A consolidated analysis of the physiologic and molecular responses induced under acid stress in the legume-symbiont model-soil bacterium Sinorhizobium meliloti

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.

A metabolomic approach to characterize the acid-tolerance response in Sinorhizobium meliloti

“…The acid-stress response in S. meliloti has proven to be a multigenic trait (Draghi et al 2016), where several molecular changes operate in the acid-challenged rhizobia.…”

“…The observed response to acidic pH was not based on a few specific genes, but rather involved whole sets of RNAs, proteins, and metabolites associated with various cellular functions. Under the low-pH conditions imposed in the chemostat, we demonstrated that the pentose-phosphate pathway (PPP) in those cultures exhibited increases in various transcripts, enzymes, and metabolites compared to S. meliloti cells grown at neutral pH (Draghi et al 2016).…”

“…Our laboratory recently contributed to an elucidation of the molecular mechanisms underlying the regulation of the crucial physiologic changes that occur in acid-stressed bacteria. In particular, Draghi et al (2016) reported an integrated analysis entailing a combination of transcriptomic, metabolomic, and proteomic analyses of the responses of S. meliloti under controlled growth rates and pH in chemostat culture. The observed response to acidic pH was not based on a few specific genes, but rather involved whole sets of RNAs, proteins, and metabolites associated with various cellular functions.…”

Characterization of Mutations That Affect the Nonoxidative Pentose Phosphate Pathway in Sinorhizobium meliloti

“…When the amount of EPS I isolated from each of these strains was compared with the intracellular concentrations of the metabolites we determined, EPS production was positively correlated with both the hexose phosphate (R 2 ϭ 0.77) and citrate (R 2 ϭ 0.99) pools, whereas it was negatively correlated with the pentose phosphate (R 2 ϭ 0.88), lactate (R 2 ϭ 0.83), and gluconate (R 2 ϭ 0.58) pools. We note that recent metabolomic characterization of acid-tolerant growth of S. meliloti also showed that there was a correlation between EPS I production and the size of the hexose phosphate pool (43,44). Enzymatic activities can be affected by allosteric effectors and posttranslational modifications such as phosphorylation events.…”

Deciphering the Symbiotic Plant Microbiome: Translating the Most Recent Discoveries on Rhizobia for the Improvement of Agricultural Practices in Metal-Contaminated and High Saline Lands

“…Studies with Mesorhizobium loti suggested heat shock results in a global downregulation of protein expression possibly to conserve energy [99], while salt stress leads to over-expression of ABC transporters and genes associated with nucleotide transport and metabolism [100]. Similarly, characterization of the acid stress response in S. meliloti suggested that this stress results in an elevated metabolic respiration rate [101]. However, to date only a few studies have examined the responses of differentiated bacteroids to environmental stress [102,103], although several studies have examined the responses of the plant partner [104].…”