Integrated Environmental Biotechnology 2572002). The microbes in the rhizosphere are, to a large extent, dependent on the
plants for a supply of many useable organic substances. As plants die and decay,
the components released by the degradative bacteria return to the soil and the
cycle begins again. Consequently, plants affect the composition of the microbial
community of the ground in which they grow, especially in soils of low fertility.
Not all the compounds released from plants are stimulants of microbial growth,
some plants may also produce inhibitors. The microbes themselves have an effect
on the plant growth characteristics. Some release into the soil, gibberellins and
cytokinins, both of which are plant growth factors, and may also affect the flow
of organic compounds, termed exudate, from the plant into the rhizosphere. The
rate at which exudate is transferred to the soil is affected by many parameters; the
presence of surrounding soil bacteria as mentioned above, the reduction of plant
mass by harvesting from above ground level, and environmental changes, for
example, variations in light or temperature. Both bacteria and fungi contribute to
the microbial population in the rhizosphere. Associations of fungi with roots of
vascular plants, called mycorrhizae, are quite common and may in some cases be
very beneficial to the plant. They may be external, ectomycorrhizal, or internal,
endomycorrhizal. Ectomycorrhizal associations more commonly occur in temper-
ate regions and often in beech, oak, birch and coniferous trees. Their association
involves a limited penetration of the root cortex by the fungi growing as a cov-
ering around the tip of the root. They aid the growth of the plants as a result of
their mycelia reaching far out into the surrounding soil, thus assisting the plant
in nutrient uptake. This quality has received commercial attention. The effect on
plant growth and subsequent predation by insect larvae, of some species of fun-
gus for example,Pisolithus tinctorius, has received particular attention (Rieske
2001). Bacteria have been found capable of encouraging this association, earn-
ing themselves the title of ‘micorrhizal helper bacteria’. Clearly, anything which
increases the efficiency of nutrient uptake by crop plants reduces the require-
ment for the addition of artificial fertilisers and thus reduces the potential for
agrichemical environmental disturbance.
The influence of microbes on the welfare of plants is not confined to the
ground and may even affect the weather. An often quoted example is that drawn
fromPseudomonas syringaewhich produces a protein known to act as a point of
nucleation of ice crystals. Plants which harbour this bacterium run an increased
risk of frost damage especially if their tissue is particularly susceptible as is the
case with strawberries.P. syringaehas been subjected to genetic engineering
which successfully reduced the problem. A description of the project is given in
Chapter 9.
Plant–microbe interactions are becoming recognised as having an immediate
and direct importance to human health in the role they can play in reducing
the effect of ‘sick buildings’. They occur principally because these buildings are
closed systems in which people work, breathing in volatile components from
plastics, paint, chemicals used in office machinery such as photocopiers and