Introduction: intracellular signalling in plant cells

Plants are able to detect and respond quickly to environmental stimuli. For example, dramatic changes in intracellular metabolism and leaf orientation occur in plants, in response to changing light conditions, that allow them to optimise their ability to harvest energy from the sun at all times of the day. Many plants are disease-resistant because they are able to detect fungal attack and respond by mounting a battery of chemical defences.

Other potential routes for signalling pathways in plant cells are those emanating from specialised internal regulators of timed events such as cell cycle or circadian rhythms (often called ‘internal clocks’). There may also be regulatory networks controlling exchange of information between different cellular compartments (e.g. between chloroplast and cytoplasm) which might be mediated by metabolites or by specialised signal molecules.

It is becoming increasingly clear that, as in animal cells (see Hardie, this volume), changes in intracellular protein (serine/threonine) phosphorylation in plants are central in transduction of extracellular signals, in control of the cell cycle and circadian rhythms and in internal metabolic control of cells (Fig. 1).

It is the critical property of the reversibility of intracellular protein phosphorylation which suits this mechanism for its role in ‘switching’ activities of target proteins from one steady state to another and allows for great sensitivity of control and integration of cellular regulation. Therefore, control of protein phosphatase activity is just as important as control of the protein kinases.