Podophyllin, an ethanolic extract of Podophyllum peltatum L. or P. emodi Wall (syn. P. hexandnum Royle), is a good source of the aryltetralin-type lignan, podophyllotoxin. The latter compound, as well as its congeners and derivatives exhibit pronounced biological activity mainly as strong antiviral agents and as antineoplastic drugs. The podophyllotoxin derivatives etoposide, etopophos (etoposide phosphate), and teniposide are thus successfully utilized in the treatment of a variety of malignant conditions. Continued research on the Podophyllum lignans is currently focused on structure optimization to generate derivatives with superior pharmacological profiles and broader therapeutic scope, and the development of alternative and renewable sources of podophyllotoxin.

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Cells of Catharanthus roseus (L.) G. Don were genetically engineered to over-express the enzymes strictosidine synthase (STR; EC 4.3.3.2) and tryptophan decarboxylase (TDC; EC 4.1.1.28), which catalyze key steps in the biosynthesis of terpenoid indole alkaloids (TIAs). The cultures established after Agrobacterium-mediated transformation showed wide phenotypic diversity, reflecting the complexity of the biosynthetic pathway. Cultures transgenic for Str consistently showed tenfold higher STR activity than wild-type cultures, which favored biosynthetic activity through the pathway. Two such lines accumulated over 200 mg.L-1 of the glucoalkaloid strictosidine and/or strictosidine-derived TIAs, including ajmalicine, catharanthine, serpentine, and tabersonine, while maintaining wild-type levels of TDC activity. Alkaloid accumulation by highly productive transgenic lines showed considerable instability and was strongly influenced by culture conditions, such as the hormonal composition of the medium and the availability of precursors. High transgene-encoded TDC activity was not only unnecessary for increased productivity, but also detrimental to the normal growth of the cultures. In contrast, high STR activity was tolerated by the cultures and appeared to be necessary, albeit not sufficient, to sustain high rates of alkaloid biosynthesis. We conclude that constitutive over-expression of Str is highly desirable for increased TIA production. However, given its complexity, limited intervention in the TIA pathway will yield positive results only in the presence of a favorable epigenetic environment.

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Rehydration of powdered tissues of Podophyllum peltatum L. prior to extraction with an organic solvent allows endogenous beta-glucosidases to hydrolyze lignan 4-O-beta-D-glucosides in situ and increase the yield of podophyllotoxin. Aqueous extraction of rhizomes and leaves of P. peltatum yielded 4- to 10-fold greater quantities of podophyllotoxin than the traditional ethanolic extraction. Most significantly, leaves were shown to contain over 52 mg of podophyllotoxin per g of dry weight (5.2%), exceeding levels previously reported from any source. These results point to the use of leaves harvested from cultivated P. peltatum as an attractive alternative to the destructive collection of natural populations.

We have used a transgenic cell line of Catharanthus roseus (L.) G. Don to study the relative importance of the supply of biosynthetic precursors for the synthesis of terpenoid indole alkaloids. Line S10 carries a recombinant, constitutively overexpressed version of the endogenous strictosidine synthase (Str) gene. Various concentrations and combinations of the substrate tryptamine and of loganin, the immediate precursor of secologanin, were added to suspension cultures of S10. Our results indicate that high rates of tryptamine synthesis can take place under conditions of low tryptophan decarboxylase activity, and that high rates of strictosidine synthesis are possible in the presence of a small tryptamine pool. It appears that the utilization of tryptamine for alkaloid biosynthesis enhances metabolic flux through the indole pathway. However, a deficiency in the supply of either the iridoid or the indole precursor can limit flux through the step catalyzed by strictosidine synthase. Precursor utilization for the synthesis of strictosidine depends on the availability of the cosubstrate; the relative abundance of these precursors is a cell-line-specific trait that reflects the metabolic status of the cultures.

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Pummelo (Citrus maxima [Burm.] Merrill) cDNAs encoding mitochondrial citrate synthase (mCS) were cloned by reverse transcription of juice-sac poly(A)+ mRNA, followed by Taq Polymerase-mediated amplification. The nucleotide sequence of the citrus gene (cit1) is 77% conserved relative to plant mRNAs for mCS. The encoded polypeptide includes a mitochondrial targeting signal at its amino terminus; all 20 putative active-site residues of the citrus enzyme are conserved. Southern hybridization showed that citrus cit1 is a single-copy gene. A polymorphism associated with cit1 did not cosegregate with fruit acidity indicating that acitric, the gene causing the acidless phenotype of pummelo 2240, is not an allele of cit1 locus. Quantitative detection of cit1 mRNA showed that transcript levels are not developmentally regulated in juice sacs; no differences were observed between high- and low-acid genotypes.