Effects of polyamines, polyamine precursors, and polyamine biosynthetic inhibitors on somatic embryogenesis from eggplant (Solarium melongena) cotyledons

Abstract

Eggplant (Solarium melongena L.) cotyledons grown on Murashige and Skoog medium with naphthaleneacetic acid formed callus, roots, and somatic embryos. Low levels of naphthaleneacetic acid (0.1 – 0.5 mg L−1) favoored rhizogenesis, intermediate levels (1.0 – 5.0 mg L−1) favoured embryogenesis, and high levels (10 – 50 mg L−1) favoured callogenesis. Addition of polyamines or their precursors did not induce morphogenesis on medium containing no growth regulator, nor did it affect embryogenesis on medium containing naphthaleneacetic acid, except at the highest concentrations tested, which were inhibitory. Enzyme-activated inhibitors of putrescine synthesis significantly reduced embryogenesis and stimulated rhizogenesis. α-Difluoromethylornithine was more potent in inhibiting embryogenesis and stimulating rhizogenesis than was α-difluoromethylarginine. α-Difluoromethylarginine did not inhibit growth and α-difluoromethylornithine stimulated growth. Addition of putrescine with α-difluoromethylornithine restored embryogenesis to control levels and reduced rhizogenesis. Competitive inhibitors of polyamine synthesis had nonspecific effects. Compared with seedling cotyledons, expiants grown on 5.0 mg naphthaleneacetic acid per litre contained slightly less free soluble putrescine and about the same amount of spermidine. At day 8, free putrescine and spermidine levels were higher in explants grown on naphthaleneacetic acid than in those grown on medium containing no growth regulator. Addition of α-difluoromethylornithine greatly reduced the putrescine and spermidine titres of the explants. Application of putrescine with α-difluoromethylornithine dramatically increased putrescine titres but not spermidine titres. Although the results suggest a role for polyamines in eggplant somatic embryogenesis, they do not support the hypothesis that polyamines act as auxin- or cytokinin-like growth regulators or as second messengers for auxin in this system.