Emerging trends in research on spatial and temporal organization of terpenoid indole alkaloid pathway in Catharanthus roseus: a literature update

Abstract

Catharanthus roseus (The Madagaskar Periwinkle) plant is commercially valued for harbouring more than 130 bioactive terpenoid indole alkaloids (TIAs). Amongst these, two of the leaf-derived bisindole alkaloids-vinblastine and vincristine-are widely used in several anticancer chemotherapies. The great pharmacological values, low in planta occurrence, unavailability of synthetic substitutes and exorbitant market cost of these alkaloids have prompted scientists to understand the basic architecture and regulation of biosynthesis of these TIAs in C. roseus plant and its cultured tissues. The knowledge gathered over a period of 30 years suggests that the TIA biosynthesis is highly regulated by developmental and environmental factors and operates through a complex multi-step enzymatic network. Extensive spatial and temporal cross talking also occurs at inter- and intracellular levels in different plant organs during TIA biogenesis. A close association of indole, methylerythritol phosphate and secoiridoid monoterpenoid pathways and involvement of at least four cell types (epidermis, internal phloem-associated parenchyma, laticifers and idioblasts) and five intracellular compartments (chloroplast, vacuole, nucleus, endoplasmic reticulum and cytosol) have been implicated with this biosynthetic mechanism. Accordingly, the research in this area is primarily advancing today to address and resolve six major issues namely: precise localization and expression of pathway enzymes using modern in situ RNA hybridization tools, mechanisms of intra- and intercellular trafficking of pathway intermediates, cloning and functional validation of genes coding for known or hitherto unknown pathway enzymes, mechanism of global regulation of the pathway by transcription factors, control of relative diversion of metabolite flux at crucial branch points and finally, strategising the metabolic engineering approaches to improve the productivity of the desired TIAs in plant or corresponding cultured tissues. The present literature update has been compiled to provide a brief overview of some of the emerging developments in our current understanding of TIA metabolism in C. roseus.

Cell-specific localization of alkaloids in Catharanthus roseus stem tissue measured with Imaging MS and Single-cell MS

“…Extensive studies have revealed that more than 130 TIAs are produced from the central precursor strictosidine in C. roseus ( Fig. 1) (9).…”

Alkaloid transporters in plants

“…Recent advances in transcriptomics and proteomics have led to the identification of genes and enzymes involved in MIA biosynthesis. Detailed analyses of the expression patterns of MIA biosynthesis genes indicated that at least four cell types in the leaf, i.e., IPAP, epidermal cells, laticifers, and idioblasts, are involved in MIA biosynthesis (De Luca et al 2014;Verma et al 2012), as illustrated in Figure 2B. This indicated the existence of mechanisms for intercellular trafficking of pathway intermediates, i.e., loganic acid from the IPAP to the epidermis and 16-methoxytabersonine from the epidermis to the laticifers and idioblasts.…”

“…Molecular and cellular studies of alkaloid biosynthesis have revealed that enzymes and metabolites related to the same alkaloid biosynthetic pathways do not necessarily co-localize at the tissue, cellular, and subcellular level, implying that alkaloids and their biosynthetic intermediates are dynamically transported in plant tissues (Shitan and Yazaki 2007;Verma et al 2012). Three modes of alkaloid transport exist: inter-organ, intercellular, and intra-cellular (Shitan and Yazaki 2007).…”

“…2007; Verma et al 2012). Elucidating the mechanisms underlying alkaloid transport in plants is important for understanding the in planta formation and accumulation of this class of compounds, and is necessary for developing plants with improved alkaloid production.…”

Space matters

“…The reason might be found in the incredibly complex spatiotemporal organisation of metabolism. Using the example of the precious Madagascar Periwinkle alkaloid vinblastine Verma et al (2012) in the current issue summarise the state of the art and discuss how at least four cell types (epidermis, internal phloëm associated parenchyma, laticifers and idioblasts) and five intra-cellular compartments (chloroplast, vacuole, nucleus, endoplasmic reticulum and cytosol) have to cooperate to produce a total of more than 130 bioactive terpenoid indole alkaloids and provide an important base on new strategies to get access to these compounds that have great pharmacological value, are only scarcely produced in planta, cannot be synthetically substituted and therefore are traded at exorbitant market costs.…”