HCHL expression in hairy roots of Beta vulgaris yields a high accumulation of p-hydroxybenzoic acid (pHBA) glucose ester, and linkage of pHBA into cell walls

Hairy Root Cultures as a Source of Phenolic Antioxidants: Simple Phenolics, Phenolic Acids, Phenylethanoids, and Hydroxycinnamates

Plant-derived antioxidants are intrinsic components of human diet and factors implicated in tolerance mechanisms against environmental stresses in both plants and humans. They are being used as food preservatives and additives or ingredients of cosmetics. For nearly forty years, Rhizobium rhizogenes-transformed roots (hairy roots) have been studied in respect to their usability as producers of plant specialized metabolites of different, primarily medical applications. Moreover, the hairy root cultures have proven their value as a tool in crop plant improvement and in plant secondary metabolism investigations. Though cultivated plants remain a major source of plant polyphenolics of economic importance, the decline in biodiversity caused by climate changes and overexploitation of natural resources may increase the interest in hairy roots as a productive and renewable source of biologically active compounds. The present review examines hairy roots as efficient producers of simple phenolics, phenylethanoids, and hydroxycinnamates of plant origin and summarizes efforts to maximize the product yield. Attempts to use Rhizobium rhizogenes-mediated genetic transformation for inducing enhanced production of the plant phenolics/polyphenolics in crop plants are also mentioned.

Bioproduction of glucose conjugates of 4-hydroxybenzoic and vanillic acids using bamboo cells transformed to express bacterial 4-hydroxycinnamoyl-CoA hydratase/lyase

Rational metabolic-flow switching, which we proposed recently, is an effective strategy to produce an exogenous high-value natural product using transformed plant cells; the proof of this concept was demonstrated using bamboo (Phyllostachys nigra; Pn) cells as a model system. Pn cells were transformed to express 4-hydroxycinnamoyl-CoA hydratase/lyase of Pseudomonas putida KT2440 (PpHCHL), which catalyzes the formation of 4-hydroxybenzaldehyde and vanillin from p-coumaroyl-CoA and feruloyl-CoA, respectively. The PpHCHL-transformed cells accumulated glucose conjugates of 4-hydroxybenzoic acid and vanillic acid, indicating that the PpHCHL products (aldehydes) were further metabolized by inherent enzymes in the Pn cells. The production titers of 4-hydroxybenzoic acid glucose ester, vanillic acid glucose ester, and 4-hydroxybenzoic acid glucoside reached 1.7, 0.17, and 0.14 g/L at the maximum, respectively. These results proved the versatility of Pn cells for producing vanillin-related compounds based on rational metabolic-flow switching.

Genomic Analysis of Microbulbifer sp. Strain A4B-17 and the Characterization of Its Metabolic Pathways for 4-Hydroxybenzoic Acid Synthesis

The marine bacterium Microbulbifer sp. A4B-17 produces secondary metabolites such as 4-hydroxybenzoic acid (4HBA) and esters of 4HBA (parabens). 4HBA is a useful material in the synthesis of the liquid crystal. Parabens are man-made compounds that have been extensively used since the 1920s in the cosmetic, pharmaceutical, and food industries for their effective antimicrobial activity. In this study, we completed the sequencing and annotation of the A4B-17 strain genome and found all genes for glucose utilization and 4HBA biosynthesis. Strain A4B-17 uses the Embden-Meyerhof-Parnas (EMP), hexose monophosphate (HMP), and Entner-Doudoroff (ED) pathways to utilize glucose. Other sugars such as fructose, sucrose, xylose, arabinose, galactose, mannitol, and glycerol supported cell growth and 4HBA synthesis. Reverse transcriptional analysis confirmed that the key genes involved in the glucose metabolism were functional. Paraben concentrations were proportionally increased by adding alcohols to the culture medium, indicating that strain A4B-17 synthesizes the 4HBA and the alcohols separately and an esterification reaction between them is responsible for the paraben synthesis. A gene that codes for a carboxylesterase was proposed to catalyze this reaction. The temperature and NaCl concentration for optimal growth were determined to be 35°C and 22.8 g/L.

Integrated Transcript and Metabolite Profiles Reveal That EbCHI Plays an Important Role in Scutellarin Accumulation in Erigeron breviscapus Hairy Roots

Scutellarin, a flavonoid 7-O-glucuronide, is an essential bioactive compound of Erigeron breviscapus (Vaniot) Hand.-Mazz. used for the treatment of cerebrovascular diseases. However, due to overexploitation and overuse, E. breviscapus is facing the problems of extinction and habitat degradation. In this study, a correlation analysis between the transcript and metabolite profiles of methyl jasmonate (MeJA)-treated E. breviscapus at different time points indicated that chalcone isomerase (EbCHI) was the primary contributor to scutellarin accumulation during flavonoid biosynthesis. EbCHI was then further characterized as a chalcone isomerase that efficiently converted chalcone to naringenin in vitro. Optimal parameters derived by comparing different culture conditions were successfully used to establish hairy root cultures of E. breviscapus with a maximum transformation rate of 60% in B5 medium. Furthermore, overexpression of EbCHI significantly enhanced scutellarin accumulation in E. breviscapus hairy roots with a maximum content of 2.21 mg g^-1 (dw), 10-fold higher than that of natural roots (0.21 mg g^-1 dw). This study sheds new light on a method of effective gene-based metabolic engineering by accurate and appropriate strategies and provides a protocol for hairy root cultures that accumulate high levels of scutellarin, providing a promising prospect for relieving the overexploitation and unavailability of E. breviscapus in the future.

Enhanced triterpene accumulation in Panax ginseng hairy roots overexpressing mevalonate-5-pyrophosphate decarboxylase and farnesyl pyrophosphate synthase

To elucidate the function of mevalonate-5-pyrophosphate decarboxylase (MVD) and farnesyl pyrophosphate synthase (FPS) in triterpene biosynthesis, the genes governing the expression of these enzymes were transformed into Panax ginseng hairy roots. All the transgenic lines showed higher expression levels of PgMVD and PgFPS than that by the wild-type control. Among the hairy root lines transformed with PgMVD, M18 showed the highest level of transcription compared to the control (14.5-fold higher). Transcriptions of F11 and F20 transformed with PgFPS showed 11.1-fold higher level compared with control. In triterpene analysis, M25 of PgMVD produced 4.4-fold higher stigmasterol content (138.95 μg/100 mg, dry weight [DW]) than that by the control; F17 of PgFPS showed the highest total ginsenoside (36.42 mg/g DW) content, which was 2.4-fold higher compared with control. Our results indicate that metabolic engineering in P. ginseng was successfully achieved through Agrobacterium rhizogenes-mediated transformation and that the accumulation of phytosterols and ginsenosides was enhanced by introducing the PgMVD and PgFPS genes into the hairy roots of the plant. Our results suggest that PgMVD and PgFPS play an important role in the triterpene biosynthesis of P. ginseng.

Enhanced accumulation of phytosterol and triterpene in hairy root cultures of Platycodon grandiflorum by overexpression of Panax ginseng 3-hydroxy-3-methylglutaryl-coenzyme A reductase

3-Hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) catalyzes the rate-limiting step in the mevalonate pathway. To elucidate the functions of HMGR in triterpene biosynthesis, Platycodon grandiflorum was transformed with a construct expressing Panax ginseng HMGR (PgHMGR). We used PCR analysis to select transformed hairy root lines and selected six lines for further investigation. Quantitative real-time PCR showed higher expression levels of HMGR and total platycoside levels (1.5-2.5-fold increase) in transgenic lines than in controls. Phytosterols levels were also 1.1-1.6-fold higher in transgenic lines than in controls. Among these lines, line T7 produced the highest level of total platycosides (1.60 ± 0.2 mg g(-1) dry weight) and α-spinasterol (1.78 ± 0.16 mg g(-1) dry weight). These results suggest that metabolic engineering of P. grandiflorum by Agrobacterium-mediated genetic transformation may enhance production of phytosterols and triterpenoids.

Expression of a bacterial, phenylpropanoid-metabolizing enzyme in tobacco reveals essential roles of phenolic precursors in normal leaf development and growth

Tobacco plants (Nicotiana tabacum cv XHFD 8) were genetically modified to express a bacterial 4-hydroxycinnamoyl-CoA hydratase/lyase (HCHL) enzyme which is active with intermediates of the phenylpropanoid pathway. We have previously shown that HCHL expression in tobacco stem resulted in various pleiotropic effects, indicative of a reduction in the carbon flux through the phenylpropanoid pathway, accompanied by an abnormal phenotype. Here, we report that in addition to the reduction in lignin and phenolic biosynthesis, HCHL expression also resulted in several gross morphological changes in poorly lignified tissue, such as abnormal mesophyll and palisade. The effect of HCHL expression was also noted in lignin-free single cells, with suspension cultures displaying an altered shape and different growth patterns. Poorly/non-lignified cell walls also exhibited a greater ease of alkaline extractability of simple phenolics and increased levels of incorporation of vanillin and vanillic acid. However, HCHL expression had no significant effect on the cell wall carbohydrate chemistry of these tissues. Evidence from this study suggests that changes in the transgenic lines result from a reduction in phenolic intermediates which have an essential role in maintaining structural integrity of low-lignin or lignin-deprived cell walls. These results emphasize the importance of the intermediates and products of phenylpropanoid pathway in modulating aspects of normal growth and development of tobacco. Analysis of these transgenic plants also shows the plasticity of the lignification process and reveals the potential to bioengineer plants with reduced phenolics (without deleterious effects) which could enhance the bioconversion of lignocellulose for industrial applications.

Overexpression of G10H and ORCA3 in the hairy roots of Catharanthus roseus improves catharanthine production

A number of genes that function in the terpenoid indole alkaloids (TIAs) biosynthesis pathway have been identified in Catharanthus roseus. Except for the geraniol 10-hydroxylase (G10H) gene, which encodes a cytochrome P450 monooxygenase, several of these genes are up-regulated by ORCA3, a jasmonate-responsive APETALA2 (AP2)-domain transcript factor. In this study, the G10H gene was transformed independently, or co-transformed with ORCA3 into C. roseus, using Agrobacterium rhizogenes MSU440. Hairy root clones expressing the G10H gene alone, or both the G10H and ORCA3 genes, were obtained. Alkaloid accumulation level analyses showed that all transgenic clones accumulated more catharanthine, with the highest accumulation level in the transgenic clone OG12 (6.5-fold higher than that of the non-expression clone). Following treatment with ABA, accumulation of catharanthine reached 1.96 mg/g DW in the transgenic clone OG12. The expression levels of TIAs biosynthesis genes in transgenic and non-transgenic clones were also investigated.