In vitro binding of auxin to particulate fractions from the shoots of dark-grown wheat seedlings

A binding site for auxins was found in the 50,000g pellet from a homogenate of shoots from dark-grown wheat seedlings. The optimum conditions for the binding of native auxin, IAA, were within the range of physiological conditions of growth (pH 5.2, temperature 20° C). The binding site displayed a high affinity to IAA (affinity constant about 10⁷ M ^–1, i.e. dissociation constant about 10^–8 M) and low capacity, 60 p mol per 1 g of fresh weight. The binding capacity of 3.5-days-old shoots is represented by about 56% and 44% of that of leaves and coleoptiles, respectively. The more rapidly growing leaves also contained more endogenous free IAA (64%) than the coleoptiles from the same seedlings (36%). The binding site was very specific, distinguishing well between strong auxins and structurally related substances which exhibit very weak auxin activity. These physiological properties of this binding site indicate that it may have a certain role in the regulation of physiological processes, such as elongation growth and cell division.