A role for G proteins in plant hormone signalling?

The G protein signalling pathway is one of the most highly conserved mechan isms that enables cells to sense and respond to changes in their environmen t. Essential components of this are cell surface G protein-coupled receptor s (GPCRs) that perceive extracellular ligands, and heterotrimeric G protein s (G proteins) that transduce information from activated GPCRs to down-stre am effecters such as enzymes or ion channels. It is now clear from a range of biochemical and molecular studies that some potential G protein signalli ng components exist in plants. The best examples of these are the seven tra nsmembrane receptor homologue GCR1 and the G(alpha) (GPA1) and G(beta) (G b eta 1) subunit homologues of heterotrimeric G proteins. G protein agonists and antagonists are known to influence a variety of signalling events in pl ants and have been used to implicate G proteins in a range of signalling pa thways that include the plant hormones gibberellin and auxin. Furthermore, antisense suppression of GCR1 expression in Arabidopsis leads to a phenotyp e that supports a role for this receptor in cytokinin signalling. This revi ew considers the current evidence for and against functional G protein sign alling pathways in higher plants and questions whether or not these might b e involved in the action of certain plant hormones. (C) Elsevier, Paris.