Extracellular accumulation secondary products

Secondary metabolites in plant cell culture are typically stored within the vacuolar compartments of the cells. Small amounts of metabolites are usually excreted into the medium or may appear in the medium due to cell lysis. In some cases, active transport of metabolites adjusts intracellular and extracellular levels in response to cellular conditions. Poor yields of secondary substances released into the medium may be caused by several factors. In those cases where low yield is due to cellularly mediated regulation of the ratio between intracellular and extracellular product concentrations, processes which reduce net medium concentrations such as enzymatic or non-enzymatic degradation, or volatility, should increase net production by depleting the level of the secondary substances in the culture medium. By using the so-called in situ extraction method, the accumulation of a secondary substance inside the cell, in the culture medium, and in the extraction phase should approach an equilibrium, which... 

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). 

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