Peculiarities of two different IAA binding sites functioning in kidney bean hypocotyl cell plasmalemma

Abstract

Peculiarities of the interaction of phytohormone indole-3-acetic acid (IAA or auxin) and auxinbinding proteins (ABP) in the plasmalemma of kidney bean (dicot plant) hypocotyl cells according to different physiological responses to IAA have been studied. Two different IAA binding sites (optimal pH 5.5 and 7.5) were discovered in the plasmalemma of cells responding to IAA by elongation. The first site is characterized as IAA-receptor mediating cell response to IAA by elongation. It has a structure of the IAA binding site – /-H-RH-S-C-E-/. This site is not functioning in hypocotyl cells not responding to IAA by elongation. The activity of the second site (optimal pH 7.5) is dependent on IAA transport. Basing on the characteristics of this site, the following suggestions may be made: one residue of histidine and amino acid(s) containing the -SH- group are functioning in this site; the IAA influx inhibitor 3-chloro-4-hydroxyphenylacetic acid (CHPAA) is capable of a very active competition with IAA in the binding site: it reduces IAA–ABP interaction by 92%. Our results confirmed that several different ABP may be functioning in the cell and that for each separate response an individual receptor may be required. Keywords: indole-3-acetic acid (IAA), auxin-binding protein (ABP), auxin influx inhibitors, auxin efflux inhibitors