Agriculture Reference
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plant root exudation could be engineered precisely to stimulate specific microbial
colonization on the roots. It has also been observed that genetically engineered
plants producing opine, for example, have an altered rhizosphere community
compared to their wild counterparts. Furthermore, due to several chemical factors
in the rhizosphere of different plants, roots are colonized by microbes out of
indefinite pool of soil microbial diversity.
Another important factor that may affect the rhizospheric microbiota has been
recognized as the cultivation practices in different production systems. Agriculture
management strategies can induce clear shifts in the structures of plant-associated
microbial communities. For example, plant genotypes can exert strong effects on
the bacterial communities associated with the plants. Growth stage of plant is
another important factor that alters the rhizobacterial community structure, and as
reported in case of potato rhizosphere, it has been identified as one of strongest
factors affecting the bacterial communities (van Overbeek and van Elsas 2008 ).
Besides, land use, soil history, and cultivation practices are some of the other
factors which govern the structure of plant-associated microbial communities
(Sharma et al. 2013 ).
7.3 Mechanism of P Solubilization
Phosphorus-solubilizing activity is determined by the ability of microbes to release
metabolites such as organic acids, which through their hydroxyl and carboxyl
groups chelate the cation bound to phosphate, the latter being converted to soluble
forms (Sagoe et al. 1998 ). General sketch of P solubilization in soil is shown in
Fig. 7.1 . A wide range of microbial P solubilization mechanisms exist in nature, and
much of the global cycling of insoluble organic and inorganic soil phosphates is
attributed to bacteria and fungi (Banik and Dey 1982 ).
Phosphorus solubilization is carried out by a large number of bacteria and fungi
acting on sparingly soluble soil phosphates, mainly by chelation-mediated mecha-
nisms (Whitelaw 2000 ). Inorganic P is solubilized by the action of organic and
inorganic acids secreted by PSB in which hydroxyl and carboxyl groups of acids
chelate cations (Al, Fe, and Ca) and decrease the pH in basic soils (Kpomblekou
and Tabatabai 1994 ; Stevenson 2005 ; Jha et al. 2013 ). The PSB dissolve the soil P
through production of low molecular weight organic acids mainly gluconic and
keto gluconic acids (Goldstein 1995 ; Deubel et al. 2000 ), in addition to lowering the
pH of rhizosphere. The pH of rhizosphere is lowered by proton/bicarbonate release
(anion/cation balance) and gaseous (O 2 /CO 2 ) exchanges. Release of root exudates
such as organic ligands can also alter the concentration of P in soil solution
(Hinsinger 2001 ). Inorganic acids like hydrochloric acid (HCl) can also solubilize
phosphate, but they are less effective compared to organic acids at the same pH
(Kim et al. 1997 ). In certain cases phosphate solubilization is induced by phosphate
starvation (Gyaneshwar et al. 1999 ).
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