228 Environmental Biotechnology
(De Cosaet al. 2001) which, because of their prokaryotic ancestry, have ‘protein
synthesising machinery’ more in keeping with prokaryotes than the eukaryotic
cell in which they cohabit.
Attempts to improve virus resistance have led to the introduction, by
A. tumefaciens, of the genes expressing antibodies to the coat protein of Tobacco
Mosaic Virus (TMV). Expression of these in the plant led to complete immunity
against TMV (Bajrovicet al. 2001).
Improved resistance to disease
Bacteria communicate with each other by way of small diffusible molecules
such as the N-acylhomoserine lactones (AHLs) of Gram negative organisms. In
this way, described as ‘quorum sensing’, they are able to detect when a critical
minimum number of organisms is present, before reacting. These responses are
diverse and include the exchange of plasmids and production of antibiotics and
other biologically active molecules. Plants are susceptible to bacterial pathogens
such asErwinia carotovora, which produces enzymes capable of degrading its
cell walls. The synthesis of these enzymes is under the control of AHLs and
so they are made only once the appropriate threshold level of this chemical has
been reached. The rationale behind using AHLs for plant protection is to make
transgenic plants, tobacco in this case, which express this signal themselves. The
consequent high level of AHL presented to the pathogenic bacteria, wrongly
indicates a very high number of similar organisms in the vicinity, and triggers
the bacteria into responding. As a consequence, they produce enzymes able to
degrade the plant cell walls and continue infection. The plant will mount its
normal response to invasion but on a far greater scale than necessary to destroy
the few bacteria actually causing the infection, thus improving the plant’s resis-
tance to the disease. It seems complicated, but research into the validity of the
hypothesis is under way (Frayet al. 1999).
Improved tolerance
Plant–microbe interactions are addressed in Chapter 10. Among the examples
given are that ofPseudomonas syringaewhich colonises the surface of leaves.
This example is of bacterial rather than plant modification but impinges on
interaction between the two.Pseudomonas syringaeproduces a protein which
promotes the formation of ice crystals just below 0◦C thus increasing the risk
of frost damage. Lindowet al. (1989) have identified and isolated the gene for
this protein. They transferred it to the bacteriumEschericia colito simplify the
genetic manipulations. This required the deletion of sufficient regions so that a
truncated, and therefore nonfunctional, ice mediating protein was expressed. They
reintroduced this mutated gene intoPseudomonas syringaeand selected forice−
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