Agriculture Reference
In-Depth Information
nitrogen fixers.
Rhizobium
and
Frankia
belong to symbiotic N
2
fixers that associate
with legumes, whereas nonsymbionts are free-living N
2
fixers which interacts with
nonleguminous plants (Ahemed and Kibret
2014
). Numerous PGPR are also known
to possess this attribute although the mechanism responsible for their growth
promotion is not N
2
fixation. Some of these PGPR are
Azotobacter
(Kizilkaya
2009
),
Bacillus
(Ding et al.
2005
),
Clostridium
,
Klebsiella
(Iniguez et al.
2004
),
Alcaligenes
,
and Arthrobacter
(Mohammadi and Sohrabi
2012
).
11.3.1.3
Increased Mineral Uptake
Plant growth-promoting rhizobacteria are reported to provide nutrients to plants via
mineralization/solubilization of unavailable minerals like P (Khan et al.
2007
).
Also, the siderophores, secreted by PGPR strains, play important roles in mineral
transport (Vessey
2003
; Ahmad et al.
2013
). Mineralization process involves
conversion of organic P into soluble forms through enzymes like phytases and
phosphatases (Walpola and Yoon
2012
), whereas in solubilization the inorganic P
is transformed into soluble forms via organic acid production, acidification of
medium (Park et al.
2009
; Khan et al.
2010
), chelation, and exchange reactions
(Walpola and Yoon
2012
). Both solubilization and mineralization mechanism can
occur in one bacterial species also. Some of the phosphate solubilizing (PS) bacteria
include
Acinetobacter
(Rokhbakhsh-Zamin et al.
2011
),
Burkholderia
(Gupta
et al.
2012
),
Enterobacter
(Gupta et al.
2012
; Maheshwari and Sudha
2013
),
Klebsiella
(Ahemad and Khan
2011
),
Pseudomonas
(Rajkumar and Freitas
2008
),
and
Stenotrophomonas
(Mehnaz et al.
2010
)
.
Numerous studies have been
conducted globally to analyze the effects of various P solubilizers on growth,
yield, and other important parameters of plants (Khan et al.
2009
; Ahmad
et al.
2012a
). Some of the examples supporting the effectiveness of these micro-
organisms against different crops are listed in Table
11.1
.
11.3.2
Indirect Mechanisms
11.3.2.1 Antibiotic Production
Antibiotics are defined as heterogenous low molecular weight organic compounds
secreted by microorganism, having destructive/inhibitory effects on the growth and
metabolism of other microorganism/s (Duffy
2003
; Beneduzi et al.
2012
). PGPR
are also known to produce antibiotics and other small molecules preventing plants
from damage caused by the plant pathogens. These antibiotics are categorized as
(A) nonvolatiles including polyketides (e.g., pyoluteorin), heterocyclic nitrogenous
compounds such as phenazine derivatives, phenylpyrrole (e.g., pyrrolnitrin),
lipopeptides (e.g., bacillomycin), aminopolyols (e.g., zwittermicin A) and
(B) volatile antibiotics such as hydrogen cyanide (HCN), aldehydes, sulfide,
