Background

The transfer of intercellular and intracellular information is essential for the control of plant growth and development. In the form of cellular proteins, a framework is present for the reception, amplification and transmission of such information through the plant. The most widely studied mechanism for the transfer of information on this network is phosphorylation. Studies of signal transduction via phosphorylation–dephosphorylation began more than three decades ago in animal cells (Sutherland & Wosilait, 1955), and have reached new intensities in the last decade. As a result, a complex pattern of interacting elements has been identified which, beginning with the receipt of a defined stimulus, results in the modulation of cellular function. In plant cells, where clearly defined agonist–receptor–stimulus models are not so readily available for study, progress has not been so rapid, and investigation has concentrated on finding systems to match those in animals. Significant progress has been achieved, from the identification of a number of plant protein kinases in the early 1970s (Ralph et al., 1972; Keates & Trewavas, 1974), to the identification of several plant GTP-binding proteins (G proteins) (see, for example, Drobak et al., 1988; Blum et al., 1988), and the recent cloning of a calcium-dependent protein kinase (CDPK) (Harper et al., 1991). Progress has been aided by the use of tools developed to elucidate more clearly defined animal pathways, such as anti-G-protein antibodies and specific promoters and inhibitors of phosphorylation.