Screening and optimization of indole-3-acetic acid production and phosphate solubilization by rhizobacterial strains isolated from Acacia cyanophylla root nodules and their effects on its plant growth

Abstract

Background Plant growth-promoting rhizobacteria (PGPR) are known to improve plant growth and are used as biofertilizers, thanks to their numerous benefits to agriculture such as phosphorus solubilization and phytohormone production. In this paper, four rhizospheric bacteria (Phyllobacterium sp., Bacillus sp., Agrobacterium sp., and Rhizobium sp.) isolated from surface-sterilized root nodules of Acacia cyanophylla were tested for their ability to solubilize inorganic phosphate and to produce indole-3-acetic acid (IAA) under laboratory conditions. Then, the best IAA producer (Rhizobium sp.) was selected to test optimized conditions for IAA production. Finally, the effect of the four strains on plant growth for A. cyanophylla was evaluated in vivo. Results The results showed that the totality of the tested isolates had solubilized inorganic phosphate (P) in both NBRIP (National Botanical Research Institute Phosphate) and PVK (Pikovskaya) media. Bacillus sp. was a high P-solubilizer and showed maximum solubilization in PVK (519 μg ml-1) and NBRIP (782 μg ml-1). The optimization of maximum phosphate solubilization was done using different sources of carbon (1%) and nitrogen (0.1%). Glucose and ammonium sulfate were selected to be the best carbon and nitrogen source for phosphate solubilization by all tested strains, except for Phyllobacterium sp., which recorded the highest phosphate solubilization with ammonium nitrate. The IAA production by the tested strains indicated that Rhizobium sp. produced the highest amount of IAA (90.21 μg ml-1) in culture media supplemented with L-tryptophan. The best production was observed with L-Trp concentration of 0.2% (116.42 μg ml-1) and at an initial pH of 9 (116.07 μg ml-1). The effect of NaCl on IAA production was tested at concentrations of 0 to 5% and the maximum production of 89.43 μg ml-1 was found at 2% NaCl. The extraction of crude IAA from this strain was done and purity was confirmed with Thin Layer Chromatography (TLC) analysis. A specific spot from the extracted IAA production was found to correspond with a standard spot of IAA with the same Rf value. Finally, the tested PGPR demonstrated growth stimulatory effects on Acacia cyanophylla seedlings in vivo, with a great increase of shoots’ and roots’ dry weights, and shoot length compared to control. The rhizobacterial isolates were identified by 16S rDNA sequence analysis as Agrobacterium sp. NA11001, Phyllobacterium sp. C65, Bacillus sp. CS14, and Rhizobium sp. V3E1. Conclusion This study highlights the importance of the use of phosphate solubilizing and IAA producer microorganisms as biofertilizers to increase crop yields. The studied strains showed a significant phosphate solubilization potential and IAA production. The use of selected strains as inoculants would be interesting, in particular with a view of promoting sustainable agriculture. However, further studies to verify the efficacy of the best isolates in situ is certainly required.

Modeling and Optimization of Phosphate Solubilizing Bacteria Isolated from Rhizospheric Soils of the Coffee Plant using Artificial Neural Network (ANN) and Response Surface Methodology (RSM).

“…Similarly, previous studies found that the maximum amount of IAA was produced at neutral to slightly alkaline pH [49][50][51]. Other scholars showed that the maximum yield of IAA was obtained at pH 9 [21,52]. Maximum IAA production of 81.49 μg/mL at a pH of 7…”

“…Quantitative production of IAA was determined using the colorimetric method as described by Lebrazi et al [21].…”

“…According to the linear coefficients of the second-order equation, incubation temperature, incubation time, pH showed a positive effect, while, and inoculum size had a negative influence. Pseudomonas putida Rs-198 culture isolated from salinized soils [38] and rhizobacterial strains isolated from Acacia cyanophylla root nodules [21].…”

Potential Role of Rhizobacteria Isolated from Citrus Rhizosphere for Biological Control of Citrus Dry Root Rot

“…The phosphates solubilizing microorganisms (PSM) are considered as potential bioinoculants to increase crop productivity by transforming phosphorus of the soil (the monobasic (H 2 PO 4 − ) and diabetic (HPO 4 2− ) into soluble forms assimilable by plants, through the production of organic acids, the acidification process and chelation [ 68 ]. In this study, 14 isolates were found able to solubilize tricalcium phosphate.…”

“…Additionally, they are important in generating plant defence responses against invading pathogens [ 41 ]. IAA produced by PGPRs contribute in increasing root surface area and length, and thereby better soil nutrients uptake [ 42 , 68 ]. Interestingly, half of effective bacterial isolates produced different shades of pinkish-red, suggesting the production of this phytohormone.…”