Polyamines and plant alkaloids.

Naturally occurring alkaloids are nitrogenous compounds that constitute the pharmacogenically active basic principles of flowering plants. Alkaloids are classified into several biogenically related groups. Tobacco alkaloids are metabolised from polyamines and diamines putrescine and cadaverine. N-methyl transferase is the first enzyme in alkaloid biosynthetic pathway which drives the flow of nitrogen away from polyamine biosynthesis to alkaloid biosynthesis. Arginine decarboxylase has been suggested to be primarily responsible for providing putrescine for nicotine synthesis. Tryptophan is the precursor of indole alkaloids. However, the biosynthetic pathway of tropane and isoquinoline alkaloids are not clear. Genes for several key biosynthetic enzymes like arginine decarboxylase, ornithine decarboxylase, putrescine N-methyl transferase and spermidine synthase, hyoscyamine 6 beta hydroxylase,tryptophan decarboxylase etc have been cloned from different plant species. These genes are regulated by plant hormones, light, different kinds of stress and elicitors like jasmonates and their strong expression is primarily in the cultured roots. In view of this, the axenic hairy root cultures induced by Agrobacterium rhizogenes have been utilised to synthesise secondary metabolites. The current development in the knowledge of alkaloid biosynthesis, particularly molecular analysis, has been discussed in this review that may help to open up new avenues of investigation for the researchers.