Biomedical Engineering Reference
In-Depth Information
not function very well, if at all, in the plant cell. For these reasons, expression
may benefit from modification of the DNA sequence to compensate for these
differences while maintaining the information and instructions. This may account
in part for the very high levels of expression and stability of the Bt proteins whose
genes have been introduced, by (biolistic) microbombardment, into chloroplasts
(De Cosa et al ., 2001) which, because of their prokaryotic ancestry, have 'protein
synthesising machinery' more in keeping with prokaryotes than the eukaryotic
cell in which they co-habit.
Attempts to improve virus resistance have lead to the introduction, by A. tume-
faciens , of the genes expressing antibodies to the coat protein of Tobacco Mosaic
Virus (TMV). Expression of these in the plant lead to complete immunity against
TMV (Bajrovic et 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 as Erwinia 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 early research into the validity
of the hypothesis (Fray et al ., 1999) has ultimately led to the development and
successful patenting of a means to control bacterial infection (Zhang et al ., 2007).
Improved tolerance
Plant - microbe interactions are addressed in Chapter 10. Among the examples
given are that of Pseudomonas syringae which colonises the surface of leaves.
This example is of bacterial rather than plant modification but impinges on
interaction between the two. Pseudomonas syringae produces a protein which
promotes the formation of ice crystals just below 0 C thus increasing the risk
of frost damage. Lindow et al . (1989) have identified and isolated the gene for
this protein. They transferred it to the bacterium Escherichia coli to simplify
Search WWH ::




Custom Search