| Microbial exopolysaccharides(EPS)are a class of bioactive macromolecules secreted by microorganisms,which are widely used in food and pharmaceutical industries because of their structural diversity and various biological activities and good rheological properties.In order to investigate the potential of EPS in agricultural applications,this study was conducted to investigate the physiological and biochemical changes of Enterococcus faecalis CX2-6 under cadmium stress by whole genome sequencing and transcriptome sequencing,followed by the purification and structural analysis of the extracellular polysaccharides of E.faecalis CX2-6.The effects of the EPS of E.faecalis on seed germination and seedling growth under Cd stress were investigated by purification and structural analysis.The main results of the study were as follows.(1)The genome sequence of strain CX2-6 was obtained by whole-genome sequencing and bioinformatics analysis.The genome size of the strain was 2559204 bp,encoding a total of 2475 genes,and the genome data were also compared with GO,COG,CAZy and other databases to complete the functional annotation of the genome of strain CX2-6,which was used to further study the physiological metabolic mechanism and genetic analysis of E.faecalis CX2-6.This will support further research on the physiological and metabolic mechanisms and genetic characteristics of Enterococcus faecalis CX2-6.(2)E.faecalis CX2-6 has good tolerance to the heavy metal Cd,and Cd stress has significantly inhibited the growth and metabolism of the bacterium,and the growth of the bacterium was very slow at the Cd concentration of 4g-L-1,and the morphology of the bacterium showed obvious wrinkling.With the increase of Cd concentration,the inhibition of physiological processes such as growth,acid production and nutrient metabolism of the bacterium became more and more obvious.The transcriptome GO differential analysis showed that Cd stress mainly promoted the expression of genes related to protein translation in E.faecalis CX2-6,while the differential expression of genes in E.faecalis was mainly concentrated in carbohydrate and anion transporter genes between different concentrations of Cd stress.The effects of different concentrations of Cd stress on differential metabolic genes in E.faecalis were mainly concentrated in the ABC transporter protein metabolic pathway.(3)The EPS of E.faecalis CX2-6 was purified by DEAE-52 and dextran gel G-100 column chromatography,and two pure fractions,EPS-1 and EPS-2,were isolated.Scanning electron microscopy revealed that the surface of EPS-1 appeared as rough flakes,while EPS-2 was irregularly twisted flakes.The UV scan results showed that both EPS-1 and EPS-2 did not contain nucleic acids and proteins.Infrared spectroscopy analysis showed that both EPS-1 and EPS-2 had the typical characteristic absorption peaks of polysaccharides.(4)The effect of E.faecium EPS on rice seed germination and seedling growth under Cd stress was investigated.Different concentrations of EPSwere added to the culture medium that contained Cd to investigate the metabolic changes of rice seed germination and seedling growth.The results showed that Cd stress significantly inhibited the germination efficiency of rice seeds and decreased the development of various organs of the seedlings,whereas the addition of different concentrations of EPS significantly improved the germination potential,germination rate,and root development of rice seeds under Cd stress;however,the germination of rice buds was significantly inhibited after germination.This inhibition was not observed during the growth stage of the rice seedlings.Under Cd stress,the content of flavonoids and anthocyanins in the rice seedlings treated with EPS increased 18.6%–208.7% and the activity of superoxide dismutase and peroxidase also increased,whereas the malondialdehyde content decreased significantly.Results from the Fourier transform infrared test showed that the contents of carbohydrates,proteins,lipids,and carboxylates in rice leaves were increased with the addition of EPS under Cd stress,which might decrease the toxic effect of Cd in rice by enhancing the osmotic effect in leaves.The results of this study indicate that the EPS of E.faecium can significantly mitigate the toxic effects of Cd stress on rice seed sprouting and seedling growth.(5)In this study,we found that the addition of different concentrations of EPS of E.faecalis could alleviate the toxic effects of heavy metal Cd on rice roots to some extent,increase the contents of soluble sugar and soluble proteins,APX enzyme activity,and pectin in rice roots,and reduce the degree of cell damage and the content of reactive oxygen species in rice roots.By measuring the Cd content in different parts of rice,we found that the EPS inhibited the transport of Cd to the aboveground by immobilizing more Cd’s in the rice roots,which finally alleviated the toxic effect of Cd on the aboveground rice. |
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