In the growth condition(s) of plants, numerous secondary metabolites (SMs) are produced by them to serve variety of cellular functions essential for physiological processes, and recent increasing evidences have implicated stress and defense response signaling in their production. The type and concentration(s) of secondary molecule(s) produced by a plant are determined by the species, genotype, physiology, developmental stage and environmental factors during growth. This suggests the physiological adaptive responses employed by various plant taxonomic groups in coping with the stress and defensive stimuli. The past recent decades had witnessed renewed interest to study abiotic factors that influence secondary metabolism during in vitro and in vivo growth of plants. Application of molecular biology tools and techniques are facilitating understanding the signaling processes and pathways involved in the SMs production at subcellular, cellular, organ and whole plant systems during in vivo and in vitro growth, with application in metabolic engineering of biosynthetic pathways intermediates.

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Stress and defense responses in plant secondary metabolites production

Higher plants undergo a variety of stresses and to combat those stresses they acclimatize themselves by producing diverse secondary metabolites. These secondary metabolites also have a wide range of industrial applications and hence they serve as candidates for commercialization. Owing to the constraints faced by natural plant extraction, plant cell/tissue culture has emerged as an alternative platform for the in vitro production of value added bioactive secondary metabolites. Implementation of several productivity enhancement strategies, including elicitation, can overcome the limitations faced by plant cell technology that hampers its extensive commercialization. Elicitation is a technique that involves exogenous addition of elicitors (abiotic or biotic) in the growth medium which consequently triggers stress response with concomitant enhancement in secondary metabolite production. Elicitor induced stress results in the activation of several defense-related genes or inactivation of non-defense-related genes, transient phosphorylation/dephosphorylation of proteins, expression of enzymes whose information can be used to ascertain the biosynthetic pathways of many secondary metabolites. Furthermore, integration of transcriptomics, proteomics and metabolomics with system biology can aid in discovery of novel genes, transcriptional factors and several biosynthetic pathways which in turn can serve as a valuable tool for metabolic engineering and gene manipulation for enhancing the yield and productivity of secondary metabolites.

Elicitation: a stimulation of stress in in vitro plant cell/tissue cultures for enhancement of secondary metabolite production

Plant secondary metabolites (SMs) play important roles in plant survival and in creating ecological connections between other species. In addition to providing a variety of valuable natural products, secondary metabolites help protect plants against pathogenic attacks and environmental stresses. Given their sessile nature, plants must protect themselves from such situations through accumulation of these bioactive compounds. Indeed, secondary metabolites act as herbivore deterrents, barriers against pathogen invasion, and mitigators of oxidative stress. The accumulation of SMs are highly dependent on environmental factors such as light, temperature, soil water, soil fertility, and salinity. For most plants, a change in an individual environmental factor can alter the content of secondary metabolites even if other factors remain constant. In this review, we focus on how individual environmental factors affect the accumulation of secondary metabolites in plants during both biotic and abiotic stress conditions. Furthermore, we discuss the application of abiotic and biotic elicitors in culture systems as well as their stimulating effects on the accumulation of secondary metabolites. Specifically, we discuss the shikimate pathway and the aromatic amino acids produced in this pathway, which are the precursors of a range of secondary metabolites including terpenoids, alkaloids, and sulfur- and nitrogen-containing compounds. We also detail how the biosynthesis of important metabolites is altered by several genes related to secondary metabolite biosynthesis pathways. Genes responsible for secondary metabolite biosynthesis in various plant species during stress conditions are regulated by transcriptional factors such as WRKY, MYB, AP2/ERF, bZIP, bHLH, and NAC, which are also discussed here.

Plant Secondary Metabolite Biosynthesis and Transcriptional Regulation in Response to Biotic and Abiotic Stress Conditions

Biotechnology is playing a vital alternative role in the production of pharmaceutical plant secondary metabolites to support industrial production and mitigate over-exploitation of natural sources. High-value pharmaceuticals that include alkaloids, flavonoids, terpenes, steroids, among others, are biosynthesized as a defensive strategy by plants in response to perturbations under natural environmental conditions. However, they can also be produced using plant cell, tissue, and organ culture techniques through the application of various in vitro approaches and strategies. In the past decades, efforts were on the clonal propagation, biomass and secondary metabolites production in the in vitro cultures of medicinally important plants that produce these molecules. In recent years, the effort has shifted towards optimizing culture conditions for their production through the application of cell line selection, elicitation, precursor feeding, two-phase co-culture among cell, tissue, and organ culture approaches. The efforts are made with the possibility to scale-up the production, meet pharmaceutical industry demand and conserve natural sources of the molecules. Applications of metabolic engineering and production from endophytes are also getting increasing attention but, the approaches are far from practical application in their industrial production.

Secondary metabolism of pharmaceuticals in the plant in vitro cultures: strategies, approaches, and limitations to achieving higher yield

Establishment of cell suspension cultures of Ocimum basilicum L. and enhanced production of pharmaceutical active ingredients

Production of camptothecin (CPT), an anticancer compound was enhanced in the cell suspension cultures of Ophiorrhiza mungos Linn. through elicitor treatment. Cell suspension culture was established using the friable callus tissues induced from the field grown leaf explants cultured in MS solid media supplemented with 3 % sucrose, 3 mg L−1 1-Naphthaleneacetic acid (NAA), 1 mg L−1 2,4-Dichlorophenoxyacetic acid (2,4-D) and 0.5 mg L−1 kinetin (KIN). The callus tissues were used for establishing cell suspension culture in half-strength MS (1/2X MS) liquid media supplemented with the same hormone concentration. NAA was found to be essential for the prolific growth of O. mungos cells in suspension culture. Influence of different elicitors such as yeast extract (YE) and silver nitrate (AgNO3) on cell growth, CPT accumulation and cell viability was studied and found that YE and AgNO3 caused a significant increase in biomass and CPT yield according to their concentration, incubation time and feeding time. A maximum of 13.3-fold increment in CPT production and threefold increase in cell growth were recorded in cell cultures elicited with 50 mg L−1 YE on the 10th day of incubation. Cell growth and CPT level were found to decrease in the cultures treated with high concentration of elicitors. CPT was estimated using high performance liquid chromatography (HPLC). The results obtained in the present investigation suggest the use of elicitation as a promising alternative method to increase CPT production and cell growth in the cell suspension cultures of O. mungos.

Enhanced camptothecin production induced by elicitors in the cell suspension cultures of Ophiorrhiza mungos Linn.

Effects of major nutrients, growth regulators and inoculum size on enhanced growth and camptothecin production in adventitious root cultures of Ophiorrhiza mungos L.

Ophiorrhiza mungos is a herbaceous medicinal plant which contains a quinoline alkaloid, camptothecin (CPT), an anticancer compound. A high-yielding cell line, O. mungos cell line-3 (OMC3) was selected from cell suspension cultures of O. mungos using cell aggregate cloning method and established cell suspension culture. OMC3 cell suspension produced significantly high biomass (9.25 ± 1.3 g/flask fresh weight (FW)) and CPT yield (0.095 ± 0.002 mg g−1 dry weight (DW)) compared with the original cell suspension. Inoculum size of OMC3 cell suspension culture was optimised as 14 g L−1. Media optimisation has shown that 5 % (w/v) sucrose and an increased ammonium/nitrate concentration of 40/20 mM favoured CPT production, whereas 3 % (w/v) sucrose, an ammonium/nitrate concentration of 20/40 mM and 1.25 mM of phosphate favoured biomass accumulation. Jasmonic acid, chitin and salicylic acid was used to elicit CPT production in the original cell suspension culture and achieved significantly high CPT production with jasmonic acid (JA) elicitation. Further, OMC3 cell suspension culture was elicited with JA (50 μM) and obtained 1.12 ± 0.08 mg g−1 DW CPT and 9.52 ± 1.4 g/flask FW (190.4 g L−1 FW). The combination of cell line selection and elicitation has produced 18.66-fold increases in CPT production together with significantly high biomass yield. The study is helpful in the scale-up studies of O. mungos cell suspension culture in suitable bioreactor systems for the production of CPT.

S. Deepthi

Papers

Enhanced camptothecin production induced by elicitors in the cell suspension cultures of Ophiorrhiza mungos Linn.

Effects of major nutrients, growth regulators and inoculum size on enhanced growth and camptothecin production in adventitious root cultures of Ophiorrhiza mungos L.

Cell line selection combined with jasmonic acid elicitation enhance camptothecin production in cell suspension cultures of Ophiorrhiza mungos L