Molecular characterization of two anthranilate synthase alpha subunit genes in Camptotheca acuminata

Accumulation of camptothecin and 10-hydroxycamptothecin and the transcriptional expression of camptothecin biosynthetic genes in Camptotheca acuminata cambial meristematic and dedifferentiated cells

Cambial meristematic cells (CMCs) of C. acuminata were isolated and compared with dedifferentiated cells (DDCs). The expression levels of seven genes encoding key enzymes involved in CPT and HCPT biosynthesis was significantly upregulated in CMCs compared with DDCs.

Sarpagine and Related Alkaloids

The sarpagine-related macroline and ajmaline alkaloids share a common biosynthetic origin, and bear important structural similarities, as expected. These indole alkaloids are widely dispersed in 25 plant genera, principally in the family Apocynaceae. Very diverse and interesting biological properties have been reported for this group of natural products. Isolation of new sarpagine-related alkaloids and the asymmetric synthesis of these structurally complex molecules are of paramount importance to the synthetic and medicinal chemists. A total of 115 newly isolated sarpagine-related macroline and ajmaline alkaloids, along with their physicochemical properties have been included in this chapter. A general and efficient strategy for the synthesis of these monomeric alkaloids, as well as bisindoles, has been presented, which involves application of the asymmetric Pictet-Spengler reaction (>98% ee) as a key step because of the ease of scale up of the tetracyclic template. Also included in this chapter are the syntheses of the sarpagine-related alkaloids, published since 2000.

Cell-specific expression of tryptophan decarboxylase and 10-hydroxygeraniol oxidoreductase, key genes involved in camptothecin biosynthesis in Camptotheca acuminata Decne (Nyssaceae)

BACKGROUND

Camptotheca acuminata is a major natural source of the terpenoid indole alkaloid camptothecin (CPT). At present, little is known about the cellular distribution of the biosynthesis of CPT, which would be useful knowledge for developing new strategies and technologies for improving alkaloid production.

RESULTS

The pattern of CPT accumulation was compared with the expression pattern of some genes involved in CPT biosynthesis in C. acuminata [i.e., Ca-TDC1 and Ca-TDC2 (encoding for tryptophan decarboxylase) and Ca-HGO (encoding for 10-hydroxygeraniol oxidoreductase)]. Both CPT accumulation and gene expression were investigated in plants at different degrees of development and in plantlets subjected to drought-stress. In all organs, CPT accumulation was detected in epidermal idioblasts, in some glandular trichomes, and in groups of idioblast cells localized in parenchyma tissues. Drought-stress caused an increase in CPT accumulation and in the number of glandular trichomes containing CPT, whereas no increase in epidermal or parenchymatous idioblasts was observed. In the leaf, Ca-TDC1 expression was detected in some epidermal cells and in groups of mesophyll cells but not in glandular trichomes; in the stem, it was observed in parenchyma cells of the vascular tissue; in the root, no expression was detected. Ca-TDC2 expression was observed exclusively in leaves of plantlets subjected to drought-stress, in the same sites described for Ca-TDC1. In the leaf, Ca-HGO was detected in all chlorenchyma cells; in the stem, it was observed in the same sites described for Ca-TDC1; in the root, no expression was detected.

CONCLUSIONS

The finding that the sites of CPT accumulation are not consistently the same as those in which the studied genes are expressed demonstrates an organ-to-organ and cell-to-cell translocation of CPT or its precursors.

Molecular cloning, expression profiling and functional analysis of a DXR gene encoding 1-deoxy-D-xylulose 5-phosphate reductoisomerase from Camptotheca acuminata

As the second enzyme of the non-mevalonate terpenoid pathway for isopentenyl diphosphate biosynthesis, DXP reductoisomerase (DXR, EC: 1.1.1.267) catalyzes a committed step of the MEP pathway for camptothecin (CPT) biosynthesis. In order to understand more about the role of DXR involved in the CPT biosynthesis at the molecular level, the full-length DXR cDNA sequence (designated as CaDXR) was isolated and characterized for the first time from a medicinal Nyssaceae plant species, Camptotheca acuminata. The full-length cDNA of CaDXR was 1823 bp containing a 1416 bp open reading frame (ORF) encoding a polypeptide of 472 amino acids. Comparative and bioinformatic analyses revealed that CaDXR showed extensive homology with DXRs from other plant species and contained a conserved transit peptide for plastids, an extended Pro-rich region and a highly conserved NADPH binding motif in its N-terminal region owned by all plant DXRs. Phylogenetic analysis indicated that CaDXR was more ancient than other plant DXRs. Tissue expression pattern analysis revealed that CaDXR expressed strongly in stem, weak in leaf and root. CaDXR was found to be an elicitor-responsive gene, which could be induced by exogenous elicitor of methyl jasmonate. The functional color complementation assay indicated that CaDXR could accelerate the biosynthesis of carotenoids in the Escherichia coli transformant, demonstrating that DXP reductoisomerase plays an influential step in isoprenoid biosynthesis.

Transport of camptothecin in hairy roots of Ophiorrhiza pumila

We have investigated the subcellular accumulation and transport of camptothecin (CPT), a monoterpene indole alkaloid, in hairy roots of Ophiorrhiza pumila. This hairy root produces high amounts of CPT and excretes it into the culture medium. When the hairy roots were exposed to UV radiation, autofluorescence emitted from CPT showed subcellular localization of CPT in the vacuole. Treatment with several inhibitors suggested that CPT excretion is a transporter-independent passive transport controlled by the concentration gradient of the compound. Interestingly, the hairy roots treated with brefeldin A, a vesicle transport inhibitor, showed increased CPT excretion. This could be explained by an increased transport rate of CPT from the endoplasmic reticulum (ER) to the cytoplasm when transport of CPT to the vacuole is blocked. The much higher concentration of CPT in the cytoplasm resulted in the increased excretion rate. This result indicates that CPT is biosynthesized at the ER and transported to accumulate in the vacuole by the same machinery that is used for vacuolar protein sorting. How O. pumila is insensitive to CPT is discussed.