Gibberellin perception and the Avena fatua aleurone: do our molecular keys fit the correct locks?

The plant hormones GA, ABA, and auxin differ from the majority of animal hormones in that they are hydrophobic weak acids. They are soluble in the inter- and intra-cellular environments of plant tissues and their neutral species can cross the plasma membrane by passive diffusion. Auxin transport is mediated by specific uptake and efflux carriers in plasma membranes, and there is some evidence for carrier-mediated uptake of GA and ABA. Because these plant hormones can cross the plasma membrane it is not a prerequisite that receptors for them should be at the protoplast surface. Nevertheless, there is substantial evidence that auxin acts at the plasma membrane, and evidence suggesting that GA may be perceived at the plasma membrane of A. fatua aleurone protoplasts has been reviewed here. It is conceivable that the plant plasma membrane might provide the means to integrate, transduce, and amplify these signals, and that such properties of the plasma membrane, rather than the permeability characteristics of these ligands, may determine the site of perception. Further progress in our understanding of signal transduction pathways that may be involved in the actions of plant hormones is likely to shed light on these questions. It has been proposed that GA receptors involved in cell elongation may be soluble rather than membrane bound. The soluble 50 kDa GA-binding protein observed in aleurone by GA4 photoaffinity labelling may be a good candidate for a soluble GA receptor.(ABSTRACT TRUNCATED AT 250 WORDS)