Organogenesis fromSolanum melongena L. (eggplant) cotyledon expiants is associated with hormone-modulated enhancement of polyamine biosynthesis and conjugation

Eggplant (Solanum melongena L. cv. Violetta lunga 2) cotyledon expiants grown on hormone-free medium (controls) or on medium containing either naphthaleneacetic acid alone (root forming) or in combination with zeatin riboside (shoot forming) showed minor differences in free polyamine titres during culture. In contrast, conjugated polyamines (particularly those in the trichloroacetic acid-soluble fraction) accumulated only in hormonetreated explants, but not in controls. The extent and the temporal changes in soluble-conjugate levels differed between root-forming and shoot-forming expiants; in the former, accumulation began earlier (within 1 day of culture) and reached the highest levels. In both organogenic programmes, maximum conjugate accumulation occurred just before and during organ emergence. Adventitious roots and shoots were formed along the cut surfaces. The regions closest to these (“borders”) displayed a significantly higher ratio of conjugated to free spermidine and/or putrescine than the nonorganogenic regions (“centres”) of the explant. Ornithine decarboxylase activity was higher than arginine decarboxylase activity both in control and hormone-treated explants. However, both activities increased markedly on day 2 of culture in the presence of hormones. Thereafter ornithine decarboxylase activity remained high in shoot-forming explants, but not in root-forming ones. Putrescine oxidising activity was also enhanced by exogenously supplied hormones starting from day 4 of culture. This activity remained high up to day 12 in the presence of auxin plus cytokinin, whereas it peaked on day 6 in auxin-treated explants. Spermidine oxidising activity was the only enzyme activity which was consistently higher in controls than in hormone-treated tissue. Differences between the two organogenic programmes with respect to temporal changes in polyamine content, and putrescine biosynthetic and oxidative activities are discussed in relation to the timing of organ formation. The latter was monitored both histologically and macroscopically.