[Binding of indole acetic acid and 2-methyl-4-chlorophenoxy acetic acid to fractions from epicotyles and roots of Pisum sativum L]

Abstract

The binding behaviour of 3-indoleacetic acid (IAA) and of (2-methyl-4-chlorophenoxy) acetic acid (MCPA), (2-chlorophenoxy) acetic acid (2-Cl-PA) and (4-chlorophenoxy) acetic acid (4-Cl-PA) in subcellular fractions of Pisum epicotyls and roots, which are rich in plasma membranes, has been investigated. Binding parameters are determined by means of centrifugation experiments or equilibrium dialysis using (14)C-labelled auxins. The experiments prove the existence of saturable (specific) binding sites and the reversibility of auxin binding. For the binding of IAA and MCPA to the particle fraction the binding constant K≧10(6) M(-1) and the number of the binding sites n·R≧10(-13) M per mg pellet-protein have been estimated. In displacement experiments MCPA or 4-Cl-PA displace IAA, whereas IAA can not displace phenoxyacetic derivatives. These results indicate that with concentrations in the range of herbicide action phenoxyacetic acids bind to a site of the auxin receptor which is not identical with the binding site of IAA. In this way the binding affinity of receptor for IAA is decreased allosterically, which results in an apparent dispalcement of IAA.