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Mechanism of acid tolerance in a rhizobium strain isolated from Pueraria lobata (Willd.) Ohwi

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The Rhizobium sp. strain PR389 was isolated from the root nodules of Pueraria lobata (Willd.) Ohwi, which grows in acidic (pH 4.6) yellow soil of the Jinyun Mountains of Beibei, Chongqing, China. While rhizobia generally have a pH range of 6.5–7.5 for optimum growth, strain PR389 grew in a liquid yeast extract – mannitol agar medium at pH 4.6, as well as in a pH 4.1 soil suspension, suggesting acid tolerance in this specific strain of rhizobium . However, at pH 4.6, the lag phase before vigorous growth was 40 h compared with 4 h under neutral conditions (pH 7.0). For PR389, the generation time after the lag phase remained the same at different pH levels despite the different durations of the lag phase. Except in the pH 4.4 treatment, the pH of the culturing media increased from 4.6, 4.8, 5.0, and 5.5 to neutral and slightly alkaline after 70 h of culture. Chloramphenicol was added to determine if protein production was involved in the increasing pH process. Chloramphenicol significantly inhibited PR389 growth under acid stress but had little effect under neutral conditions. Proton flux measured during a short acid shock (pH 3.8) revealed that this strain has an intrinsic ability to prevent H+ from entering cells when compared with acid-sensitive rhizobia. We propose that the mechanism for acid tolerance in PR389 involves both intracellular and extracellular processes. When the extracellular pH is lower than pH 4.4, the cell membrane blocks hydrogen from entering the cell. When the pH exceeds 4.4, the rhizobium strain has the ability to raise the extracellular pH, thereby, potentially decreasing the toxicity of aluminum in acid soil.

Keywords: Pueraria lobata (kudzu); acid tolerance mechanism; criblage; intracellular and extracellular process; mécanisme de tolérance à l’acidité; processus intracellulaire et extracellulaire; rhizobium; screening

Document Type: Research Article


Affiliations: 1: Centre of Microbiology, College of Resources and Environment, Southwest University, Chongqing 400716, People’s Republic of China. 2: Chongqing Normal University, Chongqing 400047, People’s Republic of China. 3: Southern Fruit Tree Institute, Chongqing 402260, People’s Republic of China. 4: College of Animal Science, Southwest University, Chongqing 400716, People’s Republic of China.

Publication date: 2011-07-01

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  • Published since 1954, this monthly journal contains new research in the field of microbiology including applied microbiology and biotechnology; microbial structure and function; fungi and other eucaryotic protists; infection and immunity; microbial ecology; physiology, metabolism and enzymology; and virology, genetics, and molecular biology. It also publishes review articles and notes on an occasional basis, contributed by recognized scientists worldwide.
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