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Current position
Professor
Research Interests
The physiology, biophysics and biochemistry of photosynthesis, including: photoprotection and photoinactivation of photosystem II, and thylakoid structure and function.
Entropy as a novel determinant of photosynthetic structure and function
Chloroplasts, the powerhouse of higher plants and recently-evolved green algae, almost invariably have a granal structure: flattened thylakoid sacs stacked up to form orderly grana, interconnected by non-stacked thylakoids and bathed in an aqueous stroma phase, all enclosed in a double-membrane envelope. Obviously, chloroplasts are an open system through which both energy and mass flow.
But why are grana so ubiquitous in the plant world? What is Nature’s driving force for selecting a granal structure? This research seeks to (1) demonstrate the involvement of disorder (entropy) in thylakoid stacking, (2) investigate the functional implications of entropy-assisted thylakoid stacking, (3) establish that the functional consequences in turn enable chloroplasts to increase entropy production, a thermodynamic imperative, and (4) relate the evolution of a granal structure to the evolutionary increase in complexity, defined as the energy flow through an open system per unit time per unit mass.
Dynamic architecture of plant thylakoid membranes between light and dark
Recent intermediate atomic resolution of the protein complexes of plant thylakoid membranes reveals an intriguing aspect of their organization into highly organized oligomeric assemblies; these supercomplexes enhance structural and functional stability compared to monomers. In contrast, little is known about the dynamic interactions of thylakoid supercomplexes in vivo inthe dark, in limiting light, in saturating light or in excess light where photosystem II becomes progressively photoinactivated. As well as long-term adjustments of thylakoid composition by gene expression in response to environmental cues (photosynthetic acclimation), existing supercomplexes are rapidly reorganized by dynamic changes in protein structure and/or supramolecular organization within the membrane in response to light. The spectral quality of light induces limited state transitions, while light quantity induces flexible partition of absorbed energy between use in photosynthesis and dissipation as heat over widely fluctuating light irradiance that involves non-photochemical quenching, photosystem II photoinactivation and D1 protein repair. Our goal is to study the structural dynamics of the supramolecular organization of the thylakoid protein complexes, particularly the dynamic structure of photosystem II in the light and in the dark, and how dynamic structural changes in grana stacking in higher plants assist function.
Pathways of photosynthetic electron flow under environmental stress
The two photosystems in the chloroplast work in series to deliver electrons from the substrate water to NADP+ in two light-driven uphill steps, conserving the light energy as chemical energy in NADPH and ATP. In addition to this linear electron flow, there is a cyclic electron pathway mediated by ferredoxin around Photosystem I, another Photosystem I cyclic pathway involving NADPH, a Q cycle around the cytochrome bf complex, as well as electron donation from stromal reductants. A challenge is to quantify these separate electron fluxes, all except one of which pass through P700, the primary electron donor in Photosystem I, and to do so in leaves without having to isolate the chloroplasts. How do these fluxes vary under different environmental conditions? What roles do the electron fluxes play in the protection of the photosynthetic apparatus under environmental stress, such as high light and/or drought? The overall goal is to decipher the various electron fluxes in situ, with leaves functioning in defined environmental conditions.
Photoprotection and photoinactivation of photosystem II
Light, as the energy source for photosynthesis, is essential for plant life. During normal photosynthesis, however, there is a small but significant probability of photosystem (PS) II complexes being damaged by light, leading to a loss of ability to evolve oxygen. This damage arises partly because, in order to split water molecules, it is necessary to generate by the use of light energy strong oxidants; these oxidants may inadvertently damage PS II. Recovery from photoinactivation of PS II requires new synthesis of the D1 protein in the PS II reaction centre. Under sunny conditions, the entire population of D1 protein is renewed approximately once a day. Given the continual damage to PS II which occurs during normal photosynthesis and which is exacerbated under environmental stress, plants have evolved a number of photoprotective strategies to minimize photoinactivation of PS II. One of these strategies is the inactive-photosystem II mediated quenching of excitation energy, a kind of last-ditch defence mechanism. Research is directed at understanding the mechanisms of photoprotection and photoinactivation of PS II, using biochemical, biophysical and physiological techniques.
Principal Collaborators
Prof. J. M. Anderson, A/Prof. B. Pogson (BaMBi), Prof. A. B. Hope (Flinders), A/Prof. B. Hankamer (University of Queensland); Prof J Barber (Imperial College London); Prof. P. Horton (The University of Sheffield); A/Prof. J. He (Nanyang Technological University, Singapore); Dr D. Fan (Inst. of Botany, Chinese Academy of Sciences)
Selected Publications
Chow, W.S., Kim, E.-H., Horton, P. and Anderson, J.M. (2005) Stacking of thylakoid membranes in chloroplasts: the physicochemical forces at work and the functional consequences that ensue. Photochemical & Photobiological Sciences 4: 1081-1090.
Chow W.S., Lee, H.-Y., He, J., Hendrickson, L., Hong, Y.-N. and Matsubara, S. (2005) Photoinactivation of Photosystem II in leaves. Photosynthesis Research 84: 35-41.
Chow W.S. and Hope A.B. (2004) Electron fluxes through Photosystem I in cucumber leaf discs probed by far-red light. Photosynthesis Research 81: 77-89.
Matsubara S. and Chow W.S. (2004) Populations of photoinactivated photosystem II reaction centers characterized by chlorophyll a fluorescence lifetime in vivo. Proceedings of the National Academy of Science U.S.A.101: 18234-18239.
Chow, W. S., Lee, H.-Y., Park, Y.-I., Park, Y.-M., Hong, Y.-N. and Anderson, J. M. (2002) The role of inactive photosystem II-mediated quenching in a last-ditch, community defence against high-light stress in vivo. Philosophical Transactions of the Royal Society of London. 357: 1441-1450.
Chow, W.S., Melis, A. and Anderson, J.M. (1990) Adjustments of photosystem stoichiometry in chloroplasts improve the quantum efficiency of photosynthesis. Proceedings of the National Academy of Science U.S.A. 87: 7502-7506.
Chow, W.S., Hope, A.B. and Anderson, J.M. (1989) Oxygen per flash from leaf disks quantifies photosystem II. Biochimica et Biophysica Acta 973: 105-108.
Chow WS (1984) The extent to which the spatial separation between Photosystems I and II associated with granal formation limits non-cyclic electron flow in isolated lettuce chloroplasts. Archives of Biochemistry and Biophysics 232: 162-171.
Chow, W.S., Wagner, G. and Hope, A.B. (1976) Light-dependent redistribution of ions in isolated spinach chloroplasts. Australian Journal of Plant Physiology 4: 853-861.
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