The Effects of Over-Expressing the Yeast Ornithine Decarboxylase Gene upon the Nicotine and Polyamine Levels in Transformed Roots of Nicotiana Rustica

Putrescine is an important precursor of alkaloids such as nicotine in Nicotiana and scopolamine and hyoscyamine in Datura. It acts at the interface between primary and secondary metabolism and its primary role is as an intermediate in the formation of polyamines which have important roles in the growth of animal and microbial as well as plant cells. In animals and microbes the formation of putrescine and polyamines is regulated by effects on the expression of the enzymes ornithine decarboxylase (ODC) and S-adenosyl methionine decarboxylase (SAM dC)1 both at the nucleic acid and protein levels. In plants, the situation is complicated by the occurrence of an alternative route of formation of putrescine from arginine and arginine decarboxylase (ADC) as well as from ornithine and ornithine decarboxylase and the presence of large buffering pools of putrescine and polyamine conjugates. Nevertheless, the role of polyamines in growth-related processes in plants is well established2,3,4. In tissues where putrescine-derived alkaloids are found, there is clearly competition for putrescine as substrate between the polyamine and alkaloid pathways. Feeding low levels of putrescine (1–5mM) can have a stimulatory effect upon the production of nicotine in N. rustica transformed root cultures5. This suggests that increasing the levels of this intermediate, by genetic manipulation techniques, may allow a greater flux down the metabolic pathway leading to nicotine and thus increase the secondary product concentrations of these cultures. We have investigated these interactions by attempting to over-express a yeast ornithine decarboxylase (yODC) gene in transformed roots of Nicotiana rustica to increase internal pools of putrescine and to study its partitioning between polyamines and alkaloids.