application of callus is to inducesomaclonalvariation through which desired mutants
can be selected.
5.6.1.1. Somaclonal Variation. Plant cells undergo varying degrees of cytological and
genetic changes during in vitro growth. Some of the changes are derived from preexisting
aberrant cells in the explants used for culture. Others represent transient physiological and
developmental disturbances caused by culture environments. Still others are a result of epi-
genetic changes, which can be relatively stable but are not transmitted to the progeny. Some
variations are a result of specific genetic change or mutation and are transmitted to the
progeny. Such genetically controlled variability is known as somaclonal variation.
Somaclonal variation serves as both a boon and a bane in tissue culture. It may hamper
clonal propagation, but at the same time generate desirable somaclonal variants that can
be selected for the development of novel cell lines. Induced somaclonal genetic variability
of callus can give rise to genetically variable plantlets regenerated from callus and are of
immense importance in the development and selection of various stress tolerant cell
lines. Salt-tolerant (Ochatt et al. 1999), heavy-metal-tolerant (Chakravarty and Srivastava
1997), disease-resistant (Jones 1990), and herbicide-resistant (Smith and Chaleff 1990)
cell lines have been selected via somaclonal mutations using callus tissue.
5.6.2 Cell Suspension Culture
Lose friable callus can be broken down to small pieces and grown in a liquid medium to
form cell suspension cultures. Cell suspensions can be maintained as batch cultures
grown in flasks for long periods of time. Somatic embryos have been initiated from cell sus-
pension cultures (Augustine and D’Souza 1997). Cell cultures have also been employed for
the production of valuable secondary metabolites.
5.6.2.1. Production of Secondary Metabolites and Recombinant Proteins
Using Cell Culture.Plant cell cultures can be useful for the production of secondary
metabolites and recombinant proteins.Secondary metabolitesare chemical compounds
that are not required by the plant for normal growth and development but are produced
in the plant as “byproducts” of cell metabolisms. That is not to say that secondary meta-
bolites serve no function to the plant; many do. Some are used for defense mechanism
or for reproductive purposes such as color or smell. Some important secondary metabolites
present in plants are flavonoids, alkaloids, steroids, tannins, and terpenes. Secondary metab-
olites have been produced using cell cultures in many plant species and have been reviewed
by Rao and Ravishankar (2002). The process can be scaled up and automated using bio-
reactors for commercial production. Many strategies such as biotransformation, cell per-
meabilization, elicitation, and immobilization have been used to make cell suspension
cultures more efficient in the production of secondary metabolites. Secondary metabolite
production can be increased by metabolic engineering, in which enzymes in the pathway
of a specific compound can be overexpressed together, thereby increasing the production
of a specific compound.
Transgenic plant cell cultures are gaining popularity in the large-scale production of
recombinant proteins, thus making them integral parts of molecular farming. What
makes molecular farming economically attractive is that production costs can potentially
be much lower than those of traditional pharmaceutical production. Plant cell cultures
122 TISSUE CULTURE: THE MANIPULATION OF PLANT DEVELOPMENT