Metabolic signalling in defence and stress: the central roles of soluble redox couples

Plant growth and development are driven by electron transfer reactions. Modifications of redox components are both monitored and induced by cells, and are integral to responses to environmental change. Key redox compounds in the soluble phase of the cell are NAD, NADP, glutathione and ascorbate--all of which interact strongly with reactive oxygen. This review takes an integrated view of the NAD(P)-glutathione-ascorbate network. These compounds are considered not as one-dimensional 'reductants' or 'antioxidants' but as redox couples that can act together to condition cellular redox tone or that can act independently to transmit specific information that tunes signalling pathways. Emphasis is placed on recent developments highlighting the complexity of redox-dependent defence reactions, and the importance of interactions between the reduction state of soluble redox couples and their concentration in mediating dynamic signalling in response to stress. Signalling roles are assessed within the context of interactions with reactive oxygen, phytohormones and calcium, and the biochemical reactions through which redox couples could be sensed are discussed.