Integrated Environmental Biotechnology - Environmental Biotechnology: Theory and Application - page 256

Microbes internal to the plant
Two categories fulfil this description. The first are the internal fungi or endomy-
corrhizae, referred to in the earlier section, together with the endophytic bacteria
and the second comprises plant pathogens, which may be bacterial or viral in
form. Although the term 'endophytic' seems unambiguous, it is sometimes used
to describe only bacteria which may be isolated from plants which have been
superficially cleansed with disinfectant, or isolated from within plant tissue and
which cause no discernible harm to the plant. Thus defined, plant pathogens are
excluded from this description. There are endophytic bacteria, called commen-
sals, which neither benefit nor harm the plant, but there are also those which are
beneficial to plant growth. These are symbiots which achieve this status either
by promoting plant growth or by protection against plant pathogens.
Symbiotic nitrogen fixation
The classic example of plant growth stimulation by plant/microbe symbiosis is
nitrogen fixation by Rhizobium bacteria within plant root tissue. A full expo-
sition of nitrogen fixation may be found in many classical textbooks and so
will be described here only in outline. The process of root nodule formation is
shown diagrammatically in Figure 10.5. In total, there are a limited number of
organisms able to fix nitrogen, all of which are prokaryotes. All living organisms
are dependent ultimately on such organisms, due to a universal and essential
requirement for nitrogen, normally in the form of nitrate, ammonia or ammo-
nium ion or as amino acids from which the amino group may be transferred as
required. Nitrogen is fixed by reduction to ammonia either by free living organ-
isms or by plant symbiots. In both cases, it is essential to have an oxygen free
environment as the enzymes involved in the process are irreversibly inactivated
by the presence of oxygen. Of the free living organisms able to fix nitrogen,
some achieve this naturally while others have to create such an environment.
Clostridium and Klebsiella achieve this since they are both anaerobes and so
are already adapted to life in an oxygen free atmosphere, while Cyanobacteria
and Azobacter have developed means of creating one for themselves. Azobac-
ter does this by having a very high oxygen consumption rate thus effectively
creating an oxygen free environment. Other nitrogen fixing bacteria include the
filamentous bacteria such as the Corynebacterium species, and photosynthetic
bacteria referred to elsewhere, such as Rhodospirillum . The latter makes use of
photosynthesis to provide the energy for these reactions and so is presented with
the problem of removing the oxygen produced during photosynthesis away from
nitrogen fixation reactions. Although these free living organisms have a vital
role to play in their particular niches, approximately 10 times more nitrogen is
fixed by plant symbiots. Presumably this is because the plant is better able to
provide the necessary levels of ATP to meet the high energy demands of the
process than are free living bacteria. In addition, the plant supplies the endo-
phytes with dicarboxylic acids, such as malate and succinate and other nutrients,