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164 Hairy Root Culture

rich hairy roots of Atropa belladonna L. by Hashimoto et al. (1993), leading to almost

exclusive production of scopolamine and increased yields in both species. Using the same

approach, Palazon et al. (2003b) transformed Duboisia hybrids, selected for initially high

scopolamine contents, using A. rhizogenes carrying the 35S-H6H construct and further

regenerated plants with even higher scopolamine contents.

Similarly, Moyano et al. (2007) reported the establishment of an N. tabacum cell

suspension culture, containing the 35S-H6H gene from Hyoscyamus niger, which showed

considerable capacity for the bioconversion of hyoscyamine into scopolamine. This

approach has also been used to alter levels of alkaloids normally produced in roots via the

expression of yeast, rather than plant, genes. In one such study, a yeast cDNA encoding

ornithine decarboxylase (ODC) was introduced into Nicotiana rustica L. hairy roots using

an Agrobacterium-derived vector (Hamill et al., 1990). Although the hairy root cultures

produced more nicotine than wild-type roots, the increase in ODC activity was

proportionately higher than the increase in nicotine levels, suggesting that ornithine

decarboxylation is not a rate-determining step in the nicotine pathway. In addition,

transformed Lithospermum erythrorhizon root cultures expressing a bacterial gene encoding

chorismate pyruvate-lyase, which converts chorismate to the shikonin precursor 4-

hydroxybenzoate (4HB), did not produce higher levels of shikonin (Sommer et al., 1999).

These results suggest that the availability of 4HB does not normally limit the biosynthesis

of shikonin.

Cinchona officinalis hairy roots constitutively expressing constructs of cDNAs

encoding the enzymes tryptophan decarboxylase and strictosidine synthase from

Catharanthus roseus, two key enzymes in terpenoid indole and quinoline alkaloid

biosynthesis, have high amounts of tryptamine and strictosidine, as well as quinine and

quinidine (Geerlings et al., 1999). These findings show that genetic engineering with

multiple genes is certainly feasible in hairy roots of C. officinalis. Similarly, the

introduction of a heterologous tryptophan decarboxylase gene into hairy root cultures of

Peganum harmala (Table 10.2) resulted in the accumulation of two biosynthetically related,

tryptamine-derived secondary metabolites: serotonin and -carboline alkaloids (Berlin et

al., 1993). Although serotonin accumulation in transgenic root cultures with elevated TDC

activity was higher than in control cultures, -carboline alkaloid levels were not affected;

thus, the tryptamine supply was shown to be limiting for serotonin, but not for -carboline

alkaloid, biosynthesis. In some cases it might therefore be necessary to augment the

medium with a precursor to obtain higher yields. For example, terpenoid indole alkaloid

production in transgenic Catharanthus roseus cells can be stimulated by addition of the

precursor loganin (Canel et al., 1998; Whitmer et al., 1998), and transformed hairy root

cultures would likely respond in a similar manner. In accordance with this hypothesis, C.

roseus hairy root cultures expressing various forms of anthranilate synthase and/or TDC

exhibited higher fluxes through the tryptophan branch of the monoterpene indole alkaloid

pathway in a study by Hughes et al. (2004). Hairy root cultures expressing these genes

under the control of constitutive or inducible promoters accumulated higher levels of

tryptamine than alkaloids. In another attempt to alter the isoprenoid pathway, a hamster 3-

hydroxy-3-methylglutaryl CoA reductase gene was expressed in hairy roots and a line was

generated that accumulated up to seven-fold higher levels of serpentine than controls.

However, no effects on catharanthine levels were detected (Ayora-Talavera et al., 2002). In

addition, the genetic engineering and expression of the enzyme catalyzing the terminal step

of vindoline biosynthesis, deacetylvindoline-4-O-acetyltransferase (DAT), in Catharanthus

roseus hairy root cultures has been recently reported (Magnotta et al., 2007). C. roseus

produces the powerful anticancer drugs vinblastine and vincristine, both of which are

Hairy Root Culture 165

derived from dimerization of the monoterpenoid indole alkaloids, vindoline and

catharanthine. Metabolite analysis of transgenic hairy root cultures in the cited study

established that hairy root extracts had an altered alkaloid monoterpenoid indole profile,

possibly useful for the production of the desired compounds.

Overall, there is still little detailed knowledge of complete biochemical pathways

involved in the synthesis of secondary metabolites in plants. In addition, the regulatory

features are not well understood. Therefore, progress in metabolic engineering remains

slow and is frequently hampered (Verpoorte et al., 2002; Oksman-Caldentey and Inze,

2004). For example, relatively few ESTs are available for alkaloid-producing plants, except

for tobacco, compared with the numbers available for model plants such as Arabidopsis,

rice and tomato (Facchini and De Luca, 2008). However, extensive EST databases have

now been compiled for opium poppy and C. roseus. In addition, functional genomic

approaches (including transcriptomics, proteomics and metabolomics) are being used to

accelerate comprehensive investigations of biological systems (Oksman-Caldentey and

Inze, 2004). For example, Rischer et al. (2006) recently attempted to combine genome-

wide transcript profiling and metabolic profiling (by cDNA-amplified fragment-length

polymorphism and LC-MS analysis, respectively) of MeJA-elicited Catharanthus roseus

cell cultures and identified sets of both known and previously undescribed transcript tags

(417 gene sequence tags) and metabolites (178) associated with terpenoid indole alkaloids.

The analysis and subsequent modification of such gene-to-metabolite networks seem to

provide a promising approach for adjusting plant biochemical machinery to improve the

production of useful compounds and/or new compounds with novel and/or more desirable

biological activities (Goossens and Rischer, 2007).

In addition to modifying naturally occurring pathways, the expression of completely

foreign genes can lead to the production of desired secondary metabolites. For instance, de

novo synthesis of poly(3-hydroxybutyrate) has been reported from hairy root cultures of

Beta vulgaris, transformed with A. tumefaciens, harbouring the root-inducing plasmid

pRi15834 together with the pBI ABC plasmid containing the phaA, phaB and phaC genes

of Cupriavidus necator (Menzel et al., 2003). High amounts of the mammalian

immunomodulator murine interleukin-12 have also been produced from transgenic

Nicotiana tabacum hairy root cultures (Table 10.2) grown in mist bioreactors (see the

section on bioprocessing).

The reports described above all present more or less successful attempts to genetically

engineer hairy root cultures in order to improve the production of targeted metabolites, but

in numerous cases overexpression of a gene encoding a specific enzyme involved in a

biosynthetic pathway of interest has not led to accumulation of the desired metabolite, and

sometimes it has even led to reductions in its production (Palazon et al., 2008 and

references cited therein). This could be desirable for a side metabolite (Fig. 10.2), but not

for the main targeted compound. The cited authors list several examples related to the

production of tropane alkaloids. One problem associated with homologous gene expression,

or even expression of a heterologous gene with high homology to native genes, is co-

suppression, as mentioned above. Moreover, secondary metabolite synthesis is highly

compartmented, thus tissue-specific expression may be required, which is not always

achieved even in hairy root cultures. In conclusion, more factors need to be considered for

successful transgenic metabolite production than merely the required genes.

166 Hairy Root Culture

Fig. 10.2. (A) Aspects of metabolic pathways that need to be considered when manipulating

a specific pathway, starting with precursor P, by metabolic engineering, including: (1) the

rate limiting step(s), (2) addition of rate-limiting intermediates, (3) alternative pathways using

the same intermediates, (4) transport to different compartments, (5) availability of cofactors,

and (6) feedback inhibition. Bold arrows indicate overexpression of genes encoding proteins

required for the indicated steps (modified after Verpoorte et al., 2000). (B) Possibilities to

change pathways in plants by heterologous gene expression, antisense inhibition or co-

suppression of undesired pathways or the expression of genes encoding transcription

factors regulating one or several steps in the pathway. The changes compared to the wild

type situation are shown underlined.

Table 10.2. Summary of engineered hairy root cultures that have been used for secondary metabolite production.

Plant species Gene(s) transformed Product Reference

Homologous genes

Hyoscyamus

Hyoscyamine 6-hydroxylase Scopolamine Jouhikainen et al., 1999

Atropa

Hyoscyamine 6-hydroxylase Scopolamine Yun et al., 1992

Catharanthus

Deacetylvindoline-4-O-acetyltransferase Horhammericine Magnotta et al., 2007

Catharanthus

Anthranilate synthase and/or tryptophan

decarboxylase

Tryptamine Hughes et al., 2004

Foreign plant genes

Duboisia

Hyoscyamus h6h Scopolamine Palazon et al., 2003b

Nicotiana

Hyoscyamus h6h Scopolamine Moyano et al., 2007

Lotus

Antirrhinum dihydroflavonol reductase Condensed tannins Bavage et al., 1997

Peganum

Catharanthus tryptophan decarboxylase Serotonin Berlin et al., 1993

Cinchona

Catharanthus tryptophan decarboxylase,

strictosidine synthase

Tryptamine, strictosidine,

quinine, quinidine

Geerlings et al., 1999

Medicago

Maize Sn gene (TF)

Anthocyanin Damiani et al., 1998

Lotus

Maize Sn gene (TF) Anthocyanin Damiani et al., 1998

Foreign genes from other kingdoms

Lithospermum

Bacterial ubiC Shikonin Sommer et al., 1999

Nicotiana

Yeast ornithine decarboxylase Nicotine Hamill et al., 1990

Beta

Bacterial poly(3-hydroxybutyrate) synthesis

genes

Poly(3-hydroxybutyrate) Menzel et al., 2003

Catharanthus

Hamster 3-hydroxy-3-methylglutaryl

CoA reductase gene

Serpentine Ayora-Talavera et al.,

2002

Nicotiana

Sequences, encoding a single-chain form of

murine IL-12

Murine Interleukin-12 Liu et al., 2009

Trichosanthes

Ribosome-inactivating protein gene Ribosome-inactivating

protein

Thorup et al., 1994

TF - transcription factor

Hairy Root Culture 167

168 Hairy Root Culture

Bioprocessing of Hairy Root Cultures

Bioprocessing aspects of transformed root cultures for mass production of desired

metabolites have been intensively studied during the last two decades. Developing

economically feasible commercial processes requires detailed knowledge of the physiology

of the hairy root strains, understanding of the interactions of the roots with the culture

environment, and tight control of the whole process (from the solid culture stage to the end

product; Fig. 10.3).

Fig. 10.3. Hairy root culture of Harpagophytum procumbens (Devil’s claw, Pedaliaceae):

from Petri dish to product formation.

An important, and in several cases limiting, stage is the bioreactor cultivation of

transformed root cultures. Diverse bioreactor configurations have been used for cultivating

transformed root cultures, including (inter alia) mechanically driven reactors (e.g. stirred

tank reactors, wave reactors and rotating drum reactors), pneumatically driven reactors (e.g.

bubble column reactors and airlift reactors) and bed reactors (e.g. trickle bed reactors and

mist reactors). It would be difficult, if not impossible, to pick any single, ideal type of

bioreactor for cultivating transformed root cultures for all purposes, although use of

ordinary, mechanically agitated reactors is not recommended because of the high stress-

sensitivity of the transformed roots (Georgiev et al., 2008).

However, slight changes in the stirred tank reactor design, such as separating the

impeller from the culture (using a stainless steel mesh) or simply reducing the agitation

speed has allowed successful cultivation of transformed root cultures of Atropa belladonna

and Beta vulgaris (Lee et al., 1999; Georgiev et al., 2006). Table 10.3 lists various types of

bioreactor that have been used to cultivate transformed root cultures of several plant

species, the final densities obtained and the biomasses productivity.

The disposable (often called single-use) wave bioreactors (which have a wave-induced

working motion that ensures mixing and aeration), originally developed for cultivating

highly stress-sensitive animal cell cultures, have been successfully modified by Prof Eibl’s

group from Switzerland for cultivating transformed root cultures from Hyoscyamus

muticus, Panax ginseng (see Table 10.3) and Harpagophytum procumbens (Eibl and Eibl,

2008; Eibl et al., 2009). The advantages of the disposable reactors over classical reactors

include tremendous flexibility, ease of handling and high biosafety (contamination levels

<1% are guaranteed; Eibl and Eibl, 2006). Thus, use of disposable bioreactors in current

Good Manufacturing Practices (cGMP) production processes can minimize complex

cleaning, sterilization, validation and process costs, with consequent reductions in

development times and times-to-market for new products (Eibl and Eibl, 2006). The

commercially available Wave reactor™ and BioWave

reactors (with 250 and 300 l

working volumes, respectively) may also facilitate the establishment of large-scale

processes (sources: www.wavebiotech.com and www.wavebiotech.net).

Hairy Root Culture 169

Mist and trickle-bed reactors seem to offer perfect environments for cultivating

transformed root cultures. Mist reactors are types of gas-phase reactors in which ultrasonic

systems are used to spray a fine mist of water and nutrients on surfaces of hairy root

cultures (in the form of a thin film of liquid medium), which are suspended in a plastic bag.

The nutrient solution is collected at the bottom of the bag and recycled through the system.

In this way, all of the materials are completely contained and isolated from the

environment. Gas-phase reactors can virtually eliminate any oxygen deficiency in dense

root beds, while providing a low shear stress environment (Kim et al., 2002). In

experiments performed in Prof Weathers’ lab in the USA with hairy roots of A. annua and

N. tabacum, grown in 1.5-l and 4-l mist reactors (respectively), biomasses of 14.4 g/l and

5.2 g/l have been attained, respectively (Table 10.3). Although, in the case with transgenic

N. tabacum the accumulated dry biomass was not considered as high, the amount of murine

interleukin-12 was higher, compared to amounts produced in other tested cultivation

systems. Ramakrishnan and Curtis (2004) reported very high growth of H. muticus hairy

roots in 14-l trickle-bed reactors, with final biomass densities significantly exceeding

previously reported values (Table 10.3). The bioreactor characterization (experiments

performed at 1.6 l and 14 l scales) was sufficient for the cited authors to carry out

preliminary design calculations and they concluded that scale-up to at least 10,000 l would

be feasible (Ramakrishnan and Curtis, 2004).

Table 10.3. Selected examples of bioreactor configurations (operated in batch mode) used

for cultivating hairy root cultures.

Bioreactor type

Hairy root

culture

Final density

(g DW/l)/

Productivity

(g DW/l/day)

Reference

Mechanically driven

Stirred tank reactor (5 l)

Beta vulgaris

12.9 / 0.68 Georgiev et al.,

2006

Stirred tank reactor with

separate impeller (25 l)

Atropa belladonna

6.02 / 0.20 Lee et al., 1999

Wave reactor (0.5 l)

Panax ginseng

11.6 / 0.41 Palazon et al.,

2003a

Pneumatically driven

Bubble column reactor (2 l)

Harpagophytum

procumbens

6.6 / 0.31 Ludwig-Müller et al.,

2008

Bubble column reactor (2 l)

Beta vulgaris

12.7 / 0.79 Pavlov et al., 2007

Airlift reactor (2 l)

Nicotiana

tabacum

4.8 / 0.34 Liu et al., 2009

Bed reactors

Mist reactor (4 l)

Nicotiana

tabacum

5.2 / 0.37 Liu et al., 2009

Mist reactor (1.5 l)

Artemisia annua

14.4 / 0.38 Kim et al., 2002

Trickle bed (14 l)

Hyoscyamus

muticus

36.2 / 1.45 Ramakrishnan and

Curtis, 2004

Bioreactor working volumes are given in parenthesis;

DW – dry weight

It should be noted that transformed root cultures have not been used in any commercial

production process to date, although the large-scale cultivation of transformed roots (at 500

l scale) was achieved more than a decade ago (Wilson, 1997). A major issue, which should

170 Hairy Root Culture

be addressed before commercialization, is the inoculation of large-scale reactors. Unlike in

plant cell suspension-based processes, in which the inoculum can be easily transferred

pneumatically, transferring inocula of hairy root cultures from small reactors to large-scale

reactors might be difficult.

A possible solution was offered by Pavlov et al. (2007), who applied fed-batch

operation mode for cultivating Beta vulgaris hairy roots in a bubble column bioreactor. The

recently developed commercial system (involving use of 10,000 l balloon type reactors) for

biomass and ginsenoside production from normal (adventitious) roots of Panax ginseng in

South Korea (Sivakumar et al., 2005), may also provide a solution to the inoculum

problem; the inoculation steps prior to the 10,000 l reactors are likely to be readily

applicable to hairy roots given the high morphological similarities of adventitious root and

transformed root cultures.

Conclusion and Future Prospects

In recent years transformed root cultures have become increasingly serious alternatives for

the mass production of high-value plant-derived metabolites, as well as suitable systems for

producing recombinant proteins. Today, hairy root cultures can be induced from any plant

species, although some are more difficult to transform than others (e.g. monocotyledons).

This raises the possibility of inducing transformed root cultures from rare and threatened

plants (e.g. some medicinal plant species), which would contribute to the preservation of

global biodiversity. In addition, transformed root cultures show promising potential for

other ecologically valuable applications in phytoremediation, and metabolic engineering

offers powerful tools for deliberate adjustment of desired metabolites’ biosynthetic

pathways. Finally, recent trends in bioreactor design (e.g. the development of single-use

bioreactor systems and novel methods for agitating cultures) should lead to significant

reductions in process costs and facilitate fulfilment of cGMP requirements. Thus, hairy root

cultures have high and increasing potential applicability for diverse purposes.

Acknowledgements

This work has been supported by grants from National Science Fund of Bulgaria (under

contract number DO-02-261/2008).

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