Enhanced production of antimicrobial sesquiterpenes and lipoxygenase metabolites in elicitor-treated hairy root cultures of Solanum tuberosum

Hairy root culture: a potent method for improved secondary metabolite production of Solanaceous plants

Secondary metabolites synthesized by the Solanaceous plants are of major therapeutic and pharmaceutical importance, many of which are commonly obtained from the roots of these plants. 'Hairy roots', mirroring the same phytochemical pattern of the corresponding root of the parent plant with higher growth rate and productivity, are therefore extensively studied as an effective alternative for the in vitro production of these metabolites. Hairy roots are the transformed roots, generated from the infection site of the wounded plants with Agrobacterium rhizogenes. With their fast growth, being free from pathogen and herbicide contamination, genetic stability, and autotrophic nature for plant hormones, hairy roots are considered as useful bioproduction systems for specialized metabolites. Lately, several elicitation methods have been employed to enhance the accumulation of these compounds in the hairy root cultures for both small and large-scale production. Nevertheless, in the latter case, the cultivation of hairy roots in bioreactors should still be optimized. Hairy roots can also be utilized for metabolic engineering of the regulatory genes in the metabolic pathways leading to enhanced production of metabolites. The present study summarizes the updated and modern biotechnological aspects for enhanced production of secondary metabolites in the hairy root cultures of the plants of Solanaceae and their respective importance.

Hairy root culture technology: applications, constraints and prospect

Hairy root (HR) culture, a successful biotechnology combining in vitro tissue culture with recombinant DNA machinery, is intended for the genetic improvement of plants. This technology has been put to use since the last three decades for genetic advancement of medicinal and aromatic plants and also to harvest the economical products in the form of secondary metabolites that are significantly important for their ethnobotanical and pharmacological properties. It also provides an efficient way out for the quicker extraction and quantification of the valuable phytochemicals. The current review provides an account of the in vitro HR culture technology and its wide-scale applications in the field of research as well as in pharmaceutical industries. Different facets of HR with respect to the culture establishment, phytochemical production as well as research investigations concerning the areas of gene manipulation, biotransformation of the secondary metabolites, phytoremediation, their industrial utilisations and different problems encountered during the application of this technology have been covered in this appraisal. Eventually, an idea has been provided on HR about the recent trends on the progress of this technology that may open up newer prospects in near future and calls for further research and explorations in this field. KEY POINTS: • Genetic engineering-based HR culture aims towards enhanced secondary metabolite production. • This review explores an insight in the HR technology and its multi-faceted approaches. • Up-to-date ground-breaking research applications and constraints of HR culture are discussed.

Biotechnological Exploration of Transformed Root Culture for Value-Added Products

Medicinal plants produce valuable secondary metabolites with anticancer, analgesic, anticholinergic or other activities, but low metabolite levels and limited available tissue restrict metabolite yields. Transformed root cultures, also called hairy roots, provide a feasible approach for producing valuable secondary metabolites. Various strategies have been used to enhance secondary metabolite production in hairy roots, including increasing substrate availability, regulating key biosynthetic genes, multigene engineering, combining genetic engineering and elicitation, using transcription factors (TFs), and introducing new genes. In this review, we focus on recent developments in hairy roots from medicinal plants, techniques to boost production of desired secondary metabolites, and the development of new technologies to study these metabolites. We also discuss recent trends, emerging applications, and future perspectives.

Root Exudates Metabolic Profiling Suggests Distinct Defense Mechanisms Between Resistant and Susceptible Tobacco Cultivars Against Black Shank Disease

There is increasing evidence that root exudates play important roles in plant disease resistance. Black shank, caused by Phytophthora nicotianae, is a destructive soil-borne disease in tobacco (Nicotiana tabacum L.). The aim of the present study was to investigate the activity and composition of the root exudates from resistant and susceptible tobacco cultivars. The root exudates of the resistant cultivar Gexin 3 showed inhibitory activity against P. nicotianae, while the exudates of susceptible cultivar Xiaohuangjin 1025 stimulated the colony growth but had no effect on spore germination. Metabolic profiling using liquid chromatography/electrospray ionization-quadrupole-time-of-flight mass spectrometry depicted differing metabolic patterns of root exudates between Gexin 3 and Xiaohuangjin 1025. The activity and composition of root exudates was altered by P. nicotianae inoculation. Multivariate analysis showed that root exudates (including organic acids, alkaloids, fatty acids, and esters) were different between the two varieties. The defense substances in root exudates, such as tartaric acid, ferulic acid, and lauric acid, may represent a pre-infection prevention strategy for tobacco. Phenylpropanoids as well as inducers of salicylic acid, fatty acids, 6-hydroxyhexanoic acid, and hydrojasmonate may be involved in tobacco defense responses. Compared to the susceptible cultivar, the roots of the resistant cultivar exhibited high enzyme activities of phenylalanine ammonia-lyase, cinnamate-4-hydroxylase and 4-coumarate-CoA ligase, which may prompt the synthesis and secretion of phenylpropanoids. Our results indicated that the root exudates not only provide a pre-infection prevention strategy by exuding antimicrobial substances, but also increase tobacco disease resistance by eliciting plant defense responses. In addition, some defense compounds as well as compounds that play a role in inducing plant defense responses, showed potential for disease control application. This study provides an insight into possible disease resistance mechanisms of root exudates, and attempts the beneficial utilization of these secondary metabolites of plants.

Molecular characterization of flavonoid biosynthetic genes and accumulation of baicalin, baicalein, and wogonin in plant and hairy root of Scutellaria lateriflora

Scutellaria lateriflora is well known for its medical applications because of the presence of flavanoids and alkaloids. The present study aimed to explore the molecular aspects and regulations of flavanoids. Five partial cDNAs encoding genes that are involved in the flavonoid biosynthetic pathway: phenylalanine ammonia lyase (SlPAL), cinnamate 4-hydroxylase (SlC4H), 4-coumaroyl CoA ligase (Sl4CL), chalcone synthase (SlCHS), and chalcone isomerase (SlCHI) were isolated from S. lateriflora. Organ expression analysis showed that these genes were expressed in all organs analyzed with the highest levels correlating with the richest accumulation of wogonin in the roots. Baicalin and baicalein differentially accumulated in S. lateriflora plants, with the highest concentration of baicalin and baicalein detected in the leaves and stems, respectively. Exogenous methyl jasmonate (MeJA) significantly enhanced the expression of SlCHS and SlCHI, and accumulation of baicalin (22.54 mg/g), baicalein (1.24 mg/g), and wogonin (5.39 mg/g) in S. lateriflora hairy roots. In addition, maximum production of baicalin, baicalein, and wogonin in hairy roots treated with MeJA was approximately 7.44-, 2.38-, and 2.12-fold, respectively. Light condition increased the expression level of SlCHS, the first committed step in flavonoid biosynthesis in hairy roots of S. lateriflora after 3 and 4 weeks of development compared to the dark condition. Dark-grown hairy roots contained a higher content of baicalin and baicalein than light-grown hairy roots, while light-grown hairy roots accumulated more wogonin than dark-grown hairy roots. These results may helpful for the metabolic engineering of flavonoids biosynthesis in S. lateriflora.

Extracellular chromone derivatives in cell cultures of Pimpinella anisum. Influence of elicitation with methyl jasmonate and 2β-methyl cyclodextrins

OBJECTIVES

To explore the potentiality of undifferentiated Pimpinella anisum L. cell cultures for the production of secondary metabolites by means of elicitation.

RESULTS

Two chromone compounds were secreted to the medium of undifferentiated cultures of P. anisum: 4-methoxyfuro[3,2-g]chromen-7-one, known as bergapten, which is constitutive to anise, and 5-hydroxy-7-methoxy-2-methylchromen-4-one, the rare chromone eugenin, not yet described in P. anisum. Caffeoyl quinic acid species were also identified in the biomass. Elicitation with methyl jasmonate enhanced chromone accumulation in the medium and stimulated phenolic acid metabolism in the biomass (11 mg caffeoyl quinic acids g^-1 DW cells). The application of 2,6-dimethyl-β-cyclodextrins to cultures led to an intense accumulation of chromones, with nearly 10 mg l^-1 bergapten and 150 mg l^-1 eugenin being accumulated extracellularly after optimal elicitation conditions.

CONCLUSIONS

The significant amounts of eugenin obtained in the anise cultures and the stability of production over long periods of time can be of interest for its biotechnological production and for future studies on biosynthesis regulation.

The regulatory mechanism of fungal elicitor-induced secondary metabolite biosynthesis in medical plants

A wide range of external stress stimuli trigger plant cells to undergo complex network of reactions that ultimately lead to the synthesis and accumulation of secondary metabolites. Accumulation of such metabolites often occurs in plants subjected to stresses including various elicitors or signal molecules. Throughout evolution, endophytic fungi, an important constituent in the environment of medicinal plants, have known to form long-term stable and mutually beneficial symbiosis with medicinal plants. The endophytic fungal elicitor can rapidly and specifically induce the expression of specific genes in medicinal plants which can result in the activation of a series of specific secondary metabolic pathways resulting in the significant accumulation of active ingredients. Here we summarize the progress made on the mechanisms of fungal elicitor including elicitor signal recognition, signal transduction, gene expression and activation of the key enzymes and its application. This review provides guidance on studies which may be conducted to promote the efficient synthesis and accumulation of active ingredients by the endogenous fungal elicitor in medicinal plant cells, and provides new ideas and methods of studying the regulation of secondary metabolism in medicinal plants.

Transcriptional Profiling and Molecular Characterization of Astragalosides, Calycosin, and Calycosin-7-O-β-D-glucoside Biosynthesis in the Hairy Roots of Astragalus membranaceus in Response to Methyl Jasmonate

We used the next-generation Illumina/Solexa HiSeq2000 platform on RNA analysis to investigate the transcriptome of Astragalus membranaceus hairy roots in response to 100 μM methyl jasmonate (MeJA). In total, 77,758,230 clean reads were assembled into 48,636 transcripts (average length of 1398 bp), which were clustered into 23,658 loci (genes). Of these, 19,940 genes were annotated by BLASTx searches. In addition, DESeq analysis showed that 2127 genes were up-regulated, while 1247 genes were down-regulated by MeJA. Seventeen novel astragaloside (AST) biosynthetic genes and seven novel calycosin and calycosin-7-O-β-D-glucoside (CG) biosynthetic genes were isolated. The accumulation of ASTs, calycosin, and CG increased significantly in MeJA-treated hairy roots compared with control hairy roots. Our findings will provide a valuable resource for molecular characterization of AST, calycosin, and CG biosynthetic pathways and may lead to new approaches to maximize their production and biomass productivity in the hairy roots of A. membranaceus.

Effects of polysaccharide elicitors on secondary metabolite production and antioxidant response in Hypericum perforatum L. shoot cultures

The effects of polysaccharide elicitors such as chitin, pectin, and dextran on the production of phenylpropanoids (phenolics and flavonoids) and naphtodianthrones (hypericin and pseudohypericin) in Hypericum perforatum shoot cultures were studied. Nonenzymatic antioxidant properties (NEAOP) and peroxidase (POD) activity were also observed in shoot extracts. The activities of phenylalanine ammonia lyase (PAL) and chalcone-flavanone isomerase (CHFI) were monitored to estimate channeling in phenylpropanoid/flavonoid pathways of elicited shoot cultures. A significant suppression of the production of total phenolics and flavonoids was observed in elicited shoots from day 14 to day 21 of postelicitation. This inhibition of phenylpropanoid production was probably due to the decrease in CHFI activity in elicited shoots. Pectin and dextran promoted accumulation of naphtodianthrones, particularly pseudohypericin, within 21 days of postelicitation. The enhanced accumulation of naphtodianthrones was positively correlated with an increase of PAL activity in elicited shoots. All tested elicitors induced NEAOP at day 7, while chitin and pectin showed increase in POD activity within the entire period of postelicitation. The POD activity was in significantly positive correlation with flavonoid and hypericin contents, suggesting a strong perturbation of the cell redox system and activation of defense responses in polysaccharide-elicited H. perforatum shoot cultures.

Effective elicitors and process strategies for enhancement of secondary metabolite production in hairy root cultures

This chapter reviews the various biotic and abiotic elicitors applied to hairy root cultures and their stimulating effects on the accumulation of secondary metabolites. Elicitors generally refer to the agents that stimulate the defense responses of plants. As a major response of plants to biotic and abiotic stress, the accumulation of secondary metabolites in plant tissue cultures can be stimulated by the elicitors. Among the many elicitors applied to hairy root cultures as well as plant cell suspension cultures, the most common and effective elicitors are fungal cell extracts, polysaccharides from fungal and plant cells, and heavy metal salts. With the crude fungal cell extracts, it is essential to observe the preparation conditions carefully for achieving reproducible effects. In addition to the chemical agents, UV-radiation, hyperosmotic stress and temperature shift have been shown effective for some plant species/metabolites. Elicitor type, dose, and treatment schedule are major factors determining the effects on the secondary metabolite production. In addition to the accumulation of products in roots, elicitor treatments often stimulate the release of intracellular products. Although elicitation is mainly effective to increase specific product yield on per unit mass of roots, the incorporation of nutrient feeding strategies can be applied to enhance the volumetric product yield. The integration of in situ product recovery from the roots/liquid medium is another synergistic strategy with the elicitor treatment to improve the process.

Elicitor-induced cellular and molecular events are responsible for productivity enhancement in hairy root cultures: an insight study

A wide range of external stress stimuli triggers a plant cell to undergo a complex network of reactions that ultimately lead to the synthesis and accumulation of secondary metabolites. These secondary metabolites help the plant to survive under stress challenge. The potential of biotic and abiotic elicitors for the induction and enhancement of secondary metabolite production in various culture systems including hairy root (HR) cultures is well-known. The elicitor-induced defense responses involves signal perception of elicitor by a cell surface receptor followed by its transduction involving some major cellular and molecular events including activation of major secondary message signaling pathways. This result in induction of gene expressions escorting to the synthesis of various proteins mainly associated with plant defense responses and secondary metabolite synthesis and accumulation. The review discusses the elicitor-induced various cellular and molecular events and correlates them with enhanced secondary metabolite synthesis in HR systems. Further, this review also concludes that combining elicitation with in-silico approaches enhances the usefulness of this practice in better understanding and identifying the rate-limiting steps of biosynthetic pathways existing in HRs which in turn can contribute towards better productivity by utilizing metabolic engineering aspects.

β-Cyclodextrins enhance artemisinin production in Artemisia annua suspension cell cultures

Artemisinin is a sesquiterpene antimalarial compound produced, though at low levels (0.1-1% dry weight), in Artemisia annua in which it accumulates in the glandular trichomes of the plant. Due to its antimalarial properties and short supply, efforts are being made to improve our understanding of artemisinin biosynthesis and its production. Native β-cyclodextrins, as well as the chemically modified heptakis(2,6-di-O-methyl)-β-cyclodextrin (DIMEB) and 2-hydroxypropyl-β-cyclodextrins, were added to the culture medium of A. annua suspension cultures, and their effects on artemisinin production were analysed. The effects of a joint cyclodextrin and methyl jasmonate treatment were also investigated. Fifty millimolar DIMEB, as well as a combination of 50 mM DIMEB and 100 μM methyl jasmonate, was highly effective in increasing the artemisinin levels in the culture medium. The observed artemisinin level (27 μmol g(-1) dry weight) was about 300-fold higher than that observed in untreated suspensions. The influence of β-cyclodextrins and methyl jasmonate on the expression of artemisinin biosynthetic genes was also investigated.

Methyl jasmonate increases silymarin production in Silybum marianum (L.) Gaernt cell cultures treated with β-cyclodextrins

Silymarin (Sm) from the fruit of Silybum marianum is an isomeric mixture of pharmacologically active flavonolignans which are formed by oxidative coupling of taxifolin (Tx) and coniferyl alcohol (CA). Suspension cultures of this plant constitutively secrete small amounts of Sm into the extracellular medium. Production can be increased by inclusion of cyclodextrins (CDs) in cultures. Both hydroxylated (RHCD) and dimethylated (RMCD) CDs strongly induced prompt accumulation of CA in the medium followed by a late production of flavonolignans. Simultaneous addition of methyl jasmonate (MJ) and RMCD to cells did not significantly modify CA release or flavonolignan accumulation. Delayed addition of MJ to cultures subcultivated in medium containing RMCD markedly influenced Sm production by promoting conversion of the previously formed CA precursor.

Influence of exogenous salicylic acid on flavonolignans and lipoxygenase activity in the hairy root cultures of Silybum marianum

Silymarin is one of the most potent antioxidant so far developed from plant sources used as hepatoprotectants. Influence of different concentrations (0, 1, 2, 4, 6 and 8mg/50ml culture) and exposure time (24, 48, 72, 96 and 120h) of salicylic acid on lipoxygenase activity, linoleic acid content, growth and production of silymarin in hairy root cultures of S. marianum were investigated. Detection and identification of flavonolignans was carried out by high performance liquid chromatograph method. Salicylic acid enhanced silymarin production (1.89mgg(-1) DW). The optimal feeding condition was the addition of salicylic acid (6 mg/50 ml culture) after 24h in which the silymarin content was 2.42 times higher than the control (0.78mgg(-1) DW). The content of silybin, isosilybin, silychristin, silydianin and taxifolin were 0.703, 0.017, 0.289, 0.02 and 0.863mgg(-1) DW respectively in these samples, while in non-treated hairy roots were 0.027, 0.046, 0.23, 0.022 and 0.453 respectively. Lipoxygenase activity also affected by elicitation. lipoxygenase activity increased 24h after treatment by approximately 1.57- fold (0.21 Delta OD(234)/mgproteinmin(-1)). Upon elicitation with salicylic acid, linoleic acid content of hairy roots (38.26mgg(-1) DW) were also elevated after 24h, in which the linoleic acid content was 2.37 times higher than the control (16.1mgg(-1) DW). It is feasible that elicitation with salicylic acid regulates the jasmonate pathway, which in turn mediates the elicitor-induced accumulation of silymarin.

Harnessing the potential of hairy roots: dawn of a new era

In the past two decades, hairy root research for the production of important secondary metabolites has received a lot of attention. The addition of knowledge to overcome the limiting culture parameters of the regulation of the metabolic pathway by specific molecules and the development of novel tools for metabolic engineering now offer new possibilities to improve the hairy root technique for the production of metabolites. Furthermore, engineering hairy roots for the production of animal proteins of therapeutic interest in confined and controlled in vitro conditions is seen as one of the exciting spin-offs of the technology. Recent progress made in the scale-up of the hairy root cultures has paved the way for industrial exploitation of this system. This review highlights some of the significant progress made in the past three years and discusses the potential implications of that research.

Distinct developmental defense activations in barley embryos identified by transcriptome profiling

Proper embryo development is crucial for normal growth and development of barley. Numerous related aspects of this process--for example how the embryo establishes and sustains disease resistance for extended periods during dormancy--remain largely unknown. Here we report the results of microarray analyses of >22,000 genes, which together with measurements of jasmonic acid and salicylic acid during embryo development provide new information on the initiation in the developing barley embryo of at least two distinct types of developmental defense activation (DDA). Early DDA is characterized by the up-regulation of a specific set of genes around 20 days after flowering, including co-regulation of those for encoding 9-lipoxygenase and several oxylipin-generating enzymes, possibly leading to the formation of alpha-ketols. The same developmental phase includes an up-regulation of several defense genes, and indications of co-regulation of those for enzymes involved in the generation of phenylpropanoid phytoalexins. Late DDA is initiated prior to grain desiccation, around 37 days after flowering, with up-regulation of several genes encoding proteins with roles in antioxidant responses as well as a simultaneous up-regulation of several PR genes is notable. Throughout barley embryo development, there are no indications of an increased biosynthesis of either jasmonic acid or salicylic acid. Collectively, the results help explain how the proposed DDA enables protection of the developing barley embryo and grain for purposes of disease resistance.

Hairy root research: recent scenario and exciting prospects

High stability of the production of secondary metabolites is an interesting characteristic of hairy root cultures. For 25 years, hairy roots have been investigated as a biological system for the production of valuable compounds from medicinal plants. A better understanding of the molecular mechanism of hairy root development, which is based on the transfer of Agrobacterium rhizogenes T-DNA into the plant genome, has facilitated its increasing use in metabolic engineering. Hairy roots can also produce recombinant proteins from transgenic roots, and thereby hold immense potential for the pharmaceutical industry. In addition, hairy roots offer promise for phytoremediation because of their abundant neoplastic root proliferation. Recent progress in the scaling-up of hairy root cultures is making this system an attractive tool for industrial processes.