262 Environmental Biotechnology
protection to the plant in cases where the fungi produce alkaloids rendering the
plant less susceptible to attack by chewing insects.
Plant pathogens
There are many bacteria, fungi and viruses which may infect a plant and cause dis-
ease, fungi being one of the major causes of plant disease. Most micro-organisms
will not be pathogens for a particular plant as the manifestation of disease requires
some quite specific reactions and responses between host and infectious agent.
Infection elicits numerous responses some of which may be quite complicated,
and so has been the centre of some fairly intensive research. The result of this
has been the identification and isolation of plant genes involved in resistance
to pathogens and pathogen virulence genes. Manipulation of these to reduce the
level of environmentally damaging chemicals to protect crop plants is an active
area of research and the manipulation of these genes is discussed in the previ-
ous chapter. This section is restricted to two examples, one bacterium and one
virus, which are chosen because of their relevance to plant genetic engineering
described in Chapter 9. The bacterium isAgrobacterium tumefaciensand the
virus is Cauliflower Mosaic Virus.
Agrobacterium tumefaciens
A feature of infection caused by this Gram positive organism, is a tumour-like
growth, seen as a crown gall in plants. This is the consequence of injection by
the bacterium of a small piece of DNA which carries on it genes which code for
opines which encourage further invasion of the plant by the bacterium, and for
plant growth hormones including auxin whose activity stimulates plant growth
thus producing the characteristic tumour. It is interesting to note this example of
gene transfer from prokaryote to eukaryote occurring with moderate frequency
in nature. The significance of this is explored further in Chapter 11. The genes
coding the information required for the insertion of this small piece of DNA
including the insertion sequence itself, are carried on a plasmid, described in
Chapter 2, called Ti plasmid whose structure is shown in Figure 9.4. The process
of infection is stimulated by exudate from a plant which has been wounded by
some means not necessarily the result of infection, for example, by frost damage.
Thevirgenes are activated leading to nicks being introduced at the borders of
the T-DNA leading to the release of one of the strands of the double-stranded
DNA. A copy is made of the remaining strand by the usual methods of DNA
synthesis and repair thus restoring the status quo of the Ti plasmid. The single-
stranded piece of T-DNA is free to be transferred into the plant cell through a
wound site and on into the plant cell nucleus. Here the complementary strand
is synthesised in the normal way using the plant cell enzymes and the resulting
double-stranded DNA integrates into the plant genome. The T-DNA comprises
three genes including those for plant hormones and opines, as mentioned above.