Understanding energy limitations in oxygenic photosynthesis
Professor A William Rutherford FRS, Imperial College London, UK
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, QA- and QB-). 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.
Why is photosynthesis in nature so inefficient?
Professor Paul Falkowski, Rutgers University, USA
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.
Functions of chloroplast proteins in stress signalling
Professor Christine Foyer, Centre of Plant Sciences, University of Leeds, UK
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.
Strategies and tools to improve crop productivity by targeting photosynthesis
Dr Michael Nuccio, Syngenta Crop Protection, LLC, USA
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.