Significance of Cyclic Nucleotides in Amaranthin Synthesis by Amaranthus caudatus Seedlings

Explants of 72–76 h old Amaranthus caudatus seedlingssynthesize the betalain pigment amaranthin in response to light.Light can be replaced with a cytokinin or a cyclic nucleotidewith an N⁶-substituent. Cyclic 3'5'-AMP shows only weak activityand that only at high unphysiological concentrations. Even cyclic2'3'-AMP, which docs not act as a ‘second messenger’,induces amaranthin synthesis to a greater degree than cyclic3',5'-AMP. But N⁶-monobutyryl-cyclic 3',5'-AMP and N⁶-2'-O-dibutyryl-cyclicAMPshow high activity, higher even than kinetin at its optimumconcentration of 10^–5 M. 2'-O-Monobutyryl-cyclicAMP, onthe other hand, is considerably less active, suggesting thatN⁶-substitution of the adenine ring is responsible for the enhancedactivity. N⁶-Propionyl, butyryl and valeryladenines are allhighly active, indicating that the cyclic monophosphate moietyis unnecessary for this response. All the compounds tested,including cyclic 3',5'-AMP, show additive effects, but thereis no amplification of the response, typical of second messengeraction. Inhibition of amaranthin synthesis imposed by hadacidin, isrelieved by kinetin, DBc AMP, N⁶-monobutyryl-cAMP and N⁶-butyryladenine. Cyclic 3',5'-AMP is weakly active in this regard. Asnatural cytokinins are N⁶-substituted adenine compounds, andas only N⁶-substituted cyclic nucleotides are able to mimicthe effect of cytokinin, it is concluded that these cyclic nucleotidesfunction as cytokinin analogues and not as ‘second messengers’'. Amaranthus caudatus, amaranthin, cytokinins, cyclic nucleotides