Inhibition of polyamine biosynthesis by L-canavanine and its effect on meristematic activity, growth, and development of Zea mays roots

The effects of L-canavanine, a structural analogue of L-arginine and a competitive inhibitor of arginine decarboxylase, were studied in corn (Zea mays L.) on primary root elongation and lateral root differentiation, meristematic activity, and on polyamine content and biosynthesis. Incubation of seedlings in 10(-3) M L-canavanine for 3 h inhibited both elongation of the primary root and differentiation of lateral root primordia. A marked decrease in cell division and DNA synthesis, along with an inhibition of the activation of the primary root quiescent center, was clearly evident in L-canavanine-treated roots. Putrescine content in subapical segments of the primary root decreased significantly during the first 24 h, while spermidine content increased at 48-72 h, coinciding with lateral root emergence and elongation. A 3 h incubation with 10(-3) M L-canavanine completely inhibited the increase in spermidine content, thus modifying the ratio of putrescine to spermidine tissue concentrations. The activity of arginine decarboxylase, and less significantly that of ornithine decarboxylase, increased steadily during the 72 h period of lateral root differentiation and elongation. Activation of both enzymes was inhibited by L-canavanine. Addition of L-arginine at 10(-3) M partially relieved the inhibitory effects of L-canavanine on primary root elongation, lateral root differentiation, activation of the quiescent center, changes in polyamine content, and the activity of arginine and ornithine decarboxylases. These and previous data suggest that root development and meristematic activity in Zea mays are associated, perhaps causally, with changes in biosynthesis and content of polyamines.