4.2.2 Role of Phosphorus Solubilizing Microorganisms
The term microorganisms broadly encompass bacteria and fungi including other
mini-creature only observed by microscope. Among the microorganisms, bacteria
and fungi are more versatile to facilitate phosphate solubilization.
4.2.2.1 Phosphorus Solubilizing Bacteria and Fungi
PSM consist predominantly the bacteria and fungi among ectorhizospheric strains,
Pseudomonas,Bacillus,and endosymbiotic rhizobia have been served as effective
phosphate solubilizers (Igual et al. 2001 ). The vast majority of fungi are
non-Phosphate solubilizers except for species of Aspergillus and Penicillium
(Sagervanshi et al. 2012 ; Sahoo and Gupta 2014 ). Villegas and Fortin ( 2002 )
identified microorganism viz., Rhizobium, Klebsiella, Mesorhizobium,
Acinetobacter, Erwinia, Achrobacter,Enterobacter,Micrococcus,Pseudomonas
andBacillusisolated from different soils as efficient P solubilizing strains. Majority
of Gram-positive soil bacilli almost 95% belong to the genusBacillus(Garbeva
et al. 2003 ) and are capable to form endospores and for this reason survive beneath
detrimental conditions; some species are diazotrophs along withBacillus subtilis
(Timmusk et al. 1999 ), while others have specific PGPR capacities (Kokalis-Burelle
et al. 2002 ; Barriuso and Solano 2008 ). From rhizobial strains, two species of
nodulating chickpea,Mesorhizobium mediterraneumandMesorhizobium ciceri, are
known for their high phosphate-solubilizing efficiency (Rivas et al. 2006 ). But, it is
recognized that each aspect of nodule formation is limited due to the supply of
P. legumes like alfalfa and clover displaying a positive effect in response to P
supplementation (Gyaneshwar et al. 2002 ), however most of the supplemented P
become unavailable when its reacts with soil components. The extracellular oxi-
dation of glucose to gluconic acid via the quinoprotein glucose dehydrogenase
results in efficient phosphate-solubilizing phenotype in Gram-negative microor-
ganism (Otieno et al. 2012 ). Numerous soil microorganisms have the ability to
solubilize this unavailable P through their metabolic activities exudating organic
acids, which directly dissolve the rock phosphate, or chelating calcium ions that
release P to the solution.
4.2.3 Microbes in Biogeochemical Cycle of P in Soil
Microorganisms are fundamental to the biogeochemical cycle of phosphorus and as
such play crucial role in mediating the availability of phosphorus to flora
(Richardson et al. 2011 ; Jain and Khichi 2014 ). Biogeochemical cycling of phos-
phorus is essential for various reasons. Every living cell requires phosphorus for
nucleic acids, lipids, and a few polysaccharides. In soil, phosphorus exists in both
4 Endophytic Bacteria: Role in Phosphate Solubilization 65