MORPHOGENESIS IN CULTURED THIN-LAYERS AND PITH EXPLANTS OF TOBACCO .2. EARLY HORMONE-MODULATED POLYAMINE BIOSYNTHESIS

In order to gain deeper insight into the interrelationships between po lyamines and hormones, superficial thin layers and pith explants, exci sed from the stem of tobacco (Nicotiana tabacum L.) plants in the vege tative stage, were cultured under various hormonal conditions (IAA, IA A+BA, BA) and in a hormone-free medium, in the presence or in the abse nce of 100 mu mol/L putrescine. In thin layers at 24 h, activation of putrescine biosynthesis via both arginine- (ADC) and ornithine decarbo xylases (ODC) as well as free putrescine accumulation were detected in the IAA treatment, and via ADC alone in the presence of IAA+BA. BA ca used an inhibition of or had no effect on putrescine biosynthesis. In pith explants, although some free putrescine accumulated in IAA treatm ent, no increase in its biosynthetic activity was detectable. No DAO a ctivity could be detected in any treatment early in culture either in superficial or deep tissues. Exogenous putrescine did not affect this pattern. The modulation of polyamine biosynthesis by the different hor mones in superficial and pith explants is discussed in relation to put rescine accumulation and oxidation and to early cytological events occ urring in culture and previously described (Altamura et al., 1995).