Agriculture Reference
In-Depth Information
than microbiologists in recent times because they can reduce/minimize the depen-
dence on synthetic P fertilizers and, hence, can protect soil from chemical toxicity.
During the last couple of decades, there has been some practical progress in this
direction where some new and functionally exciting/novel PS microbes have been
identified and used for enhancing agriculture productivity in a more sustainable
manner (Khan et al.
2007
,
2010
).
1.3 Rhizosphere and PSM Colonization
Heterogeneously distributed microbial communities play an important role in the
acquisition and transfer of various nutrients in soil. For P, soil microorganisms are
involved in a range of processes that affect P transformation and thus influence the
subsequent availability of P (as phosphate) to plant roots. The rhizosphere indeed is
the narrow region of soil that is directly influenced by root secretions (Sørensen
1997
) and associated soil microorganisms (Fig.
1.1
) and plays some critical roles in
plant growth and consequently in soil fertility (Avis et al.
2008
). According to
Bringhurst et al. (
2001
), the rhizosphere includes the region of soil bound by plant
roots, often extending a few mm from the root surface. This region of soil is much
richer in bacteria than the surrounding bulk soil (Hiltner
1904
). In soil, microbes are
often limited by energy, and hence, root exudates such as organic acids, sugars and
amino acids provide energy to them and stimulate their growth and metabolic
activities which in turn influence biogeochemical cycling of nutrients in soils
(Cardoso and Freitas
1992
; Stevenson and Cole
1999
; Fontaine and Barot
2005
).
Studies based on molecular techniques have estimated more than 4,000 microbial
species per gram of soil (Montesinos
2003
). Of these, about 10
7
-10
9
colony-
forming units of culturable bacteria have been found in per gram of rhizosphere
soil (Benizri et al.
2001
), whereas the population densities in the rhizoplane have
been reported to range from 10
5
to 10
7
colony-forming units per gram of fresh
weight (Benizri et al.
2001
; Bais et al.
2006
). Furthermore, the microbial
populations first colonize the rhizosphere following soil inoculation (Gamalero
et al.
2003
) as shown by many techniques like microscopic tools, immuno-markers
or by fluorescence in situ hybridization (FISH) and by using gnotobiotic conditions.
Following colonization, bacterial cells are visualized as single cells attached to the
root surfaces and subsequently as doublets on the rhizodermis, forming a string of
bacteria (Hanson et al.
2000
). From here onwards, the whole surface of some
rhizodermal cells are colonized, and bacteria can establish even as microcolonies
or biofilms (Benizri et al.
2001
). In a similar manner, rhizoplane colonization has
been studied using both in vitro-grown plants and plants grown in natural soil
inhabiting a high microbial diversity. In order to provide benefits to plants, such
microorganisms (inoculated one/natural inhabitants of soils) thus must be rhizo-
sphere and/or rhizoplane competent (Elliot and Lynch
1984
; Compant et al.
2005
)
for an extended period of times (Whipps
2001
). Many factors can be involved in
rhizosphere and rhizoplane competence by PGPB (Albareda et al.
2006
). However,
