As compared to the global agricultural nations, Pakistan exhibits low plant productivity due to plant diseases, poor soil fertility and less efficient chemical fertilizers. These demerits can be eliminated by replacing the conventional fertilizers with eco-friendly plant growth promoting rhizobacteria (PGPR) based biofertilizers. Current study was designed to isolate PGPRs from Melia azedarach rhizosphere and evaluate their plant growth promotion characteristics in depth. The isolated bacteria were identified as Bacillus paramycoides, Bacillus cereus, Bacillus pumilus, Bacillus amyloliquefaciens, and Pseudomonas putida and characterized through morphological, molecular (16S rRNA gene sequencing), and biochemical analysis. These bacteria were evaluated for salicylic acid, indole, cellulase, biosurfactant, ammonia and exopolysaccharide production and for their ability of phosphate and zinc solubilization, salt tolerance, and antibiotic resistance. The effect of these bacteria on seed germination was also tested and finally GC-MS based metabolic profiling was performed. All of these studied bacteria were sensitive to antibiotics, demonstrated salicylic acid (50.15 µg/ml-126.94), indole and exopolysaccharide production ability. Except B. amyloliquefaciens, all the bacteria were cellulase producers. Phosphate solubilization index (PSI) was recorded as 2.38 ± 0.001–3.59 ± 0.002. All isolates were tolerant to 3 and 5% NaCl. B. pumilus was the optimal biosurfactant producer. B. paramycoides SBBPGPR1 showed optimum ammonia production. B. cereus and B. pumilus exhibited the most pronounced plant growth-promoting effects on rice seed germination. None of the isolates demonstrated zinc solubilization. GC-MS based study demonstrated identification of organosiloxane, benzoic acid derivatives, catechols, esters, organosilicon, oxaspiro and organophosphorus compounds, pyridinone, ethers, quinones, methylsiloxane, catecholamine derivatives, hydrazines, benzamides, phenols, amino acid derivatives and cyclohexadienone. In addition, metabolic intermediates of IAA and cytokinin production, and ACC deaminase pathways were also found. These bacteria might be recommended as emerging and sustainable alternatives to the synthetic chemical fertilizers.
