Enhancing photosynthesis in crop plants: targets for improvement

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.

Professor A William Rutherford FRS, Imperial College London, UK

Professor A William Rutherford FRS, Imperial College London, UK

Bill Rutherford uses physico-chemical methods to understand photosynthesis. He has particular interests in the bioenergetics and evolution of photosynthetic electron transfer and water oxidation. Having studied biochemistry at the University of Liverpool, his PhD at UCL involved biophysics of bacterial photosynthesis. As a post doc at the University of Illinois, he moved into oxygenic photosynthesis, discovering the excited triplet state of chlorophyll formed by radical-pair recombination. This state causes photo-oxidative stress that can limit plant growth, bleach coral, etc. As a post doc in Riken, Japan, he determined the origin of thermoluminescence in oxygenic photosynthesis, establishing it as research method. His work was central in establishing the common evolutionary origins of all the photosynthetic reaction centres. He was a CNRS researcher in Saclay, near Paris, before moving to Imperial College London in 2011, where he holds the Chair of Biochemistry of Solar Energy.

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.

Professor Paul Falkowski, Rutgers University, USA

Professor Paul Falkowski, Rutgers University, USA

Paul G. Falkowski is the Bennett L. Smith Professor and director of the Environmental Biophysics and Molecular Ecology Program at Rutgers University. His scientific interests include evolution of the Earth systems, paleoecology, photosynthesis, biophysics, biogeochemical cycles, and symbiosis. Falkowski earned his B.S. and M.Sc. degrees from the City College of the City University of New York and his Ph.D. from the University of British Columbia. After a post-doctoral fellowship at the University of Rhode Island, he joined Brookhaven National Laboratory in 1976 as a scientist in the newly formed Oceanographic Sciences Division. He moved to Rutgers University in 1998. He works on global photosynthesis, especially in the oceans.  He is a member of United States National Academy of Sciences. Falkowski lives in Princeton, New Jersey, with his wife, Sari Ruskin.

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.

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

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

Christine Helen Foyer is the Professor of Plant Sciences at the University of Leeds, Leeds, UK. She is also a Winthrop Professor at The University of Western Australia, Perth, Australia and a Pao Yu-Kong Chair Professor of Zhejiang University, Hangzhou, China. She is a member of the French Academy of Agriculture and the Secretary General of the Federation of European Plant Biologists. She is member of BBSRCCommittee B and the BBSRC (UK) Pool of Experts. She is an Annals of Botany Company Board Member, an Associate Editor for Plant, Cell and Environment and on the Editorial Boards of PhysiologiaPlantarum, Journal of Experimental Botany and Frontiers in Plant Sciences. She is a Scientific Advisory Board member forthe Helmholtz Center Munich. Her current H-Index is 83. Her research interests concern how primary processes (photosynthesis, respiration) alter the reduction/oxidation (redox) status of the cell and influence plant growth and defence responses to stresses such as drought, chilling, high light and aphid infestation.

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.

Dr Michael Nuccio, Syngenta Crop Protection, LLC, USA

Dr Michael Nuccio, Syngenta Crop Protection, LLC, USA

I completed a B.S. in Biochemistry and a Ph.D. in Plant Biology at Texas A&M University. I did Post-Doctoral work in Plant Metabolic Biochemistry with Andrew Hanson at the University of Florida before joining Novartis (now Syngenta) in 2000 as a staff scientist. During my time with Syngenta I’ve had a number of technical, functional and project management roles. I am currently a Principle Research Scientist focused on trait discovery. My primary responsibility is applied genetic engineering to develop water optimization and yield enhancement technology for field crops. My work includes developing trait expression control tools and computational applications to facilitate trait discovery. Over the last five years I have also been responsible for identifying and securing research collaborations to support the company’s Agronomic Trait development objectives. My professional work produced several peer-reviewed publications, patents and patent applications.

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.

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

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

Rienk van Grondelle studied physics at the VU University.He obtained his PhD under the supervision of Prof. Dr. Lou Duysens. In 1982 he returned to the VU University, in 1987 he was appointed full professor. There he has built a large group studying the early events in photosynthesis. RvG has made major contributions to elucidate the fundamental physical mechanisms that underlie light harvesting and charge separation. He has developed theoretical tools to understand complex spectroscopic data. Using multi-dimensional electronic spectroscopy he recently showed that ultrafast charge separation is driven by specific molecular vibrations that allow electronic coherences to stay alive. He proposed a molecular model for photoprotection and demonstrated that the major plant light harvesting complex operates as a nanoswitch, controlled by its biological environment. These results, of utmost importance for our understanding of photosynthesis, inspire technological solutions for artificial and/or redesigned photosynthesis, as a route towards sustainable energy production.

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.

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

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

Alexander Ruban is a Professor in Biophysics at Queen Mary University of London and holds a 'Professeur des Universites (Biochimie et biologie moleculaire)' title awarded by the French Ministry of Education. He accomplished his undergraduate and masters studies at the Kiev State University (Ukraine) and obtained First Class Honours in Biophysics. He received his PhD at the Institute of Photobiology at the Academy of Sciences of Belarus. Since then Prof Ruban has been working at the Institute of Plant Physiology in Ukraine and later at the University of Sheffield in Prof Peter Horton’s laboratory. From 2006 he moved to Queen Mary Univerity of London. His research interests are focused on the photosynthetic light harvesting and mechanisms of protection against light stress in plants and algae. Specific research interests are the role of the membrane macrostructure in the photosynthetic adaptation, study of properties of carotenoids and their role in modulation of membrane protein conformation and functions. He contributed to the fundamental understanding of the molecular design of the photosynthetic light harvesting machinery introducing the concept of light adaptation 'memory' via the allosteric action of the xanthophyll cycle.

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.

Professor Peter J Nixon, Imperial College London, UK

Professor Peter J Nixon, Imperial College London, UK

Peter Nixon is currently a Professor of Biochemistry within the Department of Life Sciences at Imperial College London and a visiting Professor within the School of Biological Sciences at Nanyang Technological University, Singapore. His research focuses on the assembly and maintenance of the photosynthetic apparatus in the thylakoid membrane and more recently the use of synthetic biology approaches to develop cyanobacterial and chloroplasts as ‘solar biorefineries’ for the production of high-value products. He is co-convenor of the Photosynthesis and Metabolism Group of the Society of Experimental Biology.

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.

Dr Anja Krieger-Liszkay, CEA Saclay, France

Dr Anja Krieger-Liszkay, CEA Saclay, France

My main research interest is acclimation of higher plants to abiotic stress with an emphasis on regulation of photosynthetic electron transport and generation of reactive oxygen species. I characterize physiologically important processes by a combination of biophysical/biochemical/physiological techniques integrating in vivo observations and studies characterizing isolated systems on a molecular level. During my PhD thesis supervised by E. Weis at the University Düsseldorf and Postdoctoral studies (with A. W. Rutherford (CEA Saclay), with U. Heber (Würzburg University) and at Freiburg University) I have characterized a molecular switch which modulates the yield of singlet oxygen formation in photosystem II. After my appointment as researcher at the CNRS (CEA Saclay) I extended this work towards the characterization of signaling pathways triggered by singlet oxygen and hydrogen peroxide. Nowadays, I have started to characterize the plastid terminal oxidase and I got interested in redox regulation at the level of photosystem I.