The cellular redox state in plant stress biology--a charging concept.

Different redox-active compounds, such as ascorbate, glutathione, NAD(P)H and proteins from the thioredoxin superfamily, contribute to the general redox homeostasis in the plant cell. The myriad of interactions between redox-active compounds, and the effect of environmental parameters on them, has been encapsulated in the concept of a cellular redox state. This concept has facilitated progress in understanding stress signalling and defence in plants. However, despite the proven usefulness of the concept of a redox state, there is no single, operational definition that allows for quantitative analysis and hypothesis testing.