Research review paper
Transgenic hairy roots: recent trends and applications

https://doi.org/10.1016/S0734-9750(99)00016-6Get rights and content

Abstract

Agrobacterium rhizogenes causes hairy root disease in plants. The neoplastic roots produced by A. rhizogenes infection is characterized by high growth rate and genetic stability. These genetically transformed root cultures can produce higher levels of secondary metabolites or amounts comparable to that of intact plants. Hairy root cultures offer promise for production of valuable secondary metabolites in many plants. The main constraint for commercial exploitation of hairy root cultures is their scaling up, as there is a need for developing a specially designed bioreactor that permits the growth of interconnected tissues unevenly distributed throughout the vessel. Rheological characteristics of heterogeneous system should also be taken into consideration during mass scale culturing of hairy roots. Development of bioreactor models for hairy root cultures is still a recent phenomenon. It is also necessary to develop computer-aided models for different parameters such as oxygen consumption and excretion of product to the medium. Further, transformed roots are able to regenerate genetically stable plants as transgenics or clones. This property of rapid growth and high plantlet regeneration frequency allows clonal propagation of elite plants. In addition, the altered phenotype of hairy root regenerants (hairy root syndrome) is useful in plant breeding programs with plants of ornamental interest. In vitro transformation and regeneration from hairy roots facilitates application of biotechnology to tree species. The ability to manipulate trees at a cellular and molecular level shows great potential for clonal propagation and genetic improvement. Transgenic root system offers tremendous potential for introducing additional genes along with the Ri T-DNA genes for alteration of metabolic pathways and production of useful metabolites or compounds of interest. This article discusses various applications and perspectives of hairy root cultures and the recent progress achieved with respect to transformation of plants using A. rhizogenes.

Introduction

Plants remain a major source of pharmaceuticals and fine chemicals. Despite considerable efforts, only a few commercial processes have been achieved using cell cultures (e.g. shikonin, berberine). The major constraint with cell cultures is that they are genetically unstable and cultured cells tend to produce low yields of secondary metabolites. A new route for enhancing secondary metabolite production is by transformation using the natural vector system Agrobacterium rhizogenes, the causative agent of hairy root disease in plants. Genetically transformed hairy roots obtained by infection of plants with A. rhizogenes, a gram-negative soil bacterium, offers a promising system for secondary metabolite production [1]. The fast growing hairy roots are unique in their genetic and biosynthetic stability and their fast growth offers an additional advantage. These fast growing hairy roots can be used as a continuous source for the production of valuable secondary metabolites. Moreover, transformed roots are able to regenerate whole viable plants and maintain their genetic stability during further subculturing and plant regeneration.

Section snippets

Agrobacterium and Ri T-DNA genes

Agrobacterium recognizes some signal molecules exuded by susceptible wounded plant cells and becomes attached to it (chemotactic response). Infection of plants with A. rhizogenes causes development of hairy roots at the site of infection. The rhizogenic strains contain a single copy of a large Ri plasmid. In the Agropine Ri plasmid T-DNA is referred to as left T-DNA (TL-DNA) and right T-DNA (TR-DNA). TR T-DNA contains genes homologous to Ti plasmid tumor inducing genes. Genes involved in

Plant regeneration

Transformed roots are able to regenerate whole viable plants; hairy roots as well as the plants regenerated from hairy roots are genetically stable. However, in some instances transgenic plants have shown an altered phenotype compared to controls. Plants regenerated from Ri transformed roots display ‘hairy root syndrome,’ combined expression of the rolABC loci of the Ri plasmid is responsible for this expression. Each locus is responsible for a typical phenotypic alteration; that is, rolA is

Acknowledgements

The financial support to A.G. by Council of Scientific and Industrial Research (CSIR) Govt. of India is duly acknowledged. The authors thank Dr C.C. Giri, Centre for Plant Molecular Biology, Osmania University, Hyderabad, India, for his critical suggestions during the preparation of the manuscript.

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