Chemically auxins are weakly acidic growth hormones that have an unsaturated ring structure. They are capable of promoting cell elongation, especially of shoots but are inhibitory to the roots. Zinc is involved in the synthesis of Indoleacetic acid in plants and act as an activator in the enzyme tryptophan synthetase. Tryptophan is the precursor of Indole-acetic acid.
Related Theory
Antiauxins are a group of chemicals that can prevent auxin-action in plants. Transcinnamic acid, ascorbic acid, 7-phenyl butyric acid are some of the antiauxins. Probably, an antiauxin competes with an auxin for the same site of reaction and thus inhibits auxin-action.