Paper Information

Journal:   IRANIAN JOURNAL OF SOIL AND WATERS SCIENCES   2003 , Volume 17 , Number 2; Page(s) 177 To 189.
 
Paper: 

EVALUATION OF SIDEROPHORE PRODUCTION BY RHIZOBIAL STRAINS NATIVE TO THE SOILS OF IRAN

 
 
Author(s):  ALIKHANI H.A., SALEH RASTIN N., ANTOUN H.
 
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Abstract: 
This study was carried out to compare the siderophore-producing ability of the native rhizobial strains for selection of superior specimens that could be used as "PGPR" for both legume and non-legume agricultural plants. For this purpose, 447 isolates of Rhizobia from different cross-inoculation groups were tested. The Rhizobia were isolated from important agricultural legumes, cultivated in different regions of Iran and identified according to the standard methods used for rhizobial authentication. In addition, 29 strains of P. fluorescens were used as positive controls (Sid^+). The potential of siderophore production of all bacterial strains was tested using chrom azurol S (CAS)-agar assay. Since the detergent HDTMA in CAS-agar has a tendency to inhibit growth of several microbial species, the usual direct culture onto this medium was compared with half- plate and overlay of two-layer methods. These media were inoculated by the drop plate method, using 7µl of fresh culture of each strain adjusted to 5X10^8 cfu ml^-1, with 3 replications per strain. After the periods of 7, 14, and 21 days of incubation, the diameter of each bacterial growth colony and the orange halo surrounding colony, were measured. The maximum diameter of halo, the ratios of halo to colony, as well as the rate of color-change (mm day^-1) were used as the criteria for comparison of the Sid-producing ability of the strains tested. The results showed that most rhizobial strains were unable to grow directly on CAS-agar plates. M. ciceri had the highest number of CAS + strains (77.3%) and S. meliloti the lowest number (7%). The half-plate method demonstrated the minimum inhibitory effect on the growth of these bacteria. On the basis of this method, around 86% of the tested strains were Sid-producing. However, their abilities were highly different, considering the measured criteria. The best Sid-producers were R__7 and R__38 strains of S. Meliloti, R__305 of R.1.bv phaseoli, R__237 and R__229 of Bradyrhizobium sp., which revealed the highest Sid-production abilities when compared with all strains of P. fluorescens used as the Sid^+ controls.
 
Keyword(s): SIDEROPHORE, RHIZOBIAL STRAINS, PGPR, CHROM AZUROL S REAGENT, CAS-AGAR ASSAY.
 
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