Plant hormone binding sites

Receptors for plant hormones are becoming identified with increasing rapidity, although a frustrating number remain unknown. There have also been many more hormone-binding proteins described than receptors. This Botanical Briefing summarizes what has been discovered about hormone binding sites, their discovery and descriptions, and will not dwell on receptor functions or activities except where these are relevant to understand binding.

Of those receptors identified, each falls into recognized protein superfamilies. Ethylene and cytokinin receptors have intracellular histidine kinase phosphorelay domains, but the ligand-binding sites are distinct, one being buried within membrane-spanning helices, the other in an extracellular loop domain. Brassinosteroid and phytosulfokine receptors are members of the leucine-rich repeat receptor-like protein superfamily and for these the ligand binding sites are likely to be in one of the loops of the extracellular leucine-rich domain. For auxin, the auxin-binding protein ABP1 is a member of the cupin superfamily and the binding site is in a hydrophobic pocket at the head of which is a zinc ion to coordinate the acid group of the ligand. Receptors for other plant hormones have still to be identified.

Plant hormone receptors have been identified through the application of many different techniques; no one technique is likely to prove more successful than any other for discovering new receptors. At present there is structural detail only for auxin binding, although a good model exists for the amino acid residues needed for Cu(I) and ethylene binding. In this respect plant biology is very poor and effort needs to be put into receptor discovery and molecular characterization. The information accumulated by such work will undoubtedly indicate many new ways in which plant growth and development can be manipulated, but knowledge-led design of new ligands or of altered sensitivities is still some way off.