Cytokinin Activity of a Group of Ureidopurine Derivatives Related to the tRNA Component, N-(Purin-6-ylcarbamoyl)threonine

Abstract

Several ureidopurine derivatives have been tested for cytokinin activity in a soya bean callus assay system. N-(Purin-6-ylcarbamoyl)threonine (Ade-CO-Thr), a naturally occurring component of tRNA, is inactive in the assay. Analogues of Ade-CO-Thr having unmodified hydrocarbon side chains are active; optimal activity is associated with tert-butylureidopurine. In the homologous straight chain series n-butylureidopurine is the most active. Introduction of −COOH and/or −OH groups to analogues with otherwise active alkyl chains negates activity.The phenylureidopurine (PUP) derivative is barely active itself, though ortho-halogenated derivatives (o-chloro-, o-fluoro-PUP) are the most potent ureidopurines known. Biological activity decreases rapidly as the electronegative halide is moved from the ortho to the meta position, and lost entirely in the para position. Replacement of the ortho halide with a non-electronegative group (methyl) greatly reduces activity. Molecular models reveal that Ade-CO-Thr and o-chloro-PUP have nearly identical side-chain bulk. The carboxyl groups of Ade-CO-Thr and the chlorine atom of o-chloro-PUP occupy the same position relative to the electronegative carboxyl oxygen of the urea bridge. It is proposed that Ade-CO-Thr has a growth regulatory role in the intact organism, but that exogenous Ade-CO-Thr may be unable to reach the active site.