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

• Published in: Journal of Genetic Engineering and Biotechnology 2020-11, Vol.18 (1), p.71-12, Article 71
• Main Authors: Lebrazi, Sara, Niehaus, Karsten, Bednarz, Hanna, Fadil, Mouhcine, Chraibi, Marwa, Fikri-Benbrahim, Kawtar
• Format: Article
• Language: English
• Subjects: Acacia cyanophylla; Acetic acid; Acid production; Acids; Agricultural chemicals; Agriculture; Agrobacterium; Ammonium; Ammonium nitrate; Ammonium sulfate; Ammonium sulphate; Analysis; Auxin; Bacillus; Bacteria; Biosynthesis; Carbon; Carbon sources; Crop yield; Culture media; Fertilizer industry; Fertilizers; In vivo methods and tests; Indole-3-acetic acid production; Indoleacetic acid; Loam soils; Microorganisms; Nitrogen; Nodules; Optimization; PGPR; Phosphate; Phosphate solubilization; Phosphates; Phosphorus; Plant growth; Polyvinyl carbazole; Rhizobium; Root nodules; rRNA 16S; Seedlings; Sequence analysis; Shoots; Sodium chloride; Solubilization; Strains (organisms); Sustainable agriculture; Thin layer chromatography; Tryptophan
• ISSN: 1687-157X; 2090-5920
• DOI: 10.1186/s43141-020-00090-2

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 [mu]g ml.sup.-1) and NBRIP (782 [mu]g ml.sup.-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 [mu]g ml.sup.-1) in culture media supplemented with L-tryptophan. The best production was observed with L-Trp concentration of 0.2% (116.42 [mu]g ml.sup.-1) and at an initial pH of 9 (116.07 [mu]g ml.sup.-1). The effect of NaCl on IAA production was tested at concentrations of 0 to 5% and the maximum production of 89.43 [mu]g ml.sup.-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