AUTHOR=Arregui Gabriela , Hipólito Pablo , Pallol Beatriz , Lara-Dampier Victoria , García-Rodríguez Diego , Varela Higinio P. , Tavakoli Zaniani Parinaz , Balomenos Dimitrios , Paape Timothy , Coba de la Peña Teodoro , Lucas M. Mercedes , Pueyo José J. 

TITLE=Mercury-Tolerant Ensifer medicae Strains Display High Mercuric Reductase Activity and a Protective Effect on Nitrogen Fixation in Medicago truncatula Nodules Under Mercury Stress

JOURNAL=Frontiers in Plant Science

VOLUME=11

YEAR=2021

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2020.560768

DOI=10.3389/fpls.2020.560768

ISSN=1664-462X

ABSTRACT=<p>Mercury (Hg) is extremely toxic for all living organisms. Hg-tolerant symbiotic rhizobia have the potential to increase legume tolerance, and to our knowledge, the mechanisms underlying Hg tolerance in rhizobia have not been investigated to date. Rhizobial strains of <italic>Ensifer medicae</italic>, <italic>Rhizobium leguminosarum</italic> bv. <italic>trifolii</italic> and <italic>Bradyrhizobium canariense</italic> previously isolated from severely Hg-contaminated soils showed different levels of Hg tolerance. The ability of the strains to reduce mercury Hg<sup>2+</sup> to Hg<sup>0</sup>, a volatile and less toxic form of mercury, was assessed using a Hg volatilization assay. In general, tolerant strains displayed high mercuric reductase activity, which appeared to be inducible in some strains when grown at a sub-lethal HgCl<sub>2</sub> concentration. A strong correlation between Hg tolerance and mercuric reductase activity was observed for <italic>E. medicae</italic> strains, whereas this was not the case for the <italic>B. canariense</italic> strains, suggesting that additional Hg tolerance mechanisms could be playing a role in <italic>B. canariense</italic>. Transcript abundance from <italic>merA</italic>, the gene that encodes mercuric reductase, was quantified in tolerant and sensitive <italic>E. medicae</italic> and <italic>R. leguminosarum</italic> strains. Tolerant strains presented higher <italic>merA</italic> expression than sensitive ones, and an increase in transcript abundance was observed for some strains when bacteria were grown in the presence of a sub-lethal HgCl<sub>2</sub> concentration. These results suggest a regulation of mercuric reductase in rhizobia. Expression of <italic>merA</italic> genes and mercuric reductase activity were confirmed in <italic>Medicago truncatula</italic> nodules formed by a sensitive or a tolerant <italic>E. medicae</italic> strain. Transcript accumulation in nodules formed by the tolerant strain increased when Hg stress was applied, while a significant decrease in expression occurred upon stress application in nodules formed by the Hg-sensitive strain. The effect of Hg stress on nitrogen fixation was evaluated, and in our experimental conditions, nitrogenase activity was not affected in nodules formed by the tolerant strain, while a significant decrease in activity was observed in nodules elicited by the Hg-sensitive bacteria. Our results suggest that the combination of tolerant legumes with tolerant rhizobia constitutes a potentially powerful tool in the bioremediation of Hg-contaminated soils.</p>