Phosphorus is considered as an essential macronutrient and a great portion of phosphorus from chemical fertilizers becomes insoluble by its conversion into calcium or magnesium salts in soils and become unavailable to plants. Soil microorganisms involve to transform the insoluble forms of phosphorus into soluble forms and thus influence the subsequent availability of phosphate to plant roots are considered essential [49, 50]. Phosphate solubilizing microorganisms have been employed in agriculture and horticulture and have been considered very important due to their potential of ecological amelioration. It is believed that microbial mediated solubilization of insoluble phosphates in soil is through the release of organic acids microbial metabolites [51–53]. However, in addition to acid production, other mechanisms can cause phosphate solubilization . Phosphate solubilization has been reported to depend on the structural complexiCity and particle size of phosphates and the quantity of organic acid secreted by microbes .
Fluorescent pseudomonads often predominant among plant rhizosphere associated bacteria [24, 56]. Plant growth promoting rhizobacteria are classified into two different groups such as strains that have the capability of synthesizing phytohormones and strains that have the ability to suppress the growth of phytopathogens . Fluorescent pseudomonads enhance plant growth by improving soil nutrient status, producing plant growth hormones, enzymes and suppressing the growth of phytopathogenic fungi [56, 58]. Plant growth promoting rhizobacterial types of fluorescent pseudomonads use one or more mechanisms of direct or indirect in improving plant growth. These mechanisms can probably be active simultaneously or sequentially at different stages of plant growth. This group of bacteria exhibits multiple functional traits such as solubilizing of inorganic phosphate and iron, production of vitamins, phytohormones and antimicrobial metabolites. They are capable of improving plant nutrients uptake, tolerance to stress, salinity, metal toxiCity and pesticide. Fluorescent pseudomonad strains such as P. fluorescens NJ101 , P. fluorescens EM85 , P. fluorescens , Pseudomonas spp. [61, 62], P. chlororaphis, P. savastanoi, P. pickettii  and P. corrugata  have been reported as phosphate solubilizers.
In the present investigation, out of 443 fluorescent pseudomonad strains screened, 80 strains have been identified as phosphate solubilizers. These strains were taxonomically described as different fluorescent pseudomonad species such as P. monteilli, P. putida, P. plecoglossicida, P. fluoresens, P. fulva, P. monteilli and P. aeruginosa on the basis of 16S rRNA gene sequencing and subsequent molecular phylogeny analysis. Phenotypic analyses as well as 16S rRNA and BOX-PCR based genotypic analyses revealed a high degree of diversity among phosphate solubilizing bacteria reported in this study. Significant decline in the pH of the culture medium by strains was observed during mineral phosphate solubilization, which suggested the microbial production of organic acids . Although phosphate solubilization is not necessarily correlated with acidity, from the data present in this study relationship could be ascertained between the acidity of medium and the release of soluble phosphates. Estimation of phosphate solubilization of strains by other methods has been reported to be between 200 to 805 μg ml-1 . In an earlier study, P. fluorescens strain NJ-101 isolated from agricultural soil was reported to release 74.6 μg ml-1 soluble phosphate from inorganic phosphate  and in the present study, up to 105.3 μg ml-1 soluble phosphate was estimated. We have found that 49% of the strains produced IAA and 16% of the strains produced ACC deaminase. It is reported that the ACC deaminase producing bacteria increase root elongation and seed germination by lowering plant ethylene levels [24, 66]. Specific strains of fluorescent pseudomonad bacteria indirectly influence the plant health by preventing the deleterious effects of phytopathogenic microorganisms through the production of antibiotics, cell wall degrading enzymes, HCN metabolite and siderophores [56, 57]. Production of HCN by P. fluorescens CHAO was recognized as a biocontrol factor, against plant pathogenic fungi .
Production of antibiotics by fluorescent pseudomonads considered important in suppression of phytopathogens. Recently, P. aeruginosa PUPa3, a new strain from rice rhizosphere with potential for fungal antibiosis and biofertilizing traits has been identified from our laboratory . All the strains reported in this study produced hydroxamate siderophores as evidenced on FeCl3 amended CAS agar medium, production of an array of phytohormones and antifungal metabolites. Microbial production of antibiotics, PCA (2 to 3 mg ml-1), DAPG (0.5 to 3 mg ml-1), PLT (1.5 to 2 μg ml-1) and PRN (0.54 mg ml-1) by biofertilizing and biocontrol strains have been reported in earlier studies [68–70]. In the present study the production of PCA (20 to 1124 μg ml-1), DAPG (13 to 93 μg ml-1), PLT (3 to 9 μg ml-1) and PRN (3 to 41 μg ml-1) by phosphate solubilizing fluorescent pseudomonads has been reported. Strain efficiency and variations in the fermentation conditions often result in an alteration in antibiotic production. Considering the quantity of antibiotics by plant growth promoting and biocontrol strains of fluorescent pseduomonads reported by other inverstigators [68–70], strains reported in this study may be considered as non-pathogenic to plants and antagonistic bacteria against phytopathogenic fungi. Strains reported in this study utilized several carbon sources as identified by Hi-carbohydrate™ kit test. Utilization of variety of carbon sources by the strains may play an important role in adapting to a variety of crop plants and soil types.
The IAA hormone is known to have dual role in influencing plant growth, by involving in the biocontrol together with glutathione-s-transferases in defense-related plant reactions and inhibits the germination of spore and growth of mycelium of different pathogenic fungi [71, 72]. Martinez Noel et al. (2001) showed that the IAA supply to excised potato leaves reduced the severity of the disease provoked by Phytophthora infestans . In present investigation, we have identified the spectrum of bacterial antagonism by measuring the inhibition zones of mycelial radial growth in plate assays. These antagonistic strains also showed production of IAA, fungal cell wall degrading enzymes, such as cellulases, proteases and chitinases which are known to be involved in antagonistic activity against phytopathogenic fungi and insects [74, 75]. Selective microbial producers of chitinase are also reported to be the efficient phosphate solubilizers . Phosphate solubilizing fluorescent pseudomonad strains reported in this study may solubilize insoluble compounds due to the excretion of organic acids. Production of antimicrobial metabolites and organic acids is essential to decrease soil pH, which plays a major role in solubilization of phosphates and other nutrients.
Characterization of phosphate solubilizing fluorescent pseudomonad bacteria is required to study their ecological role in soil. Fluorescent pseudomonad strains reported in this study with phosphate solubilization potential and ability to excrete phytohormones and antimicrobial metabolites may be used as plant growth promoting bacteria and biocontrol agents in sustainable agriculture.