Abstract:The main aim of this study was to deal with isolation, characterization and identification IAA producing bacteria from paddy rice cultivated soil samples collected in rice-shrimp farming system in salt affected areas of the Mekong Delta of Vietnam. Fifteen soil samples collected from the rice crop in the rice-shrimp farming system of 5 different provinces within the Mekong Delta, Vietnam were used to isolate the IAA producing bacteria. NBRIP media was used to isolate the potentially IAA producing bacteria. Result showed that 213 isolates were obtained from the fifteen different soil samples. Out of 45 indole acetic acid producing isolates, ten were selected as efficient producers. One out of ten, the isolate ST2-1 was identified as the most promising strain as efficient biofertilizer inoculants to promote plant growth. This strain produced 33.13 mg.L -1 as the highest concentration of IAA after 8 days of incubation. As well, this strain stimulated the growth of rice's roots and dry biomass after 7 days of experiment with Hoagland medium containing 0.3% NaCl in 100 mL glass tube. This strain produced well IAA production in pH variation range of medium between 5 and 9 and this bacterial strain can resist to salinity of environment up to 3% NaCl. However, this bacterial was highly inhibited by not only 3 commonly used antibiotics: Ampicillin, Chloramphenicol and Steptomycin at recommended rates but also 3 plant pathology fungicides: Thiophanate Methyl, Propineb and Mancozed regarding to IAA production ability. The results of the 16S rRNA gene sequence analysis showed that this IAA producer coded as ST2-1 was genetically identified as species of Bacillus megaterium ST2-9 since 99% of its sequence affiliated with Bacillus megaterium. In conclusion, the study suggests the IAA producing bacteria as efficient biofertilizer inoculants to promote plant growth.

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This study evaluated the effects of some environmental conditions on IAA biosynthesizing capacity of four nitrogen fixing bacteria, namely Paenibacillus cineris TP-1.4, Bacillus megaterium MQ-2.5, Klebsiella pneumoniae OM-17.2, and Pseudomonas boreopolis CP-18.2. Carbon source, pH, NaCl, and tryptophan supplement treatments were set to investigate the effects of those environmental factors on IAA synthesis. The IAA synthesizing capacity of bacterial strains in liquid medium was measured spectroscopically following incubation by Salkowski's reagent method. The results showed that, under the sucrose amendment, the IAA concentrations produced by all four bacterial strains were significantly higher than those of the other four carbon source added treatments. Two of the four bacterial strains produced the highest yield of IAA in liquid medium at pH 7 (TP-1.4 and OM-17.2), whereas pH 8 was optimum for the other two strains (MQ-2.5 and CP-18.2). The MQ-2.5 strain could synthesize IAA fairly well in up to 5% NaCl and produced the highest amount of IAA with 1% NaCl. Furthermore, IAA synthesizing capability of tested bacterial strains increased sharply along with increasing tryptophan content in culture medium except for the TP-1.4 strain. From the current study, these isolates emerged as possible alternatives for future IAA production for plant growth and yield enhancement. Hence, they have a great potential to be used as bio-inoculants for plant growth promotion in eco-friendly and sustainable agriculture.

This study assessed antagonistic capacity of various indigenous microorganisms (IMO) collected from different cropping systems within Soc Trang province against plant pathogenic fungus including Fusarium oxysporum and Rhizoctonia solani. Biocontrol activity of fifteen collected IMOs was investigated on PDA agar media for 5-7 days under laboratory conditions with three different scenarios. IMO and pathogenic fungus were incubated at the same time and IMO was introduced before and after inoculation of plant pathogenic fungus. The results illustrated that all surveyed IMOs were found to have highly potential biocontrol against two plant pathogenic fungi to different extents and IMOs which were introduced before the inoculation of pathogenic fungi showed the highest efficiency in biocontrol of plant pathogen. Particularly, four out of fifteen IMOs which were collected from bamboo, shallot, grapefruit and guava farms showed their highest antagonistic efficacy on Fusarium oxysporum biocontrol by completely decaying fungal hyphae of this fungal strain after seven incubation days. For Rhizoctonia solani, all IMOs displayed highly antagonistic ability with inhibitory percentages varying between 52.96% and 92.59% after two days. The antagonistic functions of all collected IMOs could be exploited for plant protection from plant-pathogenic fungus.