Abstract
Soil contamination with heavy metals is a constraint for plant establishment and development for which phytoremediation may be a solution, since rhizobacteria may alleviate plant stress under these conditions. A greenhouse experiment was conducted to elucidate the effect of toxic metals on growth, the activities of ROS (reactive oxygen species)-scavenging enzymes, and gene expression of Medicago sativa grown under different metal and/or inoculation treatments. The results showed that, besides reducing biomass, heavy metals negatively affected physiological parameters such as chlorophyll fluorescence and gas exchange, while increasing ROS-scavenging enzyme activities. Inoculation of M. sativa with a bacterial consortium of heat- and metallo-resistant bacteria alleviated metal stress, as deduced from the improvement of growth, lower levels of antioxidant enzymes, and increased physiological parameters. The bacteria were able to effectively colonize and form biofilms onto the roots of plants cultivated in the presence of metals, as observed by scanning electron microscopy. Results also evidenced the important role of glutathione reductase (GR), phytochelatin synthase (PCS), and metal transporter NRAMP1 genes as pathways for metal stress management, whereas the gene coding for cytochrome P450 (CP450) seemed to be regulated by the presence of the bacteria. These outcomes showed that the interaction of metal-resistant rhizobacteria/legumes can be used as an instrument to remediate metal-contaminated soils, while cultivation of inoculated legumes on these soils is still safe for animal grazing, since inoculation with bacteria diminished the concentrations of heavy metals accumulated in the aboveground parts of the plants to below toxic levels.
Highlights
According to FAO, which declared the past year 2015 as the International Year of the Soils [1], the pressure of the global human population, together with the increasing demand for food, feed, biodiesel, wood, fibers, etc. will force farmers to cultivate plants on poor, degraded, or moderatelypolluted soils [2,3]
Results evidenced the important role of glutathione reductase (GR), phytochelatin synthase (PCS), and metal transporter NRAMP1 genes as pathways for metal stress management, whereas the gene coding for cytochrome P450 (CP450) seemed to be regulated by the presence of the bacteria
These outcomes showed that the interaction of metal-resistant rhizobacteria/legumes can be used as an instrument to remediate metal-contaminated soils, while cultivation of inoculated legumes on these soils is still safe for animal grazing, since inoculation with bacteria diminished the concentrations of heavy metals accumulated in the aboveground parts of the plants to below toxic levels
Summary
According to FAO, which declared the past year 2015 as the International Year of the Soils [1], the pressure of the global human population, together with the increasing demand for food, feed, biodiesel, wood, fibers, etc. will force farmers to cultivate plants on poor, degraded, or moderatelypolluted soils [2,3]. Some medicinal plants have been proposed to be grown on polluted soils [6,7], since they behave as metal excluders [8,9], and the low concentrations of metals accumulated in shoots do not affect the ulterior quality of extracted oils or bioactive substances [10,11,12] In this context, legumes are optimum candidates to adapt to degraded soils, to those affected by moderate heavy metal pollution. The health and the robustness of plants are enhanced, facilitating the adaptation of plants under stressful conditions [26,27] These bacteria can contribute to the process of phytoremediation, through mechanisms including improved solubilization of metals, siderophore production, production of organic acids and biosurfactants, reduction/oxidation, methylation, precipitation, and biosorption that affect the bioavailability of metals in soils and sediments [22,28,29]. The present study was conducted with the following objectives: (1) to evaluate the consequence of inoculation with previously selected PGPR on the growth, mineral nutrition, and physiological responses of Medicago sativa to heavy metal stress; (2) to evaluate the effect of PGPR on heavy metal uptake; and (3) to analyze the effect on the activities of ROS-scavenging enzymes and expression level of marker genes belonging to different pathways related to stress management
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