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

86j 5 Diversity and Potential of Nonsymbiotic Diazotrophic Bacteria in Promoting Plant Growth
5.2.1.2 Asymbiotic Diazotrophic Bacteria
Nonsymbiotic nitrogen fixation is known to be of great agronomic significance. The
main limitation to nonsymbiotic nitrogen fixation is the availability of carbon and
other energy sources for the energy-intensive nitrogen fixation process. This limitation
can be compensated by moving closer to or inside the plants, namely in
diazotrophs present in the rhizosphere or rhizoplane, or those growing endophytically.
Some important nonsymbiotic nitrogen-fixing bacteria include Achromobacter,
Acetobacter, Alcaligenes, Arthrobacter, Azospirillum, Azotobacter, Azomonas, Bacillus,
Beijerinckia, Clostridium, Corynebacterium, Derxia, Enterobacter, Herbaspirillum,
Klebsiella, Pseudomonas, Rhodospirillum, Rhodopseudomonas and Xanthobacter [39].
Various diazotrophic bacteria including species of Azospirillum, Azotobacter,
Bacillus, Beijerinckia and Clostridium have been commonly associated with higher
plants. The widespread distribution of dinitrogen-fixing associative symbiosis
has led to interest in determining their relative importance in agricultural
systems [40].
In natural ecosystems, biological nitrogen fixation (by free-living associative and
symbiotic diazotrophs) is the most important source of nitrogen. The estimated
contribution of free-living nitrogen-fixing prokaryotes to the nitrogen input of soil
ranges from 0 to 60 kg ha 1 year 1 . The contribution of asymbiotic and symbiotic
nitrogen fixation varies greatly but in some terrestrial ecosystems asymbiotic nitrogen
fixation may be the dominant source [32,41].
Several new nitrogen-fixing bacteria associated with grasses and cereals, including
sugarcane, have been described by many workers, namely Pseudomonas sp. [42],
Enterobacter, Klebsiella, Pseudomonas sp., Azospirillum [43], Campylobacter sp., Bacillus
azotofixans [44] and Herbaspirillum seropedicae [45].
Azotobacter The family Azotobacteraceae comprises two genera, namely Azomonas
(noncyst forming) with three species (A. agilis, A. insignis and A. macrocytogenes)
and Azotobacter (cyst forming) comprising six species [46], namely A. chroococcum,
A. vinelandii, A. beijerinckii, A. nigricans, A. armeniacus and A. paspali. Azotobacter is
generally regarded as a free-living aerobic nitrogen fixer. The genus Azotobacter
comprises large Gram-negative bacteria, obligatory aerobic rods to oval shape,
capable of fixing nitrogen nonsymbiotically. Phylogenetically, it is identified as
b-proteobacteria. Azotobacter can form resting structures called cysts that are
resistant to desiccation, mechanical disintegration and ultraviolet and ionizing
radiation [47].
Application of Azotobacter and Azospirillum has been reported to improve yields of
both annual and perennial grasses. Saikia and Bezbaruah [48] reported increased
seed germination of Cicer arietinum, Phaseolus mungo, Vigna catjung and Zea mays;
however, yield improvement is attributed more to the ability of Azotobacter to produce
plant growth promoting substances such as phytohormone IAA and siderophore
azotobactin, rather than to diazotrophic activity.
Azospirillum Members of the genus Azospirillum fix nitrogen under microaerophilic
conditions and are frequently associated with the roots and rhizospheres of a