isolation of phosphate solubilizing micro-organisms from rhizosphere introduction
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Phosphorus (P) is a macronutrient that is essential for plant growth and development. It is a component of biological molecules, such as DNA, RNA, ATP, and phospholipids, and on a macro level, it affects root development, stalk and stem strength, crop maturity, and nitrogen fixation in legumes (Khan et al., 2009). Phosphorus in soils can exist in both organic (Po) and inorganic (Pi) forms; the inorganic forms of phosphorus have been calculated to account for 35 – 70% of total P in soil (Harrison, 1987). While some P minerals, like apatites and strengites, have very slow release rates, other P minerals, complexed with calcium, aluminum, or iron, have faster dissolution rates that are dependent on the pH of the surrounding soil and on the size of the particles (Pierzynski et al., 2005). Higher soil pH values (basic) cause aluminum and iron-complexed P to become more soluble, while lower soil pH values (acidic) promote the solubility of calcium-complexed P (Wang and Nancollas, 2008). Since the concentration of available P in soil is lower than what is found in healthy plant tissues, it is common agricultural practice to apply mineral P fertilizers in the form of readily-available monocalcium phosphate or monopotassium phosphate (Schachtman et al., 1998). However, with the efficiency of the fertilizer hovering between 10 – 25% (Isherwood, 1996), a large majority of that P also becomes immobilized in inorganic and organic forms (Khan et al., 2009). Consequently, P fertilizers have become the largest market for phosphorus worldwide. Due to the demand of agriculture on global stocks of P, it is estimated that the world will reach its maximum rate of quality mineral P production by 2040 at which point production will decline while agricultural demand will continue to rise (Schroder et al., 2010). Since P supplies are not easily replenished in comparison to nitrogen, it is important to better utilize P reserves in the soil and reclaim chemically-bound P (Cordell et al., 2009).
Phosphate-solubilizing bacteria (PSB) in the plant rhizosphere play a significant role in releasing P from its insoluble complexes to a form that is more readily usable by plants. The inorganic forms of P can be solubilized by microorganisms that secrete low molecular weight organic acids to dissolve phosphate-complexed minerals (Goldstein, 1995) and/or chelate cations that partner with P ions (PO43-) to release P directly into the surrounding soil solution system (Vyas and Gulati, 2009).With the current public interest in promoting more sustainable agricultural practices, using PSB, either in conjunction with or as a replacement for expensive and environmentally damaging fertilizers, would be advantageous to the agricultural industry (Barea, 2015).
Thus far, there have been decades of research on phosphate-solubilizing microorganisms which have been transferred to industrial practices since the 1950s (Krasilinikov, 1957). Specifically, soil bacteria from genera Pseudomonas, Bacillus, Rhizobium, and Enterobacter have been thought to be the most powerful P solubilizers (Hassan and Bano, 2015; Whitelaw, 1999). The less studied Pantoeagenus contains several phosphate-solubilizing bacteria such as P. agglomeran (Son et al., 2006), P.eucalypti (Castagno et al., 2011), P. ananatis (Oliveira et al., 2009), P. vagans (Rfaki et al., 2014), and Pantoea sp. LUP (Jorquera et al., 2008).
This study represents an effort to isolate bacteria with the ability to effectively solubilize phosphate for plant utilization. We isolated a bacterium, Pantoea sp. Pot1, with efficient phosphate solubilization capabilities from an organic garden. When the bacterium was used as an inoculant in greenhouse experiments with tomato plants, this strain was found to be a very effective biofertilizer.