|Plant Stress Biology at Manchester University|
We are interested in the regulation of higher plant photosynthesis under conditions of environmental stress. In particular we are trying to understand the mechanisms that allow certain plants to tolerate stress and avoid the production of reactive oxygen species (ROS) when exposed to stress. We use a range of techniqes, including spectroscopic analyses of in vivo photosynthetic performance, biochemical approaches, including quantitative proteomics, and molecular approaches, including using T-DNA insertions, RNAi and microarray technologies.
What is environmental stress: for our purposes, we define environmental (or abiotic) stress as being any conditions that give rise to imbalances in the ability of a plant to perform normal metabolic processes. This might include extremes of temperature, light, water availability etc. We are primarily interested in conditions that are liable to result in an imbalance between the absorption of light by chlorophyll and the use of the absorbed energy in photosynthetically driven metabolic processes. When such an imbalance occurs, any excess absorbed energy is liable to give rise to ROS production.
What are reactive oxygen species: Absorption of excess energy is liable to result in the production of ROS. These are highly reactive derivatives of oxygen that are capable of reacting with and destroying a wide range of biomolecules including DNA, proteins and lipids. The chloroplast is the major source of ROS in plant leaves, with these being mainly formed via one of two routes: 1) photoreduction of molecular oxygen gives rise to superoxide (O2-) in the so-called Mehler reaction. This in turn can undergo dismutation to produce hydrogen peroxide (H2O2) which can in turn form hydroxyl radicals. 2) Singlet excited oxygen can be formed by the interaction of molecular oxygen with triplet excited chlorophyll. The latter is formed either via intersystem crossing from singlet excited chlorophyll or through charge recombination reactions taking place primarily in the photosystem II reaction centre.
How can regulation avoid ROS production: when the absorption of light exceeds the capacity for photosynthetic metabolism, ROS are liable to be formed. To avoid this, the plant must ensure that the excess energy is dissipated harmlessly. Avoidance of the Mehler reaction can be achieved through controlling the flow of electrons through the electron transport chain. This is achieved by down regulating the cytochrome bf complex, between photosystems II and I. The mechanism of this regulation is uncertain, however we have evidence that this is achieved via a redox feedback mechanism, possibly involving a thioredoxin linked pathway (Johnson 2003). Avoidance of singlet oxygen formation is achieved by activating processes that dissipate excess light energy as heat - in particular high energy state quenching. This is triggered by the presence of a pH gradient across the thylakoid membrane. The control of this gradient is not fully understood, however there is growing evidence that this requires cyclic electron transport to occur, involving only the Photosystem I reaction centre (see Johnson 2005). We are seeking to understand the mechanism and regulation of cyclic electron transport.
In addition to these short term regulatory processes, plants are also able, in the longer term, to alter the composition of their chloroplasts to optimise the balance between light capture and use. We are currently investigating the molecular processes which occur following a transition from low to high light. Over about 7 days following such a transition, the capacity for photosynthesis increases. We have been examining changes in gene expression and protein levels under such conditions. We have recently identified a novel transportor localised in the chloroplast envelope, that is essential for this process.