Transgenic studies for modulating terpenoid indole alkaloids pathway in Catharanthus roseus: present status and future options

Abstract

The anti-neoplastic herb Catharanthus roseus (Madagascar periwinkle; Family Apocynaceae), known to harbor >130 bioactive terpenoid indole alkaloids (TIAs), is in global attention of transgenic research for pathway engineering. This medicinal herb has a long folklore history of its usage in the treatment of several health disorders owing to the presence of a variety of secondary metabolites, particularly the alkaloids, phenolics and anthocyanins. Among its various bioactive molecules, C. roseus is most valued for being the exclusive bio-resource of two of the most effective anti-cancerous drugs—vincristine and vinblastine that are indispensible components of several modern day chemotherapeutic regimens. The in-planta yields of these bioactive TIAs are extremely low (<0.0002 %) owing to very rigid developmental, genetical and environmental regulation controls as a part of plant’s own auto-protection strategies against these cytotoxic molecules. Consequently the high cost of production coupled with increasing market demand for these alkaloidal drugs have made TIAs pathway as one of the preferred target for plant metabolic engineering efforts to boost their commercial production. The transgenic approaches so far applied in C. roseus can be grouped into three major lines of investigations. The first line of efforts have been dominated by the genetic transformation studies carried out with wild type Ti or Ri plasmids to measure the influence of their random insertion on TIAs biogenesis. These studies, besides helping the optimization of basic transformation protocols at different levels of cellular differentiation, have largely contributed towards the better resolution and expression of TIAs pathway architecture at the level of associated genes and enzymes. Second line of metabolic engineering works is being centered around the efforts on preparation of genomic libraries and genome mapping to hunt for TIAs pathway genes and associated transcription regulators/factors, followed by their cloning, insertion and hyper-expression to modulate the carbon flux towards desired end products. Modern day transgenic attempts in C. roseus are largely focused on superimposing the concurrent influences of biotic and abiotic elicitation, T-DNA activation and RNAi or virus induced gene silencing approaches for higher TIAs biogenesis. The pace of transgenic research in C. roseus in last two decade has been rapid and demands periodic compilation and reviewing of published literature to provide a handy status up-date of the subject. The present compilation is an effort in this direction and intends to provide a state-of-art account of various transgenic approaches currently being pursued in this high value herb. More than 130 research papers have appeared on various aspects of transgenic cells, tissues and plant production in C. roseus in 30 years. The bulk of these efforts (64.96 %) were made at the level of Agrobacterium rhizogenes-mediated hairy root cultures, followed by 26.80 % in A. tumefaciens-mediated transformed tissues and 8.24 % in cells/tissue cultures subjected to particle bombardment approach. In the present compilation, these studies are further grouped into specific categories wherein over-expression of TIAs pathway specific genes, transcriptional factor regulation, elicitation/stress superimposition, transgenic plant regeneration, metabolites profiling and gene silencing etc. were investigated in different transformed tissue.