Agriculture Reference
In-Depth Information
Similar to the pathogen response, O
3
stress also activates the production of
SA, JA and ethylene.
13.7
IN VITRO SELECTION PRESSURE TECHNIQUE
In vitro culture of plant cells, tissues or organs on a medium containing
selective agents offers the opportunity to select and regenerate plants with
desirable characteristics. The technique has also been effectively used to
induce tolerance, which includes the use of some selective agents that per-
mit the preferential survival and growth of desired phenotypes (Purohit et
al., 1998). The selecting agents usually employed for in vitro selection in-
clude NaCl (for salt-tolerance), PEG or mannitol (for drought-tolerance),
heavy metal, specific fungal culture filtrate (FCF) or phytotoxin such as
fusaric acid or the pathogen itself (for disease-resistance). The explants
are exposed to a broad range of these selective agents added to the culture
medium.
Only the explaints capable of sustaining such environments survive
in the long run and are selected. Two types of selection methods has been
suggested: (a) stepwise long-term treatment, in which cultures are exposed
to stress with gradual increase in concentrations of selecting agent and
(b) shock treatment, in which cultures are directly subjected to a shock
of high concentration and only those which would tolerate that level will
survive (Purohit et al., 1998). These methods are based on the induction of
genetic variation among cells, tissues and/or organs in cultured and regen-
erated plants (Mohamed et al., 2000). The tissue culture induces variation
in regenerated plants, called somaclonal variation (Larkin and Scowcroft,
1981), can result in a range of genetically stable variations, useful in crop
improvement. In vitro selection can considerably shorten the time for the
selection of desirable traits under selection pressure with minimal envi-
ronmental interaction, and can complement field selection (Jain, 2001).
Despite many advantages, development of stress tolerant plants through in
vitro selection has some limitations like loss of regeneration ability during
selection, lack of correlation between the mechanisms of tolerance oper-
ating in cultured cell, tissue or organ and those of the whole plants, and
phenomenon of epigenetic adaptation (Tal, 1994).
