Responses of Azorhizobium caulinodans to cadmium stress

Abstract

The symbiosis of Azorhizobium caulinodans and an annul legume Sesbania rostrata was recently found to be tolerant to cadmium pollution by an unknown mechanism. In this study, A. caulinodans ORS571 and ZY-20 showed much stronger tolerance to cadmium than a mutant ORS571-X15 and a common Rhizobium sp., with minimum inhibitory concentration values as high as 4 and 5 mM (versus 1 and 0.1 mM) on yeast extract mannitol agar medium, respectively. Although Cd uptake by all three strains of A. caulinodans were mostly from absorption rather than binding (both loosely or tightly) on cell surface, in resistant strains a higher portion of extractable Cd was bound on the cell surface vs. absorbed (about 1:2.5 ratio) compared to the sensitive mutant (about 1:35.1 ratio). These results suggest that certain level of metal exclusion by a permeability barrier was involved in the mechanism of resistance to Cd by A. caulinodans ORS571 and ZY-20. Over the 12-h period of cultivation in yeast extract mannitol agar medium with Cd addition, the Cd concentrations in the outer membrane and periplasm and spheroplast were the highest at the first 3 h, and declined steadily over time. The fact that Cd concentrations in spheroplast of all three strains were many folds higher than those in outer membrane and periplasm, suggests that extracellular sequestration was not the only mechanism of Cd tolerance in A. caulinodans. The decline of Cd concentrations was significantly faster and started earlier in strains ORS571 and ZY-20 than in ORS571-X15. This suggests a second, probably more substantial, mechanism involves active transport of the metal from the cell, e.g., some efflux system for maintaining homeostasis under cadmium stress.

A novel method to alleviate arsenic toxicity in alfalfa plants using a deletion mutant strain of Sinorhizobium meliloti

“…The strain S. meliloti smk956 could tolerate a considerable amount of As (V) in MSY liquid medium and accumulated 2-3 folds more arsenic inside it than its wild type strain S. meliloti Rm1021. Similar result has been reported by Zhengwei et al (2005) in a lipopolysaccharidde mutant of Azorhizobium caulinodans strain ORS571-X15 which accumulated more cadmium inside the cells in comparison to its parental strain Azorhizobium caulinodans strain ORS571.…”

Genomic and Transcriptomic Analysis Reveal Multiple Strategies for the Cadmium Tolerance in Vibrio parahaemolyticus N10-18 Isolated from Aquatic Animal Ostrea gigas Thunberg

“…Bacterial cell membrane permeability and fluidity and cell surface hydrophobicity are key parameters of the cell membrane responding to environmental challenges [ 80 , 81 ]. Cd is a heavy metal whose cations often cause toxicity to both eukaryotic and prokaryotic cells even at low concentrations [ 82 ]. In this study, the outer cell membrane permeability was examined using the NPN probe.…”

The Role of Rhizobial ACC Deaminase in the Nodulation Process of Leguminous Plants

“…The process of symbiotic N 2 -fixation involves a large number of rhizobial genes, namely, the nif genes that encode the nitrogenase enzyme and its iron-molybdenum cofactor [6]. Unfortunately, many of the components of this symbiotic process, such as molecular signaling, rhizobial attachment, root hair curling, and infection thread formation, as well as the nodule formation and nitrogen fixation, are severely affected by various stresses [7][8][9][10][11][12][13][14]. In addition, rhizobia also have to deal with adverse conditions within the host cells, as well as with the plant's innate immunity/response, all of which may interfere with the symbiosis [15].…”