Endophytic bacteria mitigate mercury toxicity to host plants

Abstract

Plant communities growing in metal-contaminated areas can develop resistance mechanisms by establishing symbiotic associations with endophytic microorganisms. The functionality and diversity of endophytic communities depend on the amount and type of metal present in the soil. To characterise the response of endophytic bacterial communities to mercury-induced abiotic stress, we analysed the colonization frequency and number of bacterial isolates in the roots of Aeschynomene fluminensis (Joint Vetch) and Polygonum acuminatum (Smartweed), which represent the families Fabaceae and Polygonaceae, respectively. These two plant species are found in many mercury-contaminated areas. The isolates were characterised by morpho- and genotyping and identified by 16S rDNA gene sequencing. The bacteria belonged to the phyla Actinobacteria, Bacteriodetes, Firmicutes, and Proteobacteria. The Hill series and Venn diagram provided evidence that mercury affects the composition, diversity, and richness of the endophytic bacterial communities. Inoculation with Bacillus_sp_BacI34, Burkholderia_sp_BacI45, Enterobacter_sp_BacI14, Enterobacter_sp_BacI26, Enterobacter_sp_BacI18, Klebsiella_pneumoniae_BacI20, Lysobacter_soli_BacI39, Pantoea_sp_BacI16, and Pantoea_sp_BacI23 promoted the growth of corn (Zea mays) plants in mercury-supplemented substrata. It is noteworthy that Pantoea sp_BacI23 increased the host plant length (root and shoot) by 117.09 ± 0.28%. Endophytic bacterial strains may well provide important inoculants for plant growth promotion on metal-contaminated sites and in metal bioremediation programs.