Molecular recognition of auxins - a rational approach to plant growth regulation

The introduction of plant growth regulators that interfere with endogenous phytohormones (such as auxins) revolutionized agriculture and horticulture. Synthetic auxin analogues were among the first organic agrochemicals used, but designs with increased efficiency and fewer side effects would still be well received. Such improvements should be based on a rational evaluation of the existing, large set of auxin analogues and the plant proteins with which these compounds interact. The latter are still incompletely known, but the topology of their hormone-binding sites can be deduced by relating the molecular structures and biological properties of their multiple substrates. Effective comparisons require mathematical algorithms ('similarity analysis') and exact data on molecular geometry, conformational preferences, and physico-chemical properties. Both the endogenous auxins and their synthetic analogues are metabolized by plants, and biological roles are now assigned to at least some of the metabolites: conjugates including sugar, polyol, or amino acid moieties. This observation is in accord with the 'open' molecular structure of these conjugates in which both components, and the connecting bond, are accessible for recognition by cell proteins. Structure-activity studies for auxins and their derivatives point to molecular sites to which quite large substituents may be attached without jeopardizing biological recognition. This is being exploited to develop tagged auxins to assist in the identification of proteins involved in auxin physiology. Manipulating these proteins will afford new leads towards efficient, environmentally safe, plant growth regulation.