Agriculture Reference
In-Depth Information
1.1
Introduction
The major and most essential macronutrient, phosphorus (P), is required by the
plants for vital functions such as cell division, energy transfer, signal transduction,
macromolecular formation, nucleic acid synthesis, photosynthesis and respiration,
nitrogen fixation and production of oil, sugars and starches (Saber et al. 2005 ; Zaidi
et al. 2009 ; Eftekhari et al. 2010 ; Elser 2012 ). Consequently, acquisition of suffi-
cient concentration of P enhances the growth and development of plants in different
production systems (Hayat et al. 2010 ; Ahemad et al. 2009 ; Vikram and
Hamzehzarghani 2008 ). However, of the total soil P pool (0.5 %), only 0.1 % is
plant available (Scheffer and Schachtschabel 1988 ) and the remaining soil P is
inaccessible to plants (Rodr ´ guez and Fraga 1999 ). Therefore, the deficiency of P
impedes the growth and yields of plants heavily. Such P scarcity in agronomic
practices is, however, corrected through the application of synthetic phosphatic
fertilizers which indeed is expensive and hazardous. Moreover, greater portion of P
applied exogenously to soils is rapidly fixed into soil constituents (Norrish and
Rosser 1983 ; Borling et al. 2001 ; Hao et al. 2002 ) and, hence, becomes unavailable
to plants. Even though the organic P constitutes a large fraction of P (as much as
50 % in soils), yet it is not directly used up as nutrient unless degraded by soil
enzymes. Considering the high cost of chemical phosphatic fertilizers and ability of
P to form a complex with soil constituents, it has become imperative to find an
inexpensive and viable alternative to chemical P fertilizers. In this regard, the
bio-preparation containing viable and sufficient number of efficient phosphate-
solubilizing microorganisms (PSM) quite often called as “microphos” has provided
some solution to the P problems (Ahemad and Khan 2010 ; Hui et al. 2011 ; Xiang
et al. 2011 ; Khan et al. 2013 ). When applied to seed, plant surfaces or soil, PSM
colonize the rhizosphere or the interior of the plant (endophytes) and facilitate
growth by providing P to growing plants (Khan et al. 2006 ). Several PSM
inhabiting the soils (Behbahani 2010 ; Ahemad and Khan 2011a ; Marra
et al. 2011 ; Sanjotha et al. 2011 ; Yadav et al. 2011 ; Abd El-Fattah et al. 2013 ;
Saxena and Sharma 2007 ) include bacteria (Khan et al. 2010 ; Yasmin and Bano
2011 ; Oves et al. 2013 ), fungi (Khan et al. 2010 ) and actinomycetes (Franco-Correa
et al. 2010 ; Kaviyarasi et al. 2011 ; Balakrishna et al. 2012 ; Hamdali et al. 2012 ).
Several authors attribute the solubilization of inorganic insoluble P by PSM to the
production of organic acids and chelating oxo acids from sugars (Gulati et al. 2009 ;
Khan et al. 2010 ). Mechanistically, when applied to seeds and soils, PSM facilitates
plant development by (i) supplying hugely important nutrients to plants (Sashidhar
and Podile 2010 ); (ii) releasing phytohormones, for example, IAA (Naz et al. 2009 ;
Kavamura et al. 2013 ), gibberellins (Dey et al. 2004 ; Cassan et al. 2009 ) and
cytokinin and ABA (Zahir et al. 2004 ; Cassan et al. 2013 ); (iii) alleviating the
stress induced by ethylene on plants by synthesizing 1-aminocyclopropane-1-
carboxylate (ACC) deaminase to reduce ethylene level (Ahmad et al. 2012 );
(iv) producing siderophores for iron sequestration (Roca et al. 2013 ) and cyano-
genic compounds (Ghyselinck et al. 2013 ); (v) releasing antimicrobial compounds
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