Oxygen – pros and cons

Like us, plants live in a world with an oxidising environment due to the high level of oxygen (O₂) in the atmosphere. Like us they reduce oxygen to water in the process of respiration. What’s more, plants also make oxygen through photosynthesis. Living with oxygen in the environment and undertaking oxygen-related chemical reactions exposes plants to potentially toxic reactive oxygen species. These are produced in chemical reactions involving single electron reductions of oxygen. Examples of these partially reduced oxygen species are singlet oxygen, superoxide, hydrogen peroxide and hydroxyl radicals (collectively termed “oxidants”). These molecules can react with key intracellular molecules such as proteins, lipids and DNA causing damage to a myriad of cellular functions.

Plants get stressed too

Usually a balance exists between the production of these cytotoxic molecules and their degradation by a suite protective processes that limit their accumulation, detoxify excess oxidants and repair damaged cellular components. The antioxidant machinery in the cell is critical for the detoxification. However, if the balance between production and detoxification of oxidants is perturbed, oxidatively damaged components accumulate, which at worst can result in cell death and at best impair cell function, impacting negatively on plant health and yield. This is commonly termed oxidative stress.

Oxidative stress is a hallmark molecular response in cells to a wide variety of insults, ranging from harsh environmental conditions (high light, extreme temperature, salinity, drought) to invasion by bacteria, fungi or viruses. Oxidative stress in plants is assessed by measuring increases in oxidants, changes in the amount of cytotoxic by-products of oxidants, or of oxidatively inactivated cellular components. Oxidative damage plays a central role in determining the relative efficiency of cell function - and thus crop performance - in a real-world environment.

Vitamins and antioxidants

Plants have developed very sophisticated mechanisms to recognise the onset of oxidative stress and to respond to it by increasing expression of their antioxidant defences in order to recover the balance. These defences include enzymes that directly detoxify oxidants and synthesis of antioxidant molecules such as carotenoids (including provitamin A), vitamin C and vitamin E. Genetic variability in these processes has allowed the evolution or breeding of stress-tolerant and resistant cultivars and varieties of plants. These processes can also be manipulated by chemical agents, such as herbicide safeners, that prime a plant to overcome a subsequent oxidative stress.

Signalling in oxidative stress responses

The three energy organelles in plants are intimately involved in oxidative stress and the plant’s response to it. On the one hand they contain the highest concentration of antioxidant defence compounds and detoxifying enzymes, but they are also the major sites of oxidant production in the cell. However, nearly all of the genes required for antioxidative processes are encoded in the nucleus. Hence signals must travel between the organelles and the nucleus to ensure that changes in the production of oxidants in organelles, and the consequent damage, are relayed to the nucleus to generate an orderly and timely response to ensure organelle protection. Discovering and identifying the genes and metabolic networks that underpin the production and detoxification of oxidants, and discovering the nature of the signals that coordinate a cell’s response to oxidative insults are key objectives of our research. This will allow the manipulation of oxidative stress responses in plants, and pave the way for intelligent engineering of plants that are stress-tolerant.