Symbiotic Fungi: Principles and Practice (Soil Biology)

12 A. Das and A. Varma
1.3.3.9 Nitrogenase Production
Once the conditions are appropriate for nitrogen fixation, the bacteroids produce
nitrogenase enzyme, the enzyme that converts N 2 to NH 3. The enzyme nitrogenase
is reported to be present in all the nitrogenfixing organisms, viz. bacteroids,
heterocysts of cyanophyceae and vesicles of Frankia.
1.3.3.10 Senescence of Nodule and Release of Rhizobia
The ageing (senescence) process of the nodule is genetically programmed, and
leads to the release of the bacteroids and loss of nodules.
1.4 Symbiosis of Actinomycetes
Actinomycetes are gram-positive bacteria that can form branching filaments. They
may form true mycelia or produce conidiospores. Frankia is one of the representative
genera of Actinomycetes.
Characteristics of Frankia are as follows:
1. It is a type IIID cell wall type.
2. It forms nonmotile sporangiospores in sporogeneous body.
3. It grows in symbiotic association with the roots of at least eight families of
higher nonleguminous plants (e.g., alder trees).
4. It is a microaerophile, able to fix nitrogen.
The roots of the infected plants develop nodules that fix nitrogen so efficiently that a
plant such as an alder can grow in the absence of combined nitrogen when
nodulated. Within the nodule cells, Frankia forms branching hyphae with globular
vesicles at their ends. These vesicles may be the sites of nitrogen fixation. The
nitrogen fixation process resembles that of Rhizobium, in that it is oxygen-sensitive
and requires molybdenum and cobalt. Some plants (Alnus, Ceanothus) have
nodules as large as baseballs. The nodules of Casuarina approach soccer-ball size
(Prescott et al. 1996).
The actinomycetes infect the root system and forms nodules in shrubs and trees
belonging to eight dicot families except Leguminosae (Table 1.2). However, this is
not of much economic significance, as none of the crop plants is infected with
Frankia. The development of Frankia and the mechanism of nitrogen fixation is
less well-understood than legume–rhizobium symbiosis. Casurina, Eleagnus, and
Alnus are some of the important genera which form Frankia nodules. Such nodules
are long-lived when compared with rhizobium nodules of legumes. Frankia is a
free filamentous form which can also be cultured on nutrient media. The tip of the
filament is enlarged, forming the vesicle which contains the enzyme nitrogenase.