Impact of plant-associated bacteria biosensors on plant growth in the presence of hexavalent chromium

Cr(VI) is a highly toxic metal produced by anthropogenic activity which may impact the environment, affecting plants and animals. In plants, chromium both as Cr(III) or Cr(VI) can be absorbed by roots, is poorly translocated and affects negatively plant growth. Plants used in phytoremediation need to cope with chromium toxicity. This work aimed to evaluate strains of Ochrobactrum tritici and Nitrospirillum amazonense , resistant and modified in order to become chromate whole-cell biosensors, as plant-protectors enabling plants to withstand contaminated soils. In vitro tests were performed in three rice varieties and one maize variety. Initial evaluations of Cr(VI) toxicity to plants showed that plants had different sensitivities and BRS 6 CHUÍ rice variety was the most resistant. The metal affected plant growth and development, essentially in roots which were totally inhibited in rice varieties at 500 μM. This effect was plant-dependent. Modified N. amazonense proved to protect maize plants independently of the inoculation dose but O. tritici showed plant specificity and some toxicity when inoculated at high numbers, inhibiting rice development but not maize. Inoculants were directly responsible for growth improvements of specific plant varieties at 1.25 ppm Cr(VI), a concentration which corresponds to a weak soil contamination. Improvements were observed relatively to the Cr(VI)-treated controls, but also relative to the untreated controls, i.e., the benefits went beyond a simple neutralization of inhibition brought by Cr(VI) toxicity. Graphical Abstract