Alkaloid Storage Compartments

These vesicles could be detected in Berberis as well as in Coptis (ref. 27) and seem to be the general principle for the biosynthesis and the transport of protoberberines. Once reticuline has entered this vesicle its fate to be metabolized to protoberberine alkaloids is determined. Since the quaternary end products are not able to pass membranes these vesicles also seem to be the transport vehicles to the final storage compartment, the vacuole. Electron microscopic pictures show that they seem to fuse with the vacuole and release their contents into it (ref. 29) which strongly supports this assumption. 

In poppy plants morphinan alkaloids are primarily accumulated in latex which in turn is contained in structurally and physiologically specialized cells, laticifers. Roberts et al. (1983 and references therein) demonstrated that 1000 g and supernatant fractions of the latex are required for alkaloid biosynthesis and that a subpopulation of dense organelles of the 1000 g sediment have a function as a storage compartment for alkaloids. Morphinan alkaloid production with poppy cell cultures, therefore, would appear to require the concurrent differentiation of latex vessels. The high levels of alkaloid accumulation would thus appear to be intriguingly correlated with the formation of latex and latex vessels. 

The fact that the various steps of the biosynthetic pathway of the terpenoid indole alkaloids occur in different cell compartments implies that transport of intermediates and products is involved. Little research has been done on this transport phenomenon, except for the transport to the final storage site of the alkaloids. Deus-Neumann and Zenk (205) postulated an active and selective transport system for Catharanthus alkaloids into the... 

Storage capacity in a cell culture is available inside the cells (e.g., vacuoles) or in the extracellular compartment. The storage facilities of the latter can be remarkedly improved by the addition of liquid organic phases, resins, or other sorbents to the medium. Addition of XAD-7 to Catharanthus roseus cultures resulted in increased yields of indole alkaloids (SS). Addition of a dimethylsiloxane polymer to Eschscholtzia californica cultures improved yields of benzophenanthridine alkaloids ( 9), and increased yields of nicotine and anabasine were obtained by adding of XAD-2 and XAD-4 resins to Nicotiana transformed roots (90,91). 

In addition to these factors, the mechanisms for product accumulation and storage are of prime importance, but little studied (James, 1950 Matile, 1978, 1984 Wink, 1987). In general, there are few data concerning the site of storage of secondary compounds within the plant. As observed above, storage at a particular site does not necessarily imply that the compound was synthesized there. For example, lupine alkaloids are accumulated in epidermal cells (which lack chloroplasts) but synthesized in mesophyll cells. The alkaloids are transported to the epidermis via the phloem. Accumulation depends on the season and developmental stage of the plant (James, 1950 Mothes, 1955). Sites of synthesis and accumulation often are separated in cells by compart-mentation. Lipophilic compounds tend to be accumulated in membranes, vesicles, dead cells, or extracellular sites. Hydrophilic compounds tend to be stored in an aqueous environment, typically the vacuole (Matile, 1978, 1984 Wink, 1987). 

At present, only a few membrane compartments, transporters and channels are known to have a defined role in alkaloid transport and compartmentation. One major challenge for research is the high individuality in the cellular organization that rests on the species-specific localization of biosynthetic enzymes and the opportunistic use of transporters, metabolite pools, storage sites and accumulation mechanisms even between taxonomically related genera. 

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