Production of Plant Secondary Metabolites: Examples, Tips and Suggestions for Biotechnologists

Abstract

Plants are sessile organisms and, in order to defend themselves against exogenous (a)biotic constraints, they synthesize an array of secondary metabolites which have important physiological and ecological effects. Plant secondary metabolites can be classified into four major classes: terpenoids, phenolic compounds, alkaloids and sulphur-containing compounds. These phytochemicals can be antimicrobial, act as attractants/repellents, or as deterrents against herbivores. The synthesis of such a rich variety of phytochemicals is also observed in undifferentiated plant cells under laboratory conditions and can be further induced with elicitors or by feeding precursors. In this review, we discuss the recent literature on the production of representatives of three plant secondary metabolite classes: artemisinin (a sesquiterpene), lignans (phenolic compounds) and caffeine (an alkaloid). Their respective production in well-known plants, i.e., Artemisia, Coffea arabica L., as well as neglected species, like the fibre-producing plant Urtica dioica L., will be surveyed. The production of artemisinin and caffeine in heterologous hosts will also be discussed. Additionally, metabolic engineering strategies to increase the bioactivity and stability of plant secondary metabolites will be surveyed, by focusing on glycosyltransferases (GTs). We end our review by proposing strategies to enhance the production of plant secondary metabolites in cell cultures by inducing cell wall modifications with chemicals/drugs, or with altered concentrations of the micronutrient boron and the quasi-essential element silicon.

Keywords: Artemisia; Coffea arabica L.; Urtica dioica L.; artemisinin; bioactivity; caffeine; cell wall; heterologous hosts; lignans; secondary metabolites; uridine diphosphate glycosyltransferases.

Figure 1

Diagram showing the biosynthesis of artemisinin via the mevalonate pathway in plant cells. Acetyl coenzyme A (acetylCoA), 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGR), 3-hydroxy-3-methyl-glutaryl-coenzyme A synthase (HMGS), mevalonate (MVA), isopentenyl pyrophosphate (IPP), farnesyl diphosphate (FPP), glyceraldehyde 3-phosphate (G3P), 1-deoxy-d-xylulose-5-phosphate (DXP), 1-deoxy-d-xylulose-5-phosphate reductoisomerase (DXR), 2-C-methyl-d-erythritol 4-phosphate (MEP), dimethylallyl pyrophosphate (DMAPP).

Figure 2

X-ray crystal structure of O-GT from Vitis vinifera (VvO-GT, PDB: 2C1Z) superimposed on the homology model of C-GT from Oryza sativa (a) and the GT-catalyzed reaction (b). (a) Yellow/light green, VvO-GT; left, N-terminal domain showing catalytic H20 and D119 residues (pink); right, C-terminal domain showing the green PSPG motif that binds the donor sugar (black). Here the donor analogue is UDP-2-deoxy-2-fluoro-d-glucose; Red, acceptor (kaempferol). Turquoise/dark green, rice-CGT; left, N-terminal domain showing H24, D120 and I121 residues (blue); right, C-terminal domain showing the dark green PSPG motif that binds donor sugar (black). Please note that the two imidazole rings of H20 and H24 are at almost right-angles to each other. The homology model was created by I-TASSER [150] and structures visualized by Swiss PDB Deepview 4.1 [151]. (b) Reactions catalyzed by O-GT and C-GT. The nucleotide sugar donor (UDP-glucose) reacts with phloretin to give the respective O- or C-glycosides.

Figure 3

Cartoon depicting the effects of exogenous stresses on the cell wall and the subsequent production of secondary metabolites in plant cell cultures. Cell wall drugs, as well as micronutrient deficiency, silicon addition and/or Al toxicity affect the plant cell wall by inducing modifications. These changes are sensed by specific receptors at the interface between plasma membrane and cell wall which unleash a signaling cascade resulting in calcium accumulation and induction of specific phytohormones (e.g., jasmonic acid). These induce the production of secondary metabolites as a response to the stress. Precursors for specific secondary metabolite biosynthesis are provided by the chloroplast; glycosylated secondary metabolites are stored in the vacuole. The dotted arrow indicates an effect mediated by different players (transcription factors are an example).