Integrated Environmental Biotechnology 261plant cell membrane. This structure then develops into nodules containing the
nitrogen-fixing bacteria. Several changes to the plant then ensue including the
synthesis of proteins associated with the nodule, the most abundant of which is
leghaemoglobin, which may reach levels of up to 30% of the total nodule protein.
The genes coding for this protein are partly bacterial and partly plant in origin
and so exemplify the close symbiosis between the two organisms. The expres-
sion of leghaemoglobin is essential for nitrogen fixation, since it is responsible
for the control of oxygen levels. The enzymes for fixation are coded for by a
plasmid ofRhizobiumand are referred to as thenifgene cluster. There are two
components each comprising a number of genes. One component isnitrogenase
reductasethe function of which is to assimilate the reducing power used by the
second component,nitrogenase, to reduce nitrogen to ammonium ion. Expression
of thenif genes is highly regulated in all organisms studied to date. In addition
to thenif and thenod genes,Rhizobiumalso carries additional genes which
are involved in the fixation of nitrogen called thefixgenes. Once ammonia, or
the ammonium ion, has been formed it may be transferred to the amino acid,
glutamate, to produce glutamine. This transfer is frequently invoked throughout
metabolism. This is not the only route for the assimilation of the amino group into
metabolic pathways; the synthesis of the purine derivative, allantoin or allantoic
acid, being a less heavily used alternative. These pathways are described in Chap-
ter 2. Nitrogen fixation involvingRhizobiumhas been the focus of the preceding
discussion, but the family of aerobic, Gram positive bacteria, Actinomycetes, is
another group carrying out the same function using a similar mechanism. These
may form a network of aerial or substrate hyphae resembling a structure com-
monly associated with that of fungi. These bacteria may form a close association,
called actinorhizae, with the roots of a number of plants which tend to be woody,
or shrub-like in nature.
Endomycorrhizae
It is not only bacteria which may live within plants, but some types of specialised
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,
actinorrhizae, has already been mentioned in the section describing symbiotic
nitrogen fixation. The fungal 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 features 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 supply 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