Agriculture Reference
In-Depth Information
et al.
2006
). Number of PSB among total PSM in north Iranian soil was found as
88 % (Fallah
2006
). Microorganisms involved in P acquisition also include mycor-
rhizal fungi (Fankem et al.
2006
). Among soil bacterial communities,
ectorhizospheric strains from
Pseudomonas
and
Bacilli
and endosymbiotic
Rhizobia
have been found as notable P solubilizers (Igual et al.
2001
). Additionally,
Enterobacter
sp.,
Bacillus megaterium
,
B. circulans
,
B. subtilis
,
B. polymyxa
,
B. sircalmous
,
Pseudomonas striata
(Subbarao
1988
; Kucey et al.
1989
),
Burkholderia
sp.,
Serratia marcescens
,
Klebsiella terrigena
, and
Aeromonas vaga
(Jha et al.
2013
) have also demonstrated the ability to solubilize phosphate rocks
(RP). Among fungi,
Penicillium
and
Aspergillus
are the most powerful P
solubilizers (Whitelaw
2000
).
High proportions of ubiquitously found PSM are concentrated generally in the
rhizosphere and are metabolically more active than those found in other habitat
(Vazquez et al.
2000
; Anamika et al.
2007
). Usually, one gram of fertile soil
contains 10
1
to 10
10
bacteria, and their live weight may exceed 2,000 kg ha
1
.
The shape of soil bacteria varies from cocci (sphere, 0.5
m) to bacilli (rod, 0.5-
μ
0.3
m). However, bacilli are the most dominant and common
form in soil whereas spirilli are very rare in natural environments (Baudoin
et al.
2002
). Population of PSB depends on different soil properties (physical and
chemical properties, organic matter, and P content) and cultural activities (Kim
et al.
1998a
,
b
). Larger populations of PSB are found in agricultural and rangeland
soils (Yahya and Azawi
1998
). For instance, in northern part of Iran, the PSB counts
ranged from 0 to 10
7
cells g
1
soil, with 3.98 % population of PSB among total
bacteria (Fallah
2006
). Further, zone/site of origin determines the capacity of
bacterial isolates to solubilize P. Among various sources, rhizoplanes harbor the
bacteria with highest capacity, rhizosphere organisms with intermediate capacity,
while those from bulk soils with the least PS activity. The survival of P solubilizers
is determined by “chemosequence” and “climosequence” of soils. Inadequate
information is, however, available about the diversity of bacterial populations in
and around the rhizosphere, possibly due to lack of appropriate techniques required
to isolate and accurately identify specific PS strains belonging to the same species.
Such limitations hinder the process to explore community dynamics, which leads to
the poor understanding of variations in microbial community dynamics in response
to soil type, plant type, or stage of plant development (McSpadden Gardener and
Driks
2004
). In fact, bacterial communities residing in the rhizosphere respond, in
particular, with respect to density, composition, and activity, to the plethora and
diversity of organic root exudates, resulting in plant species-specific microflora
which may eventually vary with the stage of plant growth. The role of plant (largely
exudates) in affecting the ability of bacteria to colonize the rhizosphere (Kumar
et al.
2011
) has been considered as one of the major factors. Among PSB, fluores-
cent pseudomonads that colonize aggressively the plant roots have been considered
as an important group of bacteria due to their biofertilizing and biocontrol proper-
ties (Naik et al.
2008
; Parikh and Jha
2012
; Jha et al.
2012
). These strains were
taxonomically described as different fluorescent pseudomonad species such as
P. monteilli
,
P. putida
,
P. plecoglossicida
,
P. fluorescens
,
P.
m) to spiral (1-100
μ
μ
fulva
, and
