Growing a glue factory: Open questions in laticifer development

Opium poppy (Papaver somniferum) is one of the world's oldest medicinal plants and remains the only commercial source for the narcotic analgesics morphine, codeine and semi-synthetic derivatives such as oxycodone and naltrexone. The plant also produces several other benzylisoquinoline alkaloids with potent pharmacological properties including the vasodilator papaverine, the cough suppressant and potential anticancer drug noscapine and the antimicrobial agent sanguinarine. Opium poppy has served as a model system to investigate the biosynthesis of benzylisoquinoline alkaloids in plants. The application of biochemical and functional genomics has resulted in a recent surge in the discovery of biosynthetic genes involved in the formation of major benzylisoquinoline alkaloids in opium poppy. The availability of extensive biochemical genetic tools and information pertaining to benzylisoquinoline alkaloid metabolism is facilitating the study of a wide range of phenomena including the structural biology of novel catalysts, the genomic organization of biosynthetic genes, the cellular and sub-cellular localization of biosynthetic enzymes and a variety of biotechnological applications. In this review, we highlight recent developments and summarize the frontiers of knowledge regarding the biochemistry, cellular biology and biotechnology of benzylisoquinoline alkaloid biosynthesis in opium poppy.

Laticifers are specialized cells that occur in over 20 plant families in several unrelated angiosperm orders. Although laticifers are likely to be of polyphyletic origin, their occurrence is considered a morphological indicator of relatedness among species. The classification of laticifers is based on developmental patterns and overall morphology. The cytoplasmic latex exuded in response to damage often includes specialized metabolites, such as cardenolides, alkaloids and natural rubber. Laticifers provide an effective location to store defense metabolites, although not all latex-bearing plants accumulate bioactive natural products. Ecophysiological studies have shown that latex and its associated metabolites are vital for the defense of plants against insects. The anatomy, development and physiology of laticifers are discussed with a focus on evolutionary and ecological perspectives.