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2012). Phosphate solubilizing bacteria (PSB) solubilize phosphate by a synthesis of low

molecular weight organic acids such as gluconic acid and citric acid. The organic acids
produced by PSBs bind phosphate with their hydroxyl and carboxyl group and induce solid

acidification too, which releases soluble phosphate phosphate (Kpomblekou-a et al., 1994).
In addition, exopolysaccharides produced by PSB participate in indirect phosphate

solubilization (Yi et al., 2008). PSBs produce phosphatase enzyme which catalyses the
hydrolysis of phosphoric esters (Rodrı guez et al., 1999).

2.1.3 Potassium Solubilization

Potassium is another macromolecule that has an important role in plant growth and

development. Suboptimal levels of K can lead to poorly developed roots, lower growth and
small seeds. Absorption of K from soil for plants is dependent on the total K content in the soil

and K dynamics. Three forms of K found in soil among which 90% to 98% of soil K is in
form of soil minerals (Sparks, 1987). A wide scope of microorganisms Pseudomonas,

Burkholderia, Acidothiobacillus ferroxidase, Bacillus mucilaginosus, Bacillus edaphicus, B.

circulans and Paenibacillus sp. have been accounted for to change over mineral type of K
to available structure in soil (Lian et al., 2002). The potassium solubilizing microscopic

organisms (KSB) were found to break down potassium, silicon and aluminium from
insoluble K-bearing minerals for example, micas, illite, and orthoclase’s, by discharging

natural acids which either straightforwardly disintegrated rock K or chelated silicon particles

to carry K into the arrangement (Aleksandrov et al., 1967).

2.1.4 Siderophores and Ion Chelation:
Iron is an essential element for all living forms. Despite iron is not in readily available because

the dominating form of Iron is Ferric, which is stingily soluble hence it is available in very
low concentration for assimilation by living forms. PGPB produces low molecular weight

compounds called siderophores to acquire ferric iron. Depending on the functional group
three main types of siderophores are produced, i.e. catecholates, hydroxymates and

carboxylates. Use of radio labeled ferricsiderophores as a sole source of iron demonstrated

that plants are able to take up labeled iron by PGPB including Aeromonas, Azadirachta,
Azotobacter, Bacillus, Burkholderia, Pseudomonas, Rhizobium, Serratia and Streptomyces

sp. (Subramoni et al., 2013). Soil bacteria improves plant iron nutrition, further more by

supplying iron to the plants, siderophores may help to alleviate the stresses imposed on plants
by high soil levels of heavy metals (Diels et al., 2002).

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