Isolation and identification of salt tolerant-plant growth promoting bacteria from the rhizosphere of halophyte plants

Salinity is a serious problem and one of the main factors in reducing agricultural productivity worldwide. The use of microorganisms to improve plant growth in low quality soil is a potential solution to address this problem. The salinity resistant microbiome improves the health of the salinity affected soils, maintains ecological functions and enhances plant growth. Salt tolerant plant growth promoting rhizobacteria (ST-PGPR) are able to adjust the salinity stress adverse conditions for their symbiotic plants through several mechanisms. This study aimed to isolation, identification and investigation the characteristics of salt tolerant-plant growth promoting rhizobacteria from the rhizosphere of halophyte plants.

Salinity tolerant plant growth promoting bacteria were isolated from the rhizosphere of native halophyte plants collected from the north of Iran. Bacterial isolates were selected according to their ability to growth in 2 M sodium chloride and traits of the motility, inorganic phosphate solubilization ability in PKV-Agar medium using phosphate solubilization index calculation, and siderophore production capacity in CAS-agar medium by calculating of its production index. Then, the top 10 isolates were selected and the ability of nitrogen fixation in Burk's medium, potassium dissolution in Aleksandrov's medium by using potassium solubilization index, as well as, the production of intrinsic IAA and the tryptophan-dependent IAA were investigated. Finally, molecular identification of selected bacterial isolates was performed based on 16S rDNA gene sequencing.

Four genera of Bacillus, Klebsiella, Proteus and Halomonas were identified after 16S rDNA gene sequencing. All isolates were motile. The most of isolates belonged to the genus Bacillus. Species belonging to Klebsiella and Halomonas genera had the highest salt tolerance. Klebsiella genus showed phosphorus and potassium solubilizing ability more than other isolates. The highest amount of siderophore production was observed in the isolate belonging to Proteus genus. All isolates were able to fix nitrogen. Among the studied isolates, Bacillus isolates had the highest rate of intrinsic IAA, and the tryptophan-dependent IAA production in B. licheniformis was higher than other isolates.

Isolation of wide range of salt-tolerant bacteria with favorable characteristics from the rhizosphere of halophyte plants indicates the microbial richness potential of this area, which provides the possibility of finding useful microorganisms that promote plant growth and reduce the adverse effects of stress in plants. Keywords: Plant growth promoting bacteria, Salinity stress, Halophyte plants, Indole acetic acid