Abstract
Benzylisoquinoline alkaloids (BIAs) are a diverse group of biologically active specialized metabolites produced mainly in four plant families. BIA metabolism is likely of monophyletic origin and involves multiple enzymes yielding structurally diverse compounds. Several BIAs possess defensive properties against pathogenic microorganisms and herbivores. Opium poppy (Papaver somniferum: Papaveraceae) has emerged as a model system to investigate the cellular localization of BIA biosynthesis. Although alkaloids accumulate in the laticifer cytoplasm (latex) of opium poppy, corresponding biosynthetic enzymes and gene transcripts are localized to proximal sieve elements and companion cells, respectively. In contrast, BIA metabolism in the non-laticiferous meadow rue (Thalictrum flavum; Ranunculaceae) occurs independent of the phloem. Evidence points toward the adoption of diverse strategies for the biosynthesis and accumulation of alkaloids as defensive compounds. Recruitment of cell types involved in BIA metabolism, both within and external to the phloem, was likely driven by selection pressures unique to different taxa. The biochemistry, cell biology, ecophysiology, and evolution of BIA metabolism are considered in this context.
Highlights
Alkaloids are low-molecular weight, nitrogenous specialized metabolites occurring in approximately 20% of plant species
Progenitors of the Papaveraceae might have harbored a capacity for benzylisoquinoline alkaloids (BIAs) biosynthesis in sieve elements or other phloem cell types, and the emergence of laticifers provided a convenient site for the accumulation of copious alkaloid quantities, which potentially enhanced evolutionary fitness
The questions of how, why and where BIA biosynthesis originated remains a matter of speculation
Summary
Alkaloids are low-molecular weight, nitrogenous specialized metabolites occurring in approximately 20% of plant species. BIAs such as berberine and sanguinarine possess antimicrobial activity, whereas others such as noscapine are potentially antineoplastic (Barken et al, 2008). More is understood about the effects of alkaloids on humans than the roles of these compounds in the plants that produce them. In opium poppy (Papaver somniferum: Papaveraceae), sieve elements and specialized laticifers of the phloem produce and accumulate BIAs, respectively (Bird et al, 2003; Samanani et al, 2006). The inclusion or exclusion of phloem or other tissues in BIA metabolism was likely driven by several factors including the potentially independent emergence of laticifers and the inherent biological activities of a persistently changing arsenal of BIAs. DEFENSIVE PROPERTIES OF BIAs Numerous studies have supported defensive roles for BIAs in plants, some of which possess anti-herbivory, antifungal and/or antibacterial properties. Quaternary ammonium salts of certain protoberberine and benzophenanthridine (e.g., sanguinarine and chelerythrine) alkaloids show antifungal and antibacterial activities toward www.frontiersin.org
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