Rhizobacterial Phosphorous Solubilization for Enhanced Yield of Potato (Solanum tuberosum L.)

Abstract

Phosphorus (P) is one of the indispensable element for plants to regulate key biochemical processes like photosynthesis, respiration, energy storage and transfer, cell division and cell enlargement. Although, sufficient amount of P is present in agricultural soils but generally it is unavailable for plant growth and development due to fixation and immobilization. Potato is used as a staple food in most part of the world and popular in Pakistan as well. Microorganisms with P-solubilizing ability have substantial ability to provide P, required for healthy plant growth. Present study was carried out to isolate P-solubilizing rhizobacteria from major potato growing areas of Punjab, Pakistan with the hypothesis that the P-solubilizing rhizobacteria would improve growth and yield of potato on inoculation. Out of 116 isolates, 31 were characterized as P-solubilizers with varying ability of tri-calcium phosphate solubilization (16.57 to 140.38 μg ml-1) in vitro. Fifteen isolates showed P-mineralizing potential with extracellular (0.9×10-10 kat to 6.07×10-10) and intra-cellular (0.32×10-10 kat to 3.65×10-10) phytase activity. Of 31 P-solubilizing rhizobacteria, 20 isolates showed IAA (0.5 to 13.84 μg ml-1) producing potential. Taxonomically these bacteria were identified on the bases of 16S rRNA gene and belongs to ten different genera. Gene considered for P-solubilization pqqE, was successfully amplified in four (P. brassicacearum KPS-5, Pseudomonas sp.KPS-7, Serratia sp. KPS-8 and S. plymuthica KPS-10) bacterial strains while ß-propeller gene (encoding alkaline phytase) was confirmed in two (P. brassicacearum KPS-5 and B. subtilis KPS-11) bacterial strains. Phylogenetic analysis based on 16S rRNA, pqqE and ß-propeller genes showed their phylogeny at genus level. Phenotypic microarray analysis of potent P-solubilizers revealed their metabolic diversity and potential. P-solubilizers exhibited organic acids (gluconic, oxalic, tartaric, malic and lactic acids) production ability in varying amount. P-solubilization and organic acid production attributes were positively correlated (r= 0.9412 to 0.9912) with pH decline in culture medium. Transmission electron microscopy (TEM) of phosphobacterial strains P. brassicacearum KPS-5, Pseudomonas sp.KPS-7 and Serratia sp. KPS-8 showed their colonization potential xv with potato roots. These bacterial strains were labelled with yellow fluorescent protein (yfp), inoculated to potato seedlings and observed under confocal laser scanning microscope (CLSM) which further confirmed their root colonization potential. Fifteen promising P-solubilizing bacterial strains were selected for plant inoculation test. All inoculations enhanced potato growth parameters. However, P. brassicacearum KPS-5, Pseudomonas sp.KPS-7 and Serratia sp. KPS-8 showed significantly (P ≤ 0.05) higher potato growth compared to other inoculated and un-inoculated plants. In another experiment, conducted in soil supplemented with phytate, two rifampicin resistant derivatives of bacterial strains P. brassicacearum KPS-5 and B. subtilis KPS-11 were successfully recovered from potato rhizosphere and rhizoplane at different time intervals up to 60 days after inoculation which showed their survival and colonization potential. Both bacterial strains significantly (P ≤ 0.05) increased the potato growth parameters in comparison with un-inoculated plants. Field experiments were conducted at three different localities (Faisalabad, Sahiwal and Rawalakot) to evaluate plant growth promoting potential of selected rhizobacterial strains under field conditions. Bacterial inoculums of P. brassicacearum KPS-5, Pseudomonas sp.KPS-7 and Serratia sp. KPS-8 supplemented with half dose of P fertilizer significantly (P ≤ 0.05) increased the growth and yield of potato when compared with un-inoculated plants with same level of P fertilizer as well as un-inoculated treatments with full dose of rock phosphate (RP). It is concluded on the basis of in vitro and in vivo studies that P. brassicacearum KPS-5, Pseudomonas sp.KPS-7 and Serratia sp. KPS-8 are potential P-solubilizing rhizobacterial strains that can enhance growth and yield of potato by making unavailable forms of phosphorus to bioavailable form. These bacterial strains can be used as P-solubilizing bio-inoculants with reduced dose of P fertilizer for potato crop.