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Phosphorous is an important mineral nutrient for plant growth and development which
plays a vital role in photosynthesis, nucleus formation and transfer of
heredity. Phosphate present in the soil is mainly of two types, insoluble mineral phosphates and organic
phosphate. Inorganic phosphates in acidic soils are the result of association with iron (Fe) and
aluminum (Al) compounds, whereas in neutral or calcareous soils, calcium (Ca)-phosphates
like tricalcium phosphate, dicalcium phosphate, hydroxyapatite and others are
predominant. Plants can take up phosphorous only in the form of soluble orthophosphate ions. Due
to less availability of soluble phosphate, plants suffer from nutrient deficiency.
Application of soluble inorganic phosphate in the form of chemical fertilizer can be hazardous
for environment; also, it soon gets immobilized after appliance. Certain bacterial species
has the ability to solubilize organic phosphate by synthesizing organic acids or by
enzymatic action of acid phosphatase and phytase (Ponmurugan and Gopi, 2006).
These
phosphate solubilizing bacteria can influence plant growth when used as biofertilizer because
they provide plants with accessible form of phosphate which was first unavailable to the
plants in an environment-friendly and sustainable manner. Frequently, they are present within
the plant rhizosphere and are termed as rhizobacteria. This group of plant growth
promoting rhizobacteria, Plant Growth Promoting bacteria (PGPR) include strains from genera
such as Alcaligenes, Acinetobacter, Arthrobacter, Azospirillum, Bacillus,
Burkholderia, Enterobacter, Erwinia, Flavobacterium, Pseudomonas,
Rhizobium and Serratia (Glick, 1995). In the lab, phosphate solubilization by these bacteria is often observed, but it is not
the same after its application (Gyaneshwar et
al., 2002). Consistent increase of phosphorus uptake by plants after inoculating the phosphate solubilizers is not much evident. The
reason for this difference in growth enhancement is the effect of buffering the capacity of
soils because solubilization of CaP complexes is mediated mainly by lowering the pH of
the medium (Sperber, 1957). Water sources for irrigation of soil have increasingly
become polluted with heavy metals due to industrial effluents in urban agriculture (Pepper
and Roane, 2000). Also, sewage sludge used in agriculture causes contamination of soil
with heavy metals like nickel, lead, chromium, cadmium, copper and zinc (Agrawal and
Singh, 2007). Some bacteria develop heavy metal resistance due to exposure to toxic heavy
metals. Certain phosphate solubilizing bacteria not only tolerate and extract heavy metal but
also produce phytohormones which aid heavy metal uptake by plants. So these organisms
can be used as inoculum to the soil to clean up heavy metal contamination. Molecular
biology techniques have been applied for improving phosphate solubilizing efficiency of
phosphate solubilizing bacteria which have enhanced agriculture productivity. From research
studies, it can be said that by appropriate regulations, genetically enhanced microorganisms
can be used safely in agriculture (Okkeri and Haltia, 1999). Benefits of developing
genetically-modified plant growth promoting bacteria over the transgenic plants for improving
plant performance are: (i) modifying a bacterium is much easier than altering complex
higher organisms; (ii) numerous plant growth-promoting traits can be combined within a
single organism; and (iii) single, engineered inoculant can be used for quite a few crops in
the place of engineering crop one by one. To avoid horizontal transfer of the introduced
genes within the rhizosphere, one of the best methods used is chromosomal insertion of the
genes (Rodrýguez et al., 2006). |
