Cytokinin activity of disubstituted aminopurines in Amaranthus

Cytokinin (CK) receptors have different affinities for certain ligands, and consequently, studies of the plantʹs response to CK analogues constitute a good approach to identify active compounds that trigger specific plant responses. In this study, N6 and N6,N6-substituted CK analogues were synthesized and their CK-like activity was examined in the Amaranthus betacyanin and the bacterial receptor assay. The compounds showed CK-like activities that were not always associated with their binding affinity to the Arabidopsis receptors AHK3 and CRE1/AHK4. The highest level of activity in both bioassays was obtained for the N6-alkylaminopurines, which showed an especially high binding affinity to AHK3. In contrast to previously published data, we found remarkable activity of N6,N6-alkylbenzylaminopurines in the Amaranthus betacyanin bioassay, which was not associated with their binding affinity to the tested receptors. The N6,N6-substituted CK that showed the highest activity at the lowest concentration, N6,N6-methylbenzylaminopurine (BAP-C1), was studied to determine its effect on different leaf parameters of whole Amaranthus plants, with benzylaminopurine (BAP) used as standard compound. The interaction with ethylene was examined in plants supplied with the ethylene-synthesis inhibitor aminooxiacetic acid (AOA). After 3 d, the CKs supplied in the solution culture exerted effects on leaf dry weight and gas-exchange parameters. These effects of exogenous CKs are suggested to be ethylene-synthesis dependent.