Plant synthetic biology for human health: advances in producing medicines in heterologous expression systems

Plant synthetic biology has the capability to provide solutions to global challenges in the production and supply of medicines. Recent advances in 'omics' technologies have accelerated gene discoveries in medicinal plant research so that even multistep biosynthetic pathways for bioactive plant natural products with high structural complexity can be reconstituted in heterologous plant expression systems more rapidly. This review provides an overview of concept and strategies used to produce high-value plant natural products in heterologous plant systems and highlights recent successes in engineering the biosynthesis of conventional and new medicines in alternative plant hosts.

Promiscuous terpene synthases from Prunella vulgaris highlight the importance of substrate and compartment switching in terpene synthase evolution

The mint family (Lamiaceae) is well documented as a rich source of terpene natural products. More than 200 diterpene skeletons have been reported from mints, but biosynthetic pathways are known for just a few of these. We crossreferenced chemotaxonomic data with publicly available transcriptomes to select common selfheal (Prunella vulgaris) and its highly unusual vulgarisin diterpenoids as a case study for exploring the origins of diterpene skeletal diversity in Lamiaceae. Four terpene synthases (TPS) from the TPS-a subfamily, including two localised to the plastid, were cloned and functionally characterised. Previous examples of TPS-a enzymes from Lamiaceae were cytosolic and reported to act on the 15-carbon farnesyl diphosphate. Plastidial TPS-a enzymes using the 20-carbon geranylgeranyl diphosphate are known from other plant families, having apparently arisen independently in each family. All four new enzymes were found to be active on multiple prenyl-diphosphate substrates with different chain lengths and stereochemistries. One of the new enzymes catalysed the cyclisation of geranylgeranyl diphosphate into 11-hydroxy vulgarisane, the likely biosynthetic precursor of the vulgarisins. We uncovered the pathway to a rare diterpene skeleton. Our results support an emerging paradigm of substrate and compartment switching as important aspects of TPS evolution and diversification.

Cytosolic lipid droplets as engineered organelles for production and accumulation of terpenoid biomaterials in leaves

Cytosolic lipid droplets are endoplasmic reticulum-derived organelles typically found in seeds as reservoirs for physiological energy and carbon to fuel germination. Here, we report synthetic biology approaches to co-produce high-value sesqui- or diterpenoids together with lipid droplets in plant leaves. The formation of cytosolic lipid droplets is enhanced in the transient Nicotiana benthamiana system through ectopic production of WRINKLED1, a key regulator of plastid fatty acid biosynthesis, and a microalgal lipid droplet surface protein. Engineering of the pathways providing the universal C5-building blocks for terpenoids and installation of terpenoid biosynthetic pathways through direction of the enzymes to native and non-native compartments boost the production of target terpenoids. We show that anchoring of distinct biosynthetic steps onto the surface of lipid droplets leads to efficient production of terpenoid scaffolds and functionalized terpenoids. The co-produced lipid droplets "trap" the terpenoids in the cells.

Metabolite Diversity in Alkaloid Biosynthesis: A Multilane (Diastereomer) Highway for Camptothecin Synthesis in Camptotheca acuminata

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-α(S)-strictosidine as its central MIA intermediate and instead uses an alternative seco-iridoid 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.

Chorismate pyruvate-lyase and 4-hydroxy-3-solanesylbenzoate decarboxylase are required for plastoquinone biosynthesis in the cyanobacterium Synechocystis sp. PCC6803

Plastoquinone is a redox active lipid that serves as electron transporter in the bifunctional photosynthetic-respiratory transport chain of cyanobacteria. To examine the role of genes potentially involved in cyanobacterial plastoquinone biosynthesis, we have focused on three Synechocystis sp. PCC 6803 genes likely encoding a chorismate pyruvate-lyase (sll1797) and two 4-hydroxy-3-solanesylbenzoate decarboxylases (slr1099 and sll0936). The functions of the encoded proteins were investigated by complementation experiments with Escherichia coli mutants, by the in vitro enzyme assays with the recombinant proteins, and by the development of Synechocystis sp. single-gene knock-out mutants. Our results demonstrate that sll1797 encodes a chorismate pyruvate-lyase. In the respective knock-out mutant, plastoquinone was hardly detectable, and the mutant required 4-hydroxybenzoate for growth underlining the importance of chorismate pyruvate-lyase to initiate plastoquinone biosynthesis in cyanobacteria. The recombinant Slr1099 protein displayed decarboxylase activity and catalyzed in vitro the decarboxylation of 4-hydroxy-3-prenylbenzoate with different prenyl side chain lengths. In contrast to Slr1099, the recombinant Sll0936 protein did not show decarboxylase activity regardless of the conditions used. Inactivation of the sll0936 gene in Synechocystis sp., however, caused a drastic reduction in the plastoquinone content to levels very similar to those determined in the slr1099 knock-out mutant. This proves that not only slr1099 but also sll0936 is required for plastoquinone synthesis in the cyanobacterium. In summary, our data demonstrate that cyanobacteria produce plastoquinone exclusively via a pathway that is in the first reaction steps almost identical to ubiquinone biosynthesis in E. coli with conversion of chorismate to 4-hydroxybenzoate, which is then prenylated and decarboxylated.

Catalytic reactions of the homogentisate prenyl transferase involved in plastoquinone-9 biosynthesis

Homogentisate solanesyl transferase (HST) catalyzes the prenylation and decarboxylation of homogentisate to form 2-methyl-6-solanesyl-1,4-benzoquinol, the first intermediate in plastoquinone-9 biosynthesis. In vitro, HST from Spinacia oleracea L., Arabidopsis thaliana, and Chlamydomonas reinhardtii were all found to use not only solanesyl diphosphate but also short chain prenyl diphosphates of 10-20 carbon atoms as prenyl donors. Surprisingly, with these donors, prenyl transfer was largely decoupled from decarboxylation, and thus the major products were 6-prenyl-1,4-benzoquinol-2-methylcarboxylates rather than the expected 2-methyl-6-prenyl-1,4-benzoquinols. The 6-prenyl-1,4-benzoquinol-2-methylcarboxylates were not substrates for HST-catalyzed decarboxylation, and the enzyme kinetics associated with forming these products appeared quite distinct from those for 2-methyl-6-prenyl-1,4-benzoquinol formation in respect of catalytic rate, substrate K(m) value, and the pattern of inhibition by haloxydine, a molecule that appeared to act as a dead end mimic of homogentisate. These observations were reconciled into a simple model for the HST mechanism. Here, prenyl diphosphate binds to HST to form at least two alternative complexes that go on to react differently with homogentisate and prenylate it either with or without it first being decarboxylated. It is supposed that solanesyl diphosphate binds tightly and preferentially in the mode that compels prenylation with decarboxylation.

Characterization of homogentisate prenyltransferases involved in plastoquinone-9 and tocochromanol biosynthesis

A cDNA of Chlamydomonas reinhardtii encoding a plastidial homogentisate prenyltransferase was identified. Functional expression studies in Escherichia coli revealed that the enzyme possessed properties similar to the prenyltransferase of Arabidopsis thaliana encoded by At3g11950 but different from the phytyltransferases of A. thaliana and Synechocystis. Unlike the phytyltransferases, the C. reinhardtii and the respective A. thaliana enzyme showed highest activities with solanesyl diphosphate, but were hardly active with phytyl diphosphate. Hence, these data provide evidence that the latter represent homogentisate solanesyltransferases involved in plastoquinone-9 biosynthesis. Overexpression of At3g11950 in A. thaliana, however, suggests that the solanesyltransferase can affect tocopherol biosynthesis as well.

Characterisation of plant tocopherol cyclases and their overexpression in transgenic Brassica napus seeds

Tocopherols, collectively known as vitamin E, are only synthesised in photosynthetic organisms. Tocopherol cyclase (TC) catalyses the formation of the chromanol headgroup of the various tocopherol isoforms. TCs from Arabidopsis and maize (Zea mays) were expressed in Escherichia coli and purified. Analysis of the enzymatic properties revealed similarities but also differences between the two enzymes. Overexpression of chimeric TC gene constructs in developing seeds of transgenic rapeseed plants enhanced and modified the relative abundance of individual tocochromanol species in the seed oil, indicating a regulatory function of the enzyme in prenyllipid metabolism.