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

92j 5 Diversity and Potential of Nonsymbiotic Diazotrophic Bacteria in Promoting Plant Growth
It is now clear that associative diazotrophs exert their positive effects on plant
growth through different mechanisms. Apart from fixing nitrogen, diazotrophs can
affect plant growth directly by the synthesis of phytohormones [13,49] and vitamins
[47], inhibition of plant ethylene synthesis [3], improved nutrient uptake (microbial
cooperation in rhizosphere), enhanced stress resistance [50], solubilization of inorganic
phosphate and mineralization of organic phosphate [8]. However, diazotrophs
are able to decrease or prevent the deleterious effects of plant pathogens mostly
through the synthesis of antibiotic and fungicidal compounds [117,77], through
competition for nutrients (for instance, by siderophore production) or by the induction
of systematic resistance to pathogens [80]. Some of the well-known mechanisms
of PGP by diazotrophic bacteria are listed in Table 5.3.
Table 5.3 Mechanisms of plant growth promotion by diazotrophic bacteria.
Mechanisms Organisms Effect on plant growth References
Production of plant growth promoting substances
Auxins Azotobacter Increased root length, number of
Azospirillum lateral roots and number of roots.
Acetobacter Significantly increased seedling
diazotrophicus
Herbaspirillum
Bacillus
Paenibacillus
Burkholderia
Bradyrhizobium
Rhizobium
(root and shoot weight)
Gibberellins Azotobacter Increased shoot growth of dwarf
Azospirillum
Acetobacter
diazotrophicus
Herbaspirillum
Bacillus
Rhizobium
plants of maize and rice.
Increased shoot growth of
alder seedlings
Cytokinins Azotobacter Affect morphology of radish
Azospirillum
Paenibacillus
Rhizobium
and maize
Phosphate
solubilization
Bacillus Enhanced growth and yield but
Paenibacillus not phosphorus solubilization
of canola
Increased percent germination
and growth emergence of
wheat
Azotobacter Increased dry matter and yield
Rhizobium but not phosphorus uptake of
lettuce and barley
[78,103,118–126]
[120,126–133]
[127–129,134–138]
[37,103,109,121,123,139]