256 Environmental Biotechnology
virusin vitro, in the laboratory. This being the case, the coding sequence for the
polyhedrin protein may be replaced by a ‘foreign’ gene where under the control
of the polyhedrin promoter it may, depending on the gene being replaced, have
a good chance of being expressed at a very high level. This methodology used
to overexpress proteins, which was described in Chapter 9, has been used with
great success but increasingly in biotechnology fields other than environmental,
notably pharmaceuticals.
Plant/microbe interactions
The microbiology of soils and plant microbe interactions are enormous topics
worthy of the many books and research papers on the subjects, some of which
are listed in the bibliography. It is not the aim of this section to give a detailed
account but simply an introduction to the complexity of plant–microbe interac-
tions in the hope of illustrating that ‘no man is an island’: disturbance of these
interactions has its consequences. Although the term ‘plant’ includes all plant
forms from trees to algae, the current discussion addresses interactions between
higher plants and micro-organisms. Such interactions fall into two basic cate-
gories: the first category being those involving microbes external to the plant,
such as soil bacteria and soil fungi. The second being microbes internal to the
plant, which include endophytic bacteria such as those involved in nitrogen fix-
ation, internal fungi, and plant pathogens examples of which areAgrobacterium
plasmodiumandAgrobacterium tumefaciens(Greene and Zambryski 1993). The
latter is now used extensively to introduce ‘foreign’ genes into plants as described
in Chapter 9. The associations may therefore involve bacteria, fungi or viruses
and in some cases, some quite complex interactions involving three or four dif-
ferent organisms often bringing great benefit to the plant in environments where
nutrients are somewhat deficient.
Microbes external to the plant
There are clearly two distinct areas of a higher plant which are inhabited by
different communities of micro-organisms: above ground around and on the sur-
face of leaves, stems, seeds and flowers and below ground in zones of increasing
distance away from the root mass. These rhizospheres, or zones around the roots,
which are more accurately envisaged as a continuous gradient of nutrients, are
the result of plant metabolic activity constantly drawing from the surrounding
soil. Nutrients may also be transferred in the reverse direction that is, to the soil,
as is the case with aerating plants exemplified byPhragmitesused in reed bed
systems described in Chapter 7. It appears that colonisation of the rhizosphere by
bacteria is stimulated by exudate from the plant. The first phase is attraction to
the plant roots, the second is a ‘settlement’ phase during which bacteria grow to
form colonies and the third is a ‘residence’ phase when a balance is established
between root mass and bacterial numbers (Espinosa-Urgel, Kolter and Ramos