Biomedical Engineering Reference
In-Depth Information
the genetic manipulations. This required the deletion of sufficient regions so that
a truncated, and therefore non-functional, ice mediating protein was expressed.
They reintroduced this mutated gene into Pseudomonas syringae and selected for
ice - mutants which were no longer able to produce the ice nucleating protein.
Many such regimes fail in practice because it is difficult to maintain a population
of mutant bacteria in a community dominated by the wild type as frequently, the
latter will soon out number the mutant by competition for nutrients, since it is
usually better adapted to the particular environment than the mutant. However,
in this case, due to massive application of Pseudomonas syringae ice - to straw-
berry plants, the mutants were able to compete with the wild type and protect
this particularly susceptible crop against frost damage.
Salt tolerance in tomatoes has been established by introducing genes involved
in Na + /H + antiport, the transport of sodium and hydrogen ions in opposite direc-
tions across a membrane. The quality of the fruit was maintained by virtue of
the fact that the sodium accumulation caused by the antiport occurred in leaves
only and not in the fruit (Zhang and Blumwald, 2001).
Improved tolerance to drought, salt and freezing in Arabidopsis has been
achieved by overexpressing a protein which induces the stress response genes.
However, if too much of this factor is produced, which was the case when the
35S CaMV was employed, severe growth retardation was observed. No such
problem existed when instead the overexpression was under the control of a pro-
moter which was only switched on when stressful conditions existed (Kasuga
et al ., 1999).
Improved plants for phytoremediation
Chapter 7 mentioned the genetic modification of a poplar to enable mercury to be
removed from the soil and converted to a form able to be released to the atmo-
sphere. This process is termed 'phytovolatilisation' (Rugh et al ., 1998). The
modification required a gene to be constructed, styled on the bacterial mer A gene,
by making a copy reflecting the codon bias found in plants using PCR technique.
The mer A gene is one of a cluster of genes involved in bacterial detoxification
of mercury, and is the one coding for the enzyme, mercuric ion reductase ,which
converts mercury from an ionic to a volatile form. Initially the constructed mer A
gene was expressed in Arabidopsis thalia (rape) where resistance to mercury was
observed, and in this study, the gene was transferred by microprojectile bombard-
ment ('gene guns') to poplar tree ( Liriodendron tulipifera ) embryogenic material.
When the resulting yellow poplar plantlets were allowed to develop, they were
found to exhibit tolerance to mercury and to volatalise it at 10 times the rate
observed in untransformed yellow poplar plantlets. This study demonstrated the
possibility that trees can be modified to become useful tools in the detoxification
of soil contaminated with mercury. These studies were pursued in Arabidopsis
thalia where it was observed that successful remediation also required the mer
B genes coding for a lyase (Bizily, Rugh and Meagher, 2000).
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