Abstract
Abiotic stresses in general and extracellular acidity in particular disturb and limit nitrogen-fixing symbioses between rhizobia and their host legumes. Except for valuable molecular-biological studies on different rhizobia, no consolidated models have been formulated to describe the central physiologic changes that occur in acid-stressed bacteria. We present here an integrated analysis entailing the main cultural, metabolic, and molecular responses of the model bacterium Sinorhizobium meliloti growing under controlled acid stress in a chemostat. A stepwise extracellular acidification of the culture medium had indicated that S. meliloti stopped growing at ca. pH 6.0–6.1. Under such stress the rhizobia increased the O2 consumption per cell by more than 5-fold. This phenotype, together with an increase in the transcripts for several membrane cytochromes, entails a higher aerobic-respiration rate in the acid-stressed rhizobia. Multivariate analysis of global metabolome data served to unequivocally correlate specific-metabolite profiles with the extracellular pH, showing that at low pH the pentose-phosphate pathway exhibited increases in several transcripts, enzymes, and metabolites. Further analyses should be focused on the time course of the observed changes, its associated intracellular signaling, and on the comparison with the changes that operate during the sub lethal acid-adaptive response (ATR) in rhizobia.
Highlights
Abiotic stresses in general and extracellular acidity in particular disturb and limit nitrogen-fixing symbioses between rhizobia and their host legumes
This information on the acidic pHlimit was used in a subsequent experiment to set up a continuous culture at pH 6.1 in order to have rhizobial cells growing just 0.1 pH unit above the condition where they had reached their limit of acid tolerance
In order to characterize the bacterial changes induced by low pH, we set up S. meliloti steady-state cultures in a chemostat, an experimental system allowing a strict control of cultivation parameters
Summary
Abiotic stresses in general and extracellular acidity in particular disturb and limit nitrogen-fixing symbioses between rhizobia and their host legumes. After performing a complementary study using proteomic tools, Reeve et al.[43] suggested that the folding, proteolysis, and transport processes were key activities in S. medicae growing in acidity While all these studies were performed on low-pH batch cultures, Hellweg et al.[44] undertook an in-depth time-course analysis of the transcriptomic response in S. meliloti after shifting the bacterial cells from neutral to acid pH. The work revealed that a short-term exposure of the bacteria to low pH was sufficient to induce significant transcriptional changes in diverse rhizobial genes All these studies provided useful—albeit fragmented—information on the phenotypes, and in certain examples, on the molecular components displayed by the rhizobia after challenge with a high concentration of extracellular hydrogen ions
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