Biochemical signal transduction is a term used for chains, or rather for networks, of reactions induced by chemical or physical signals. Chemicals as signals in plants may originate internally, for instance hormones or other compounds acting at low concentrations, or externally, for instance by cell–cell interactions (Scherer, 1990a). Typical physiological signals (besides hormones) in plants are gravity, light or touch. Usually, signal transduction reactions are triggered by a conformational change induced by binding of the signalling molecule or by other induced physical changes to a receptor structure, which are in turn transduced by conformational changes in proteins interacting with the receptor structure. A typical reaction chain in eukaryotic signal transduction leads from a membrane-bound receptor to interactions with G proteins, which may activate or inhibit second-messenger-generating enzymes (Gilman, 1984). The enzymes known to generate second messengers in animal cells are phospholipase C, phospholipase A2 and phospholipase D, all of which generate lipid breakdown products as second messengers, and adenylate cyclase and guanylate cyclase, which generate cAMP and cGMP, respectively. Cytosolic Ca2+ ions also function as second messengers, the concentration of which is regulated by several processes, including the action of other second messengers. Hence, it is more correct to envisage signal transduction in plants as a network rather than as a linear chain of reactions.

Second messengers often activate protein kinases specifically, and these in turn can regulate enzymatic activities by regulatory phosphorylation, leading eventually to cellular responses (Ranjeva & Boudet, 1987).