Plant cell culture biosynthetic precursors

To date, progress achieved clearly demonstrates the potential of cultured plant cells for secondary metabolite production. Use of concurrent immobilization/permeabilization procedures, as well as precursor and elicitor treatments, may open new avenues of increasing product yields and will consequently affect the economic aspects of plant cell culture in a positive manner. However, our understanding of the many biosynthetic pathways of desired secondary metabolites is incomplete and successful industrial scale plant cell culture processes are still limited. Results of research in the area of plant cell culture will increase our understanding of the biosynthesis of plant metabolites, enhance our knowledge of plant-microorganism or plant-plant interactions and can lead to entirely new products or product lines of desirable compounds currently not available to use. Such work can also lead to development of industrial scale production processes for products now produced and recovered by conventional methods. Also, the genetic variety of the 250,000 to 750,000 plant species available remains to be explored. Presently only 5 to 15% of these species have been subject to even... 

I. is used as a plant growth regulator. For synthesis, see Ul.. The biosynthetic precursor of I. is considered to be L- tryptophan Biosynthetic studies have been performed with Agrobacterium tumefaciens transformed plant cell cultures. For detection, see Lit.. lU. J. Heterocycl. Chem. 29,953-958 (1992) J. Am. Chem. Soc. 116, 3127 (1994). Aust. J. Plant Physiol. 20. 527-539 (1993). Plant Growth Regul. 10, 313-327 (1991). Pharis. Rood Stewart (eds.). Plant Growth Subst., p. 441 -449, Berlin Springer 1990. 

In addition to the pathway leading to the protoberberines, a second one leading to the protopine alkaloids has now been elucidated at the enzyme level utilizing plant cell cultures as the enzyme source. Since the protopines are regarded as the biosynthetic precursors of the benzazepine and benzophenanthri-dine alkaloids (refs. 36-39) these now can be the target of further enzymatic investigations. 

Based on knowledge of a biosynthetic pathway one can select certain steps which could be of interest for bioconversion of (a) readily available precursor(s). This could, for example, be stereospecific reactions, like the reduction of quinidinone in quinine or quinidine and the epoxidation of atropine to scopolamine. For the bioconversion one can consider using plant cells [e.g., the production of L-dopa from tyrosine by immobilized cells of Mucuna pruriens (10)] or isolated enzymes from the plant itself. An interesting example of the latter is the (5)-tetrahydroprotoberberine oxidase (STOX) enzyme, which oxidizes (5)-reticuline but not its stereoisomer (11). This feature can be used in the production of (i )-reticuline from a racemic mixture (see below). Immobilized strictosidine synthase has been successfully used to couple secologanin and tryptamine. The gene for this enzyme has been isolated from Rauvolfia (6) and cloned in Escherichia coli, in which it is expressed, resulting in the biosynthesis of active enzyme (7). The cultured bacteria produced 20 times more enzyme... 

Delineation of biosynthesis is usually carried out by different approaches e.g., feeding of presumptive precursors labeled by radioactive or stable isotopes. As described in this review, cell suspension cultures of higher plants opened the door to a methodology for the isolation of single biosynthetic enzymes. Then the role of each participating enzyme elucidated can give evidence for entire metabolic sequences, pathways or even metabolic networks. But these approaches are invasive and might not always reflect the in vivo situation. 

U. tomentosa is a significant traditional medicinal plant that is an important source of monoterpenoid oxindole alkaloids (MOAs) with bioactivities including immunomodulatory, cytotoxic, anti-HIV, and antileukemic activities [100]. It will be of interest to understand biosynthetic insights of bioactive compounds of this plant as an example here. Studies have reported the regulation of biosynthesis of sterols and triterpenes in U. tomentosa [100]. Administration of pectin to U. tomentosa cell suspension cultures increased the activity of isopentyl diphosphate isomerase of ursolic and oleanolic acid. The treated cells also transformed a higher percentage of labeled mevalonic acid precursors into triterpenes and resulted in the decrease in activity of far-nesyl diphosphatase by a factor of two when compared to the control. The... 

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