Evaluation and Comparative Analysis of Plant Growth-Promoting Traits in Diverse Groups of Rhizosphere Bacteria

Background and objectives: Using chemical fertilizers is the primary method of plant nutrition in agriculture, however, it has a high potential for contamination of water, soil, plants, animals and humans; therefore, it is crucial to provide solutions based on sustainable development for crop production. One of the solutions is to use the potential of beneficial soil microorganisms including plant growth-promoting rhizobacteria (PGPR). The PGPR are a large group of beneficial soil bacteria that affect plant growth through various mechanisms including, production growth-promoting substances such as auxin, solubilization of insoluble organic and inorganic phosphates, production of side­rophores, and potassium releasing from soil clay minerals such as biotite and moscuvite. Having positive impact of PGPR on plant growth has led to their use as biofertilizers or inoculants in agriculture. One of the most essential components of a biofertilizer is microorganisms. The most important and well-known PGPRs include Azotobacter, Pseudomonas, Bacillus, and Azospirillum. The aim of this research was to compare these four groups, native to Iraninan soils regarding plant growth-promoting characteristics and to identify superior strains that can be investigated in future research as biofertilizers. Such a comparison has not been done in the country on different groups of PGPR.Materials and methods: In this research, 63 strains of PGPR including 13 Azotobacter, 10 Azospirillum, 21 Bacillus, and 19 Pseudomonas strains were used obtained from the microbial culture collection of Soil and Water Research Institute which were previously isolated across from different agricultural soils of Iran. The strains were screened and compared regarding plant growth-promoting characteristics. The ability of strains to produce auxin evaluated using Salkowski reagent method at the levels 0 and 50 mg/L L-tryptophan. The siderophore assayed using the CAS-agar method, calculating the ration of halo-to-colony diameter. The extracellular polysaccharides (EPS) were measured weighting the dried precipitated substances. The ability to solubilize inorganic and organic phosphates was investigated by calculating the ratio of halo to colony diameter in Pikovskaya's agar medium. Alexandrov's method was used to investigate the ability of released potassium from muscovite and biotite soil clay minerals. The NBRIP culture medium, was used to evaluate the ability to dissolve insoluble iron phosphate. All experiments were performed in triplicate and the statistical analysis of data done using SAS software. Means were compared and grouped by the least significant difference (LSD) method at the 5% probability level.Results: The results showed that Pseudomonas strain P214 produced 51.68 μg/mL of auxin at 50 mg/L of L-tryptophan and 48.72 μg/mL in the absence of L-tryptophan (0 mg/L). For mineral and organic phosphate solubilization, Pseudomonas P187 and P186 demonstrated the highest indices, with halo-to-colony diameter ratios of 2.59 and 4.13, respectively. Siderophore production by the Pseudomonas group was significantly higher than that of the other three groups, with strain P188 exhibiting the greatest capacity, achieving a halo-to-colony diameter ratio of 3.38. Additionally, Pseudomonas exhibited the strongest ability to solubilize insoluble iron phosphate, with strain P192 recording the highest halo-to-colony diameter ratio of 4.45. Among potassium-releasing strains, Bacillus B517 showed the highest release from biotite, with a value of 33.33 g/L, while B326 exhibited the highest release from muscovite, with a value of 24.67 g/L. Finally, strain P526 produced the highest quantity of extracellular polysaccharides, with a value of 84.6 g/L.Conclusion: In this research, 63 strains belonging to four groups of plant growth-promoting rhizobacteria (PGPR) native to Iranian soils obtained from culture collection of Soil and Water Research institute including Pseudomonas (19 strains), Azotobacter (13 strains), Bacillus (21strains) and Azospirillum (10 strains) were investigated and compared in terms of plant growth-promoting characteristics. The results showed that the Pseudomonas group showed a significant difference from the other groups regarding siderophore production, solubilization of insoluble iron phosphate, extracellular polysaccharides, and solubility of mineral and organic phosphates. Pseudomonas and Azotobacter groups produced the highest indole acetic acid at 0 and 50 mg/L L-tryptophan levels. The Bacillus group had the highest ability to release potassium from biotite and Pseudomonas from muscovite. The Pseudomonas had superior strains in most of the plant growth-promoting characteristics, however, considering that other groups showed superiority in some other characteristics, it is suggested to use a Consortium of superstrains of each group should be used as inoculants or biofertilizers in the future research.