Polyamines suppress manifestation of cytokinin primary effects

Abstract

Using Amaranthus and transgenic Arabidopsis PARR5:GUS seedlings in bioassays, we studied the effects of exogenous polyamines on the development of cytokinin-induced effects based on the expression of the cytokinin primary response genes. Transgenic Arabidopsis contained a reporter GUS gene under the control of Arabidopsis ARR5 promoter. In both bioassays, we found that all polyamines tested suppressed the effects induced by 5 µM benzyladenine (BA). Spermine and putrescine were more efficient as BA antagonists than cadaverine and spermidine. Polyamines affected posttranscriptional stages of BA action but did not interfere with the accumulation of cytokinin-dependent mRNA. This fact contradicts the hypotheses about the existence of a common receptor for polyamines and BA or about polyamines as second messengers in the cytokinin signaling pathway. In Arabidopsis seedlings, exogenous polyamines suppressed BA effects at low (micromolar) concentrations comparable with BA concentrations. The incubation of Arabidopsis seedlings with aminoguanidine (an inhibitor of putrescine and cadaverine catabolism) for 24 h resulted in a significant reduction in the activity of the cytokinin primary response gene. These data indicate that endogenous polyamines can modulate considerably the amplitude of the cytokinin primary response gene expression in vivo.