Agriculture Reference
In-Depth Information
et al.
2005
). Soil microorganisms participate in processes like decomposition,
mineralization, and nutrient availability, improve soil structure (soil aggregation
by production of polysaccharides), increase the nutrient acquisition efficiency of
the plants, and improve plant health through growth hormone production (Hayat
et al.
2010
; Singh et al.
2011b
). Microbial populations having the ability to colonize
root surface and imparting beneficial effects to plants are known as plant growth-
promoting rhizobacteria (PGPR) (Kloepper and Schroth
1978
; Joshi and Bhatt
2011
). Plant growth-promoting rhizobacteria facilitate plant growth both directly
and indirectly (Glick
2012
). Some of the notable PGPR belong to genera
Arthrobacter
(Banerjee et al.
2010
),
Azotobacter
(Ponmurugan et al.
2012
),
Azospirillum
(Jacoud et al.
1999
),
Bacillus
(Kumar et al.
2011
),
Enterobacter
(Shoebitz et al.
2009
),
Pseudomonas
(Noori and Saud
2012
), and
Serratia
(Zhang
et al.
1997
). Based on the proximity with the plant roots, PGPR can be divided into
(1) extracellular PGPR, existing in rhizosphere, rhizoplane, or spaces between root
cortices, and (2) intracellular PGPR, present within roots or nodules of the plant.
Also, based on the mode of action, PGPR have been classified as (1) bio-stimulants
which promote plant growth via phytohormone production, including auxins IAA
and similar compounds like abscisic acid, gibberellic acid, cytokinins (Carmen and
Roberto
2011
); (2) biofertilizers which enable nutrient availability and acquisition
via N
2
fixation (Mohammadi and Sohrabi
2012
) and P solubilization (Khan
et al.
2007
; Zaidi et al.
2009
; Khan et al.
2010
; Das et al.
2013
); and
(3) bioprotectants which provide protection to plants against phytopathogens via
production of antibiotics (Labuschagne et al.
2011
), siderophores (Glick
2012
), and
induced systemic resistance (Figueiredo et al.
2011
).
11.3.1 Direct Mechanisms
11.3.1.1 Production of Plant Growth Regulators
Microorganisms are known to produce plant growth-stimulating substances such as
phytohormones, for example, auxins (Spaepen and Vanderleyden
2011
), cytokinins
(Nieto and Frankenberger
1990
), gibberellins and abscisic acid (Singh
2013
), etc.,
as well as certain volatiles (Ryu et al.
2003
). The phytohormone-producing micro-
organisms include
Acetobacter diazotrophicus
(Patil et al.
2011
),
Azospirillum
brasilense
(Perrig et al.
2007
),
Herbaspirillum seropedicae
(Bastian et al.
1998
),
Bacillus pumilus
and
B. licheniformis
(Gutirrez-Manero et al.
2001
), etc.
11.3.1.2 Nitrogen Uptake
Specialized microorganisms have capability to fix atmospheric N (biological nitro-
gen fixation; BNF) and maintain the balance of N in soil ecosystem. Nitrogen fixers
are categorized into two groups: (a) symbiotic nitrogen fixers and (b) nonsymbiotic
