Growth promotion traits and emission of volatile organic compounds of two bacterial strains stimulate growth of maize exposed to heavy metals

Stress alleviation of plants grown in heavy metal contaminated soil has been observed after inoculation with plant growth-promoting bacteria. Here we analyzed individual and combined effects of Pseudomonas fluorescens UM270 and Bacillus paralicheniformis ZAP17 on plant growth of maize (Zea mays L.) subjected to stress through arsenic and mercury exposure. Both strains showed high and regular levels of tolerance to arsenic and mercury metals, respectively. Interestingly, plant growth promoter mechanisms, such as the production of 3-indolacetic acid, solubilization of phosphates, secretion of siderophores and biofilm formation, were maintained under the presence of the two metals, except for phosphate solubilization and the production of siderophores that were inhibited at certain concentrations. The metallic salts modified the mix of volatiles produced by UM270 and ZAP17, with the presence of ketones and aldehydes predominating. We also found compounds like 2-butanone, 2,3-butanediol and dimethyl disulfide, nonanal, hexadecanal, 2-tetradecanon and 2-tridecanone, which have been reported as plant growth promoters. Finally, the co-inoculation of the strains UM270 and ZAP17 in maize plants grown with different concentrations of metallic salts, showed that the presence of HAsNa2O4 stimulated shoot length and plant biomass, the trials with HgCl2 in the presence of the consortium presented a greater development of plant biomass, root and shoot length, while the combination of AsNaO2 and the bacterial consortium allowed to stimulate all the evaluated parameters, as compared with the control plants (without inoculation). These results demonstrate the importance of phytoremediation processes assisted by beneficial microorganisms, further is one of the first that has tried to link the emission of volatile organic compounds with a possible growth promotion effect in conditions of stress caused by heavy metals. In conclusion, the PGPB P. fluorescens UM270 and B. paralicheniformis ZAP17 exert multiple mechanisms of growth promotion, and they represent an excellent option as biopromoters in soils contaminated with heavy metals.

Automated summary

The plant growth-promoting bacteria Pseudomonas fluorescens UM270 and Bacillus paralicheniformis ZAP17 can alleviate stress in maize plants exposed to arsenic and mercury. Both strains showed high tolerance to these metal contaminants and maintained their growth-promoting mechanisms. The presence of metallic salts also affected the production of volatile organic compounds, which are known to promote plant growth. Therefore, these beneficial microorganisms have great potential in phytoremediation of heavy metal-contaminated soils.