Enhancing phytoremediation of chromium-stressed soils through plant-growth-promoting bacteria

Abstract

Chromium, specifically hexavalent chromium is one of the most toxic pollutants that are released into soils by various anthropogenic activities. It has numerous adverse effects not only on plant system but also on beneficial soil microorganisms which are the indicators of soil fertility and health. Recent emergence of phytoremediation as an environmental friendly and economical approach to decontaminate the chromium stressed soils has received wider attention. But major drawback of this process is that it takes long time. Application of multifunctional plant-growth-promoting bacteria (PGPB) exhibiting chromium resistance and reducing traits when used as bioinoculants with phytoremediating plants, has resulted in a better plant growth and chromium remediating efficiency in a short time span. PGPB improve chromium uptake by modifying root architecture, secreting metal sequestering molecules in rhizosphere and alleviating chromium induced phytotoxicity. The purpose of this review is to highlight the plant-beneficial traits of PGPB to accelerate plant-growth and concurrently ameliorate phytoremediation of chromium contaminated soils.

Keywords: ACC deaminase; Bioremediation; Chromium; Cr(VI) reduction; Heavy metal; Phytoremediation; Plant growth promoting bacteria.

Figure 1

Chromium-induced changes and effects of different metabolites/activities of plant growth promoting bacteria (PGPB) on plants.

Figure 2

Schematic depiction of chromium resistance and toxicology in bacterial cell: (1) chromate due to the structural similarity with sulfate enters the bacterial cell through sulfate transporter encoded by the chromosomal DNA. (2) Plasmid DNA encoded efflux systems are used to expel the intracellular chromates outside the bacterial cell to resist the chromate toxicity. (3) Aerobic Cr⁶⁺ reduction into Cr³⁺ involves soluble reductase which requires NAD(P)H as an electron donor while anaerobic Cr⁶⁺ reduction occurs in the electron transport pathway by cytochrome b (cyt b) or cytochrome c (cyt c) along the respiratory chains in the inner membrane; Cr³⁺ cannot pass the bacterial cell membranes due to the insolubility of Cr³⁺ derivatives. (4) Membrane-embedded chromate reductase which is encoded by the chromosomal DNA, reduces Cr⁶⁺ anaerobically in the presence of electron donors. (5) Cr⁵⁺ produced during the redox cycle of Cr⁶⁺ produces oxidative stress by the production of reactive oxygen species (ROS). (6) To combat the ROS generated oxidative stress, protective metabolic enzymes superoxide dismutase, catalase and glutathione are secreted. Some outer membrane proteins are also involved to counter the oxidative stress. (7) Cr⁶⁺ and principally Cr³⁺ not only negatively affects DNA replication and RNA transcription by damaging DNA but also alters gene expression. In addition, Cr³⁺ also damages proteins by impairing their functions. (8) DNA repair system is activated in order to repair the damaged DNA (Source: Ahemad [3]).