Genetic Manipulation 229mutant bacteria in a community dominated by the wild type as, frequently, the
latter will soon outnumber 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 ofPseudomonas 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 inArabidopsis 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
promoter 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
atmosphere. This process is termed ‘phytovolatilisation’ (Rughet al. 1998). The
modification required a gene to be constructed, styled on the bacterialmer Agene,
by making a copy reflecting the codon bias found in plants using PCR technique.
Themer Agene 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 constructedmer
Agene was expressed inArabidopsis thalia(rape) where resistance to mercury
was observed, and in this study, the gene was transferred by microprojectile
bombardment (‘gene guns’) to poplar tree (Liriodendrn 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 inArabidopsis
thaliawhere it was observed that successful remediation also required themer
Bgenes coding for alyase(Bizily, Rugh and Meagher 2000).
A bacterial gene encodingpentaerythritol tetranitrate reductase, an enzyme
involved in the degradation of explosives, has been transferred into tobacco
plants. The transgenics have been shown to express the correct enzyme and trials
are under way to determine their ability to degrade TNT (Frenchet al. 1999).