase PTOX increase upon abiotic stress. PTOX may protect the photosynthetic apparatus when electron transport is limited. However, overexpression of PTOX in Nicotiana tabacum, increased the production of reactive oxygen species and exacerbated photoinhibition.

The active site of PTOX comprises a non-heme diiron centre that catalyses the oxidation of plastoquinol and the reduction of O₂ to H₂O. We have performed for the first time a biochemical characterization of purified PTOX. The activity of PTOX was determined to be much higher than what had been previously estimated from chlorophyll fluorescence. The main reaction of PTOX is the reduction of O₂ to H₂O but PTOX generates O₂^•- in a side reaction in a substrate- and pH-dependent manner. PTOX activity in planta was investigated using Nicotiana tabacum expressing PTOX1 from Chlamydomonas reinhardtii. PTOX competed efficiently with photosynthetic electron flow. High CO₂ concentrations inactivated PTOX most likely because of an acidification of the stroma. Immunoblots showed that PTOX detached from the membrane in dark-adapted leaves or in the presence of uncouplers. A model is proposed in which the membrane association of PTOX is controlled by stromal pH. When the stromal pH is neutral, PTOX exists as a soluble form and is enzymatically inactive avoiding the interference of PTOX with linear electron flow. When the stromal pH is alkaline and the photosynthetic electron chain is highly reduced under stress conditions, PTOX binds to the membrane, has access to its substrate and can serve as safety valve.

• Listen to audio (mp3)

09:00-09:30 Improving plant photosynthesis by its redesign in bacteria

Professor Dr Dario Leister, Ludwig-Maximilians-University Munich (LMU), Germany

Abstract

The light reactions of plant photosynthesis are an evolutionary patchwork, providing ample scope for their improvement. This particularly concerns its light-harvesting components and the susceptibility of photosystems to photodamage. Classical approaches involve genetic engineering and, more indirectly, conventional breeding. In the long-term, synthetic biology might allow the redesign or de novo creation of entire photosystems that are more efficient because they are less susceptible to photodamage and produce fewer harmful reactive oxygen species. This redesign of entire photosystems is a formidable challenge, but the gain in photosynthetic efficiency when photosystems require less repair and photoprotection will be significant. It is clear that crop plants are the least suited test systems for such approaches, given their long life cycle and inaccessibility to efficient genetic engineering. Therefore, redesigning photosystems will not only require a very deep understanding of the structure and dynamics of the natural photosynthetic light reactions, but also innovative concepts of what a perfect photosystem should look like, as well as novel model organisms in which such concepts can be realized, tested, and reiteratively improved. Obviously, bacteria are the model system of choice and corresponding concepts will be elaborated in this presentation.

• Listen to audio (mp3)

09:30-10:00 Dynamic responses of photosynthesis: minding the gap between the lab and the field

Professor David Kramer, Michigan State University, USA

Abstract

Some of the most pressing open questions about photosynthesis concern how it is adapted to function in living organisms to balance the needs for efficient energy capture and the avoidance of toxic side reactions. I will describe a series of enabling technologies designed to bridge the gaps between the lab and the field that can be and how these tools can be used to observe the biophysical machinery of photosynthesis in action under the conditions of the field. I will describe new results using these tools to explore the relationships between photochemistry and photoprotection in a wide range of wild type and mutant plants, both in the lab and the field. These results identify potential biophysical “Achilles Heels” of photosynthesis that involve the interactions of electron and proton transfer reactions. In one case, loss of thylakoid proton motive force leads to buildup of electrons on, and subsequent photodamage of, photosystem I. In another case, excessive proton motive force that drives ATP synthesis and down-regulates light capture also induces singlet oxygen production by stimulating recombination reactions within photosystem II. I will also show data that suggests that this inherent weakness in the photosynthetic process limits photosynthesis in the field, and likely drove the evolution of a large number of “ancillary regulatory components” of photosynthesis to tune its responses to the environment. Finally, I will discuss the prospects of using this basic knowledge and new phenotyping tools to improve the breeding and and management of crops, especially in the developing world.

• Listen to audio (mp3)

11:00-11:30 Structure and function of ATP synthase and its role as new dug target against tuberculosis

Professor Thomas Meier, Imperial College London

Abstract

F₁F_o-ATP synthases are paradigmatic molecular machines, which use the transmembrane electrochemical ion gradient to power ATP synthesis. The enzymes belong to the class of rotary ATPases, which all share a common architecture principle, consisting of a rotor and stator entity. While ions are shuttled through the F_o complex of the enzyme, torque is generated at the rotor/stator and transferred to the F₁-catalytic subunits for ATP synthesis. In the opposite direction, ATP hydrolysis can be used to drive ion pumping. In my talk I will present the structure of complete ATP synthase analysed by X-ray crystallography and cryo-electron microscopy. Professor Meier will also focus on biochemical and structural investigations of the ATP synthase rotor rings with respect to their cell physiological function in bioenergetics and as a novel target to treat pulmonary tuberculosis.

• Listen to audio (mp3)

11:30-12:00 Alternative carbon fixation pathways for improved crop plant productivity

Dr James W. Murray, Imperial College London, UK

Abstract

Plant productivity is limited by the oxygenation reactino of Rubisco, which leads to the energetically costly photorespiratory pathway. Plants and cyanobacteria have, to varying degree, carbon concentrating mechanisms, that concentrate carbon dioxide around Rubisco to reduce the level of the oxygenation reaction. Transferring these mechanisms to crop plants is a major goal. In the longer term, it may be possible to escape using Rubisco and the Calvin cycle completely, and to fix carbon with a new pathway that is not oxygen sensitive. One example is the autotrophic 3-hydroxypropionate (3HP) pathway of the photosynthetic Chloroflexus group of bacteria.  There are other synthetic pathways designed from individual enzymes.

I will discuss the potential and problems of transferring new autotrophic carbon fixation pathways to plants, algae and cyanobacteria. For example, under conditions of low photorespiration, the Calvin cycle is very efficient. I will describe our flux-balance modelling work on cyanobacterial metabolism under different conditions with different carbon fixation pathways in place. We have made some progress in characterising the enzymes of the 3HP pathway, and in expressing them in the model cyanobacterium Synechocystis 6803.

• Listen to audio (mp3)

13:30-14:00 Improving photosynthesis to improve crop yield

Professor Christine Raines, University of Essex, UK

Abstract

Increasing demands of the growing world population for food and fuel are putting ever greater pressure on the need to develop higher yielding crop varieties. It has been estimated that increases of 50% will be required in the yield of grain crops such as wheat and rice if food supply is to meet the demands of the increasing world population. The primary determinant of crop yield is the cumulative rate of photosynthesis over the growing season which is the result of the crop’s ability to capture light, the efficiency by which this light is converted to biomass and how much biomass is converted into the usable product e.g. grain in the case of wheat and rice. There is compelling evidence from transgenic studies that the manipulation of the Calvin-Benson cycle enzyme SBPase could increase yield in a range of species, including wheat, tobacco and Arabidopsis. More recently we have shown that overexpression of SBPase in combination with FBPaldolase, ictB (from cyanobacteria) or a component of the algal electron transport chain can lead to a further increase in photosynthesis and biomass. Our approaches to the manipulation of photosynthesis will be presented together with results from both greenhouse and field studies.

• Listen to audio (mp3)

14:00-14:30 Overcoming the limitations of Rubisco

Professor Martin Parry, Lancaster University, UK

Abstract

The light driven assimilation of CO₂ in photosynthesis is the primary determinant of crop biomass and yield. Whilst the current theoretical maximum efficiency of photosynthesis of C₃ crops is 4.5%, it averages less than 1% in the field. Improving this conversion efficiency is a key opportunity to both increase yield and also the efficiency with which other resources are used. Overcoming the limitations of Rubisco (ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase) is a simple and direct way towards improving this efficiency. This is because some characteristics of Rubisco make it surprisingly inefficient and compromise photosynthetic productivity. For example, Rubisco catalyses a wasteful reaction with oxygen that leads to the release of previously fixed CO₂ and NH₃ and the consumption of energy during photorespiration. Furthermore, Rubisco is slow and large amounts are needed to support adequate photosynthetic rates. The catalytic properties of Rubiscos isolated from diverse sources vary considerably, suggesting that changes in the speed, affinity, or specificity can be introduced to improve Rubisco performance in specific crops and environments. An analysis of natural catalytic diversity has revealed amino acids under positive selection and allowed prediction of amino acid substitutions that could improve Rubisco in different crops. Tobacco lines expressing ‘faster Rubisco’ - that are capable of higher photosynthetic rates per unit Rubisco at high CO₂ concentrations than the native enzyme - have already been generated, with further changes in progress.

• Listen to audio (mp3)

15:30-16:00 Understanding the evolution of C4 photosynthesis

Professor Julian Hibberd, University of Cambridge, UK

Abstract

Along with mimicry and the camera-like eye, the C₄ pathway is a remarkable example of the repeated evolution of a complex trait. In fact, despite its complexity, C₄ photosynthesis has evolved in at least 66 independent plant lineages. I will discuss molecular mechanisms that have likely facilitated the repeated evolution of the C₄ trait. This will include the repeated use of the same cis-elements and homologous transcription factors, to control C₄ gene expression, as well as the use of pre-existing gene regulatory networks. I will also use information from the C₃ model A. thaliana to illustrate some of these phenomena.

• Listen to audio (mp3)

16:00-16:30 Engineering carboxysome cores and bacterial microcompartments for enhancing photosynthesis

Professor Cheryl Kerfeld, Michigan State University and Berkeley National Laboratory, USA

Abstract

Dissociating the complexity of photosynthetic processes into modules is a shift in perspective from the single gene/gene product to discrete functional and evolutionary units.  Modules can be defined as semi-autonomous functional units, with relatively strong intramodule functional connectivity among components and weaker, yet important, inter-module connections and interfaces.  By virtue of their potential for “plug and play” into new contexts, modules can be viewed as units of both evolution and engineering.  By coupling modular thinking with the technical advances that have made large scale DNA fabrication affordable, the prospects of engineering organisms by installing new functional modules becomes attainable.  

Bacterial microcompartments (BMCs) such as carboxysomes illustrate biological modularity in the form of a multienzyme-containing proteinaceous organelle.  The carboxysome is a self-assembling metabolic module for CO2 fixation found in all cyanobacteria.  These large (~100-500 nm) polyhedral bodies sequester Carbonic Anhydrase and RuBisCO within a protein shell, thereby concentrating substrates and protecting RuBisCO from oxygen generated by the light reactions.  Because carboxysomes and other BMCs function to organize reactions, sequester substrates, cofactors, or toxic intermediates and to protection of oxygen sensitive enzymes, they have received considerable attention for bioengineering of heterologous pathways, in addition to the goal of installing carboxysomes directly into plants.  There are two central challenges to these goals: design and assembly of multi-enzyme cores and tailoring of shell proteins to serve as the interface between the cytosol and the encapsulated reactions.  These challenges and proof-of-concept solutions will be discussed.  

• Listen to audio (mp3)

16:30-17:00 Future directions

• Listen to audio (mp3)