Screening of soil bacteria as potential plant growth promoters

IBARRA J.G.; GODEAS, A.; LÓPEZ, N.I.

Congreso; VIII Congreso Argentino de Microbiología General Samige del Bicentenario; 2012

Screening of soil bacteria as potential plant growth promoters José G. Ibarra, Alicia M. Godeas and Nancy I. López Dpto. de Química Biológica y Dpto. de Biodiversidad y Biología Experimental Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires Plant growth promoting bacteria (PGPB) can benefit plant development through multiple mechanisms of action such as the production of substances which promote growth and increase nutrient availability in soil. The aim of this study was the isolation of bacteria able to solubilize inorganic phosphate and to produce indole 3-acetic acid (IAA) from different soils as potential PGPB. Sampling was carried out in two agricultural soil located in Ines Indart (Salto) and America (General Villegas), both belonging to Buenos Aires province. Locations were selected by different rainfall regimes and differences in soil structure. Soil samples were taken between 0 and 20 cm depth. Samples were processed and cultures were done in different culture media (mannitol, King B and nutrient agar) for the isolation of different bacterial groups. All isolates were biochemically tested to determine production of IAA and inorganic phosphate solubilization. Approximately eighty strains were subjected to chemical tests for phosphate solubilization in NBRIP culture medium, which contains calcium phosphate (Ca3(PO4)2, insoluble in water at neutral pH. The solubilizing capacity of each isolate was determined by performing a ratio between colony size and the solubilization halo around them, after 5 days of growth at room temperature. Of all the isolates tested, 18 were capable of solubilizing phosphate. Three of them showed a very good relationship between the diameter of the halo and colony size (10/3, 10/5 and 9/2, measured in mm). The IAA production ability was tested using 96-multiwell plates containing LB medium supplemented with 1 g/l tryptophan for 24 h. Positive results were detected by using Salkowski reagent.Of ninety six strains tested, 25 were positive for IAA test. Ten positive isolates were selected to quantify the IAA production. Strains were cultured for 48 h in BDN medium plus tryptophan and a standard curve was made with commercial IAA. Control cultures without tryptophan were also performed. We have obtained very promising results, with values of IAA production ranging from 3 g/ml to 92 g/ml. IAA levels detected in control cultures were near zero for the same isolates. The best strains were isolated from Inés Indart soil. Some of these were primarily isolated in KB medium, indicating a probable filiation with Pseudomonas species. P. extremaustralis, a species studied in our laboratory that was also tested, presented a high IAA production and phosphate solubilization, allowing the analysis of genes involved these processes in its genome. It is expected that bacterial strains with multiple beneficial mechanisms of action will be better PGPB. We isolated a very interesting strain, able to solubilize phosphate and produce high amounts of IAA. These characteristics allow hypothesize that this strain could be a good candidate to be tested in soils in presence of plants to confirm their potential as PGPB. The assays are currently in progress.