Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

Table 5.2 Diversity of diazotrophs.
Group of bacteria Example Nature
Cyanobacteria Anabaena Free living
Nostoc Free living
Actinobacteria Frankia Symbiotic
5.2 Rhizosphere and Bacterial Diversityj85
Gram-positive bacteria Bacillus Facultative microaerophilic
Paenibacillus Facultative microaerophilic
Clostridium Anaerobic
Proteobacteria a Acetobacter Associative nitrogen fixer (endophytic)
Azospirillum Microaerophilic, asymbiotic
Beijerinkia Asymbiotic
Bradyrhizobium Symbiotic
Rhizobium Symbiotic
b Azocarus Aerobic/microaerophilic
Burkholderia Associative nitrogen fixer (endophytic)
Herbaspirillum Associative nitrogen fixer (endophytic)
g Azotobacter Asymbiotic, free living
host primary production and root exudation and edaphic physicochemical parameters
[33], have also been intensively studied, as have the diazotrophic organisms
themselves. Various diazotrophic bacteria are listed in Table 5.2.
Due to the physiological diversity of diazotrophs and the documented unculturability
of many prokaryotes [34,35], cultivation-based strategies have severe limitations
for the description of the diversity of free-living soil diazotrophs. Therefore,
molecular approaches have been developed as discussed above. These molecular
approaches to study the diversity of diazotrophic organisms are primarily based on
PCR amplification of a marker gene (nifH) for nitrogen fixation.
5.2.1.1 Symbiotic Diazotrophic Bacteria
Two groups of nitrogen-fixing bacteria, that is rhizobia and Frankia, have been
studied extensively. Frankia forms root nodules on species of Alnus and Casuarina.
Symbiotic nitrogen-fixing rhizobia are now classified into 36 species distributed
among seven genera (Allorhizobium, Azorhizobium, Bradyrhizobium, Mesorhizobium,
Methylobacterium, Rhizobium and Sinorhizobium) [36]. Legume–rhizobia symbiosis
and nitrogen fixation are not the focal points of this chapter, however. Rhizobia have
been widely studied and their contribution to sustainable crop production is well
acknowledged. Other dimensions of rhizobial research include their application in
rice plants, as discussed in Chapter 11. However, recent trends also indicate that
Rhizobium as free-living rhizospheric bacteria can promote plant growth even in
nonlegume (maize, lettuce and pine) crops by their PGP activities. It can also produce
indole acetic acid (IAA) and siderophores and can solubilize phosphate [37,38].