Only a small fraction of the immense diversity of plant metabolism has been explored for the production of new medicines and other products important to human well-being. The availability of inexpensive high-throughput sequencing is rapidly expanding the number of species that can be investigated for the speedy discovery of previously unknown enzymes and pathways. Exploitation of these resources is being carried out through interdisciplinary synthetic and chemical biology to engineer pathways in plant and microbial systems for improving the production of existing medicines and to create libraries of biologically active products that can be screened for new drug applications.

SignificanceThe multistep assembly of catharanthine and tabersonine from strictosidine remains poorly characterized for understanding the biochemistry of anticancer monoterpenoid indole alkaloid (MIA) biosynthesis in the medicinal plant, Catharanthus roseus. The seven-step pathway from 19E-geissoschizine to four major MIA skeletons enables the assembly of catharanthine and tabersonine that complete the pathway for biosynthesis of the anticancer drugs, anhydrovinblastine and vincristine as well as for production of other biologically active MIAs.

The monoterpenoid indole alkaloids (MIAs) of Madagascar periwinkle (Catharanthus roseus) continue to be the most important source of natural drugs in chemotherapy treatments for a range of human cancers. These anticancer drugs are derived from the coupling of catharanthine and vindoline to yield powerful dimeric MIAs that prevent cell division. However the precise mechanisms for their assembly within plants remain obscure. Here we report that the complex development-, environment-, organ-, and cellspecific controls involved in expression of MIA pathways are coupled to secretory mechanisms that keep catharanthine and vindoline separated from each other in living plants. Although the entire production of catharanthine and vindoline occurs in young developing leaves, catharanthine accumulates in leaf wax exudates of leaves, whereas vindoline is found within leaf cells. The spatial separation of these two MIAs provides a biological explanation for the low levels of dimeric anticancer drugs found in the plant that result in their high cost of commercial production. The ability of catharanthine to inhibit the growth of fungal zoospores at physiological concentrations found on the leaf surface of Catharanthus leaves, as well as its insect toxicity, provide an additional biological role for its secretion. We anticipate that this discovery will trigger a broad search for plants that secrete alkaloids, the biological mechanisms involved in their secretion to the plant surface, and the ecological roles played by them.catharanthine | Catharanthus roseus | surface secretion | vindoline

Camptothecin is a monoterpene indole alkaloid (MIA) used to produce semisynthetic antitumor drugs. We investigated camptothecin synthesis in Camptotheca acuminata by combining transcriptome and expression data with reverse genetics, biochemistry, and metabolite profiling. RNAi silencing of enzymes required for the indole and seco-iridoid (monoterpene) components identified transcriptional crosstalk coordinating their synthesis in roots. Metabolite profiling and labeling studies of wild-type and RNAi lines identified plausible intermediates for missing pathway steps and demonstrated nearly all camptothecin pathway intermediates are present as multiple isomers. Unlike previously characterized MIA-producing plants, C. acuminata does not synthesize 3-a(S)-strictosidine as its central MIA intermediate and instead uses an alternative secoiridoid pathway that produces multiple isomers of strictosidinic acid. NMR analysis demonstrated that the two major strictosidinic acid isomers are (R) and (S) diastereomers at their glucosylated C21 positions. The presence of multiple diastereomers throughout the pathway is consistent with their use in synthesis before finally being resolved to a single camptothecin isomer after deglucosylation, much as a multilane highway allows parallel tracks to converge at a common destination. A model "diastereomer" pathway for camptothecin biosynthesis in C. acuminata is proposed that fundamentally differs from previously studied MIA pathways.

Hydroxylation of tabersonine at the C-16 position, catalyzed by tabersonine 16-hydroxylase (T16H), initiates the synthesis of vindoline that constitutes the main alkaloid accumulated in leaves of Catharanthus roseus. Over the last decade, this reaction has been associated with CYP71D12 cloned from undifferentiated C. roseus cells. In this study, we isolated a second cytochrome P450 (CYP71D351) displaying T16H activity. Biochemical characterization demonstrated that CYP71D12 and CYP71D351 both exhibit high affinity for tabersonine and narrow substrate specificity, making of T16H, to our knowledge, the first alkaloid biosynthetic enzyme displaying two isoforms encoded by distinct genes characterized to date in C. roseus. However, both genes dramatically diverge in transcript distribution in planta. While CYP71D12 (T16H1) expression is restricted to flowers and undifferentiated cells, the CYP71D351 (T16H2) expression profile is similar to the other vindoline biosynthetic genes reaching a maximum in young leaves. Moreover, transcript localization by carborundum abrasion and RNA in situ hybridization demonstrated that CYP71D351 messenger RNAs are specifically located to leaf epidermis, which also hosts the next step of vindoline biosynthesis. Comparison of high-and low-vindoline-accumulating C. roseus cultivars also highlights the direct correlation between CYP71D351 transcript and vindoline levels. In addition, CYP71D351 down-regulation mediated by virus-induced gene silencing reduces vindoline accumulation in leaves and redirects the biosynthetic flux toward the production of unmodified alkaloids at the C-16 position. All these data demonstrate that tabersonine 16-hydroxylation is orchestrated in an organ-dependent manner by two genes including CYP71D351, which encodes the specific T16H isoform acting in the foliar vindoline biosynthesis.

Iridoids form a broad and versatile class of biologically active molecules found in thousands of plant species. In addition to the many hundreds of iridoids occurring in plants, some iridoids, such as secologanin, serve as key building blocks in the biosynthesis of thousands of monoterpene indole alkaloids (MIAs) and many quinoline alkaloids. This study describes the molecular cloning and functional characterization of three iridoid glucosyltransfeases (UDP-SUGAR GLYCOSYLTRANSFERASE6 [UGT6], UGT7, and UGT8) from Madagascar periwinkle (Catharanthus roseus) with remarkably different catalytic efficiencies. Biochemical analyses reveal that UGT8 possessed a high catalytic efficiency toward its exclusive iridoid substrate, 7-deoxyloganetic acid, making it better suited for the biosynthesis of iridoids in periwinkle than the other two iridoid glucosyltransfeases. The role of UGT8 in the fourth to last step in secologanin biosynthesis was confirmed by virus-induced gene silencing in periwinkle plants, which reduced expression of this gene and resulted in a large decline in secologanin and MIA accumulation within silenced plants. Localization studies of UGT8 using a carborundum abrasion method for RNA extraction show that its expression occurs preferentially within periwinkle leaves rather than in epidermal cells, and in situ hybridization studies confirm that UGT8 is preferentially expressed in internal phloem associated parenchyma cells of periwinkle species.

SUMMARYIridoids are a major group of biologically active molecules that are present in thousands of plant species, and one versatile iridoid, secologanin, is a precursor for the assembly of thousands of monoterpenoid indole alkaloids (MIAs) as well as a number of quinoline alkaloids. This study uses bioinformatics to screen large databases of annotated transcripts from various MIA-producing plant species to select candidate genes that may be involved in iridoid biosynthesis. Virus-induced gene silencing of the selected genes combined with metabolite analyses of silenced plants was then used to identify the 7-deoxyloganic acid 7-hydroxylase (CrDL7H) that is involved in the 3rd to last step in secologanin biosynthesis. Silencing of CrDL7H reduced secologanin levels by at least 70%, and increased the levels of 7-deoxyloganic acid to over 4 mg g À1 fresh leaf weight compared to control plants in which this iridoid is not detected. Functional expression of this CrDL7H in yeast confirmed its biochemical activity, and substrate specificity studies showed its preference for 7-deoxyloganic acid over other closely related substrates. Together, these results suggest that hydroxylation precedes carboxy-O-methylation in the secologanin pathway in Catharanthus roseus.

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