Multiscale structural basis of photoprotection in plant light-harvesting proteins

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The project objective is a structure-based molecular theory of the fast photoprotective mechanism (qE) in the PSII antenna of plants. Although now an immediate target for crop optimization efforts, qE is poorly structurally characterized and therefore highly controversial. We aim to resolve this with a multiscale approach, first establishing the structural changes which determine light-harvesting or quenching in individual LHCII sub-units and then combining these parameters with correlated AFM/FLIM to characterize large-scale photoprotective mechanisms within membranes.

The intrinsic switch in individual LHCII is too subtle to be resolved by structural experiment and so we will use a combination of Molecular Dynamics simulations and the PI's theory of carotenoid-mediated quenching to predict it. We will characterize how external conditions determine structure, how structure determines pigment interactions, energy transfer and quenching, and how these processes determine the functional and spectral properties of LHCII. By considering conditions and mutations that inhibit or promote quenched states we will reveal which interactions are responsible for energy quenching and the dynamics of their (de)activation.

Although quenching in isolated LHCII is a reductive model system, knowing its internal mechanics and kinetic parameters yields the basis for studying the basic functional units of in vivo qE: LHCII clusters that form in the thylakoid membrane. The Co-I's novel correlated AFM/FLIM approach allows us to directly visualize these and quantify their fluorescence kinetics. Combining this with our single molecule parameters we will construct the first structurally-informed theory of qE. We will determine which energy-quenching pathways are relevant in vivo, the concentration of these quenchers in the membrane and their relationship to the size and shape of the antenna superstructure. This will provide the core structural basis currently missing from this field.

(As in the Lead Proposal:)

This project will characterize the qE mechanism that allows plants to regulate their light-harvesting processes, switching between efficiency in low-light and protection in high light. Despite its importance to plant performance there is currently no definitive theory of qE due to a lack of structural information, both in terms of how individual LHCII light-harvesting proteins control their function by altering their shape and how the network of many of these proteins (the 'antenna') are rearranged and 'rewired' according to the light environment. Using a novel combination of bio-molecular simulation, theoretical bio-physics and super-resolution microscopy we will establish the light-harvesting and photoprotective structures of LHCII, the molecular mechanism at the centre of qE, and how these properties can be altered via structural modification. We have identified 4 impact objective.

1. Interaction with agritech: Working with A Ruban (QMUL) and S Santabarbara (CNR, Italy) the PI is developing tools for analysing the fluorescence measurements typically used in the field to quantify plant performance. A Ruban has links to the Waltz Heinz GmbH, developing systems for monitoring plant productivity, light-tolerance and photodamage through measurements of qE. Relating qE to macroscopic plant function is difficult given the lack of a coherent picture of what qE actually is. This project will address this and exploitation of the results will occur through the PI's current Royal Society International Exchange Grant which initiated this collaboration.

2. Future potential for understanding NPQ for crop productivity: Recently it was shown that qE is one of the key factors determining crop-plant productivity. Crude enhancement of qE in tobacco mutants resulted in a 15-20% yield increase in field conditions. However, without understanding the core qE mechanism, more refined approaches will be difficult. The project will produce a structure-based model of fundamental molecular processes of qE and its key structural features and experimental signatures. This involves characterizing altered structures of LHCII with enhanced qE properties. During the early stages of the project we will initiate communication with key research groups working in plant engineering to establish the data requirements of these efforts. At the culmination of the project we will initiate exchange with agritech companies such as Syngenta. The aim is to disseminate the key parameters of qE and our models of LHCII with enhanced NPQ features.

3. Developing the next generation of multi-disciplinary researchers: Bio-science is becoming a truly multi-disciplinary field. In particular, young researchers must be comfortable with both experiment and theory. The project is in collaboration with R Croce (Amsterdam) an expert in the spectroscopic techniques needed to support theoretical research in light-harvesting. Moreover, they will work very closely with the Co-I P Adams (Leeds) to carry out high level visualization measurements. By the end of the project the PDRA will be an expert bio-theoretician with the knowledge of the principals of spectroscopy and microscopy to allow them to design a comprehensive programme of research needed to support theoretical work.


4. Public communication and engagement: qE is a key bioenergetic mechanism but is generally unfamiliar to young science students. Theoretical approaches are also not seen as core to biology. We intend to take part in several out-reach activities, including the Leeds Festival of Science. These presentations have to be interactive and stimulating and we intend to achieve this with a simple animation and an interactive hydrological model of qE and light-harvesting that uses a network of water pipes and valves to illustrate how plants control the flow of solar energy into their biochemical processes.