Significance of Cyclic Nucleotides in Amaranthin Synthesis by Amaranthus caudatus Seedlings

Abstract

Explants of 72–76 h old Amaranthus caudatus seedlings synthesize the betalain pigment amaranthin in response to light. Light can be replaced with a cytokinin or a cyclic nucleotide with an N6-substituent. Cyclic 3′5′-AMP shows only weak activity and that only at high unphysiological concentrations. Even cyclic 2′3′-AMP, which docs not act as a ‘second messenger’, induces amaranthin synthesis to a greater degree than cyclic 3′,5′-AMP. But N6-monobutyryl-cyclic 3′,5′-AMP and N6-2′-O-dibutyryl-cyclicAMP show high activity, higher even than kinetin at its optimum concentration of 10−5 M. 2′-O-Monobutyryl-cyclicAMP, on the other hand, is considerably less active, suggesting that N6-substitution of the adenine ring is responsible for the enhanced activity. N6-Propionyl, butyryl and valeryladenines are all highly active, indicating that the cyclic monophosphate moiety is unnecessary for this response. All the compounds tested, including cyclic 3′,5′-AMP, show additive effects, but there is no amplification of the response, typical of second messenger action. Inhibition of amaranthin synthesis imposed by hadacidin, is relieved by kinetin, DBc AMP, N6-monobutyryl-cAMP and N6-butyryl adenine. Cyclic 3′,5′-AMP is weakly active in this regard. As natural cytokinins are N6-substituted adenine compounds, and as only N6-substituted cyclic nucleotides are able to mimic the effect of cytokinin, it is concluded that these cyclic nucleotides function as cytokinin analogues and not as ‘second messengers’'.