Role of putrescine metabolic pathways in the differentiation process of maize meristematic callus

The role of putrescine and its interaction with auxin in modulating the differentiation of maize meristematic callus was studied. The effect of exogenous putrescine on differentiation depends on the endogenous polyamine level of the callus and the stage at which putrescine was applied. Plant regeneration was highest in the 1 mM putrescine-pretreated calluses that initiated the differentiation process with a higher endogenous putrescine content versus control. The enzymatic activities of arginine and ornithine decarboxylase (responsible for putrescine synthesis) were considerably lower in the presence of exogenous putrescine, indicating a feed-back or a final product inhibition, respectively. The rise observed in the perchloric acid-insoluble polyamine fraction, especially putrescine, could be related to the initiation of the differentiation process. Low doses of auxin in differentiation medium induced a general decrease in plant regeneration. However, activation of the differentiation process was observed in calluses cultured in the presence of auxin and an excess of putrescine. In these calluses, the perchloric acid-soluble conjugated fraction increased significantly. These data indicate that putrescine is involved in the endogenous regulation of callus morphogenesis.