Trends in Biotechnology
Volume 24, Issue 9, September 2006, Pages 403-409
Journal home page for Trends in Biotechnology

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

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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.

Section snippets

Underground resources of plants: a paradigm shift

Several secondary metabolites of pharmaceutical interest are accumulated in plant roots [1]. However, harvesting roots is destructive for the plants and hence there has been a growing interest to generate hairy roots from medicinal plant species (Table 1; Box 1). Hairy roots develop as the consequence of the interaction between Agrobacterium rhizogenes, a Gram-negative soil bacterium, and the host plant (Box 2). To generate hairy roots, wounded plant tissues are inoculated with A. rhizogenes,

Transcriptome analysis of elicited hairy roots and T-DNA activation tagging are used for metabolic gene discovery

Valuable secondary metabolites are often synthesized by plants for defence against pathogens, and the corresponding biosynthesis pathways are known to be induced by pathogen cell-wall-derived molecules called elicitors. Several elicitors have been successfully tested to increase the production or the secretion of secondary metabolites in hairy roots from different plant species (Table 2). For example, the stress hormone methyl jasmonate (MeJa), which often has a role as secondary messenger in

The metabolic engineering of hairy roots

Sometimes the biosynthesis of a valuable metabolite by hairy roots is limited by the availability of its precursor. One solution to address this problem is to add the desired product to the culture medium, but this can be costly if the precursor is difficult to synthesize or to obtain from other natural sources. In this context, the co-culture system (see Glossary) has proven to be a judicious alternative for the production of the anti-tumor drug podophyllotoxin by Podophyllum hexandrum cell

Genetic transformation of hairy roots can help to design metabolic traps

When the produced metabolites are secreted into the culture medium, a trapping system can enhance the biosynthesis of them, opening the way to design continuously producing culture systems. Metabolite trapping (see Glossary) depends on the chemical nature of the metabolite considered. The addition to the culture medium of an alumina:silica (ratio 1:1) trap made it possible to recover 97.2% of betalaines, the red natural pigments produced by Beta vulgaris hairy roots [23]. Similarly, adding

Hairy roots for molecular farming

The possibility to express functional animal proteins in hairy roots makes this plant material attractive for molecular farming (see Glossary), with several advantages compared with field-cultured plants. These advantages are mostly based on the fact that hairy roots are cultured in confined recipients, avoiding transgene or pharmacologically active protein dissemination in the environment, and in controlled conditions of growth, avoiding pollution of the produced proteins. Recombinant proteins

Scale-up and integration of technology to industry

When fast-growing, properly engineered hairy roots and their culture medium have been optimized, it remains to scale-up the culture for industrial production. Different bioreactor systems are suitable for hairy root cultures. The most traditional system is to adopt the airlift bioreactors used for microorganisms or plant cells to culture the transformed roots in liquid medium, as reported for the hairy roots of Beta vulgaris, Artemisia annua and Astragalus membranaceus 30, 31, 32, 33.

Hairy roots: centre stage of pharmaceutical biotechnologies

The hairy root culture is entering into a new phase, drawing benefit from the progress made in the field of the natural biodiversity of plants, metabolic engineering, animal protein expression in plant system and the conception of new bioreactor systems adapted to root culture. Concerning the natural metabolite production, the discovery of transcription factors regulating several genes of a biosynthetic pathway will stimulate the improvement of hairy roots to produce molecules that are normally

Glossary

Airlift system
the medium is mixed and gassed by introducing air, or another gas mixture, inside a tube placed at the base of the reactor.
Airlift mesh draught reactor
an airlift system equipped with a concentric draft-tube internal-loop. Gas sparging is carried out on the draft-tube bottom so that the liquid circulation is driven from riser to down corner.
Basket bubble bioreactor
a co-culture system with two identical glass domes separated by a silicon gasket. Cultures are immobilized inside the

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