Integrated Environmental Biotechnology - Environmental Biotechnology: Theory and Application - page 259

Endomycorrhizae
It is not only bacteria which may live within plants, but some types of spe-
cialised fungi may also occupy such a niche. Their presence is fairly widespread
and may be found in various grasses and a wide range of commercial crop plants
including tomatoes, apples, beans, wheat and corn. One type of plant-fungus
association, actinorhizae, has already been mentioned in Section 10.4.4. The fun-
gal hyphae penetrate the plant cells where a variety of structures may develop
such as swellings or the development of coils or small branches. Vesicles and
arbuscules which are branched structures reminiscent of a tree, are common fea-
tures of this invasion. Despite how this description may appear, such association
of plant with fungus may be very beneficial to the plant. In exchange for energy
derived from the plant through photosynthesis, the fungus may enhance the sup-
ply of available nutrients to the plant under conditions of relatively high humidity
and conversely, in dry conditions, the fungus may help the plant in the uptake of
water. In addition, some fungi have been found to afford 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
disease, fungi being one of the major causes of plant disease. Most microorgan-
isms 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 compli-
cated, 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 is Agrobacterium tumefaciens and the
virus is Cauliflower Mosaic Virus (CaMV).
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 stimulate plant growth
thus produces the characteristic tumour. It is interesting to note this example of