Introduction

Until today, mistletoe has not been a particular focus of cell and tissue culture experiments, i.e. the removal of cell groups or tissue explants from a mistletoe donor plant and their subsequent cultivation on a nutrient medium supplemented with growth regulators and other substances. In vitro techniques are a major component of plant biotechnology, since they permit to artificially control several of the parameters affecting the growth and metabolism of cultured tissues. Researchers working with V. album world wide could benefit from the establishment of tissue culture protocols for this species in the following ways:

1. By altering the culture parameters, it might be possible to control the quantity, composition and timing of production of mistletoe extracts. In this way, problems associated with the standardisation of mistletoe extracts (Wagner et al, 1986; Lorch and Tröger, this book) might be overcome. By feeding cultures with precursor substances for the biosynthesis of certain metabolites, a higher productivity may be achieved from cultured cells (in vitro) than from whole plants.

2. Potentially, entirely novel substances may be synthesised through biotransformation or by taking advantage of somaclonal variation, i.e. a transient or heritable variability of metabolic procedures induced by the procedure of in vitro culture.

3. The establishment of a callus culture is the first step required in order to obtain genetically modified cells or plants, e.g. crop plants with a viscotoxin-based resistance to pathogens such as Plasmodiophora brassicae (Holtorf et al., 1998).

4. Due to its semiparasitic nature, it is very difficult to propagate mistletoe, which is accomplished with the aid of birds or insects, carrying distantly mistletoe seeds (Becker, 1986; Grieve, 1994; Ramm et al., this book). Biotechnology could offer an alternative method for the production of considerable mistletoe biomass in a relatively short time, either through cell and callus proliferation, or plant regeneration via organogenesis or somatic embryogenesis.

Becker and Schwarz (1971) were the first to mention the possible use of mistletoe callus cultures as a source of bioactive products. In 1990, Fukui et al. reported on the

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