Polyamine-derived alkaloids in plants: Molecular elucidation of biosynthesis

Alkaloids are nitrogen-containing specialized metabolites generated from primary metabolites, including polyamines, in plants. Alkaloids include many biologically active chemicals and confer adaptive advantages to plants producing them. The tropane alkaloids include cocaine and calystegines, whereas pyrrolizidine alkaloids are a group of defense compounds that contain a necine base moiety derived from homospermidine. Nicotine and tropane alkaloids are commonly derived from the diamine putrescine, which is incorporated into the pyrrolidine ring of nicotine or the tropane ring in tropane alkaloids. Here we review the progress that has been made in understanding the biosynthesis of putrescine-derived alkaloids, which has been intensively studied in tobacco and medicinal plants of the Solanaceae. In solanaceous species, alkaloids, including clinically important hyoscyamine and scopolamine, are synthesized in specific cells in the roots and then translocated to shoots through the xylem. Homospermidine synthase, which catalyzes homospermidine formation from putrescine and spermidine, repeatedly evolved in multiple plant lineages from deoxyhypusine synthase involved in posttranslational activation of eIF5A protein. The evolution of these alkaloid biosynthesis enzymes and pathways is discussed in light of the newly available information in this field.