Interactions of plants with the environment: optimization of phytoremediation and mechanisms of phytotoxicity of contaminating chemicals

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Plants are exposed to environmental contaminants such as polycyclic aromatic hydrocarbons (PAH), petroleum hydrocarbons (PHC), metals and salt. These contaminants are toxic to plants, and many act via reactive oxygen species (ROS). Based on toxicity mechanisms, strategies for stress alleviation can be used to improve phytoremediation. Two on-going projects are optimization of phytoremediation and mechanisms of phytotoxicity. ***Optimization of Phytoremediation: Due to persistence of contaminants, remediation is often required. Phytoremediation holds great promise because plant roots can provide large sinks for partitioning of contaminants out of soil. Phytoremediation has faced many obstacles; prominent among these is phytotoxicity of the contaminants. Based on our toxicology work, we have developed strategies to achieve tolerance to contaminants - a key aspect is plant growth with plant growth promoting rhizobacteria (PGPR). This led us to develop PGPR-Enhanced Phytoremediation Systems (PEPS), which show accelerated plant growth and remediation in contaminated soils. For PHC and PAH, plants support microbial communities in the soil and the microbes metabolize the PHC and PAH. For salt, the ions are taken up into foliage and can be removed from the site along with the foliage. Phytoremediation is being optimized at the laboratory and field levels for PHC and salt remediation. To make PEPS more effective, we need to better understand the mechanisms of plant stress and acclimation during phytoremediation. To achieve this we will study the performance of grasses and cereals during phytoremediation by examining photosynthesis, flavonoids, ROS scavenging and gene expression. We will also examine enhancement of phytoremediation of PHC using calcium peroxide and emulsifiers. The former will increase soil O2 levels for contaminant metabolism and the latter will render the PHC more bioavailable.***Mechanisms of phytotoxicity of environmental contaminants: We have shown that PAH and PHC are phytotoxic and their toxicity is enhanced by UV radiation. In many instances, damage is to the photosynthetic apparatus via ROS. Further, we have found that metals synergize PAH and PHC toxicity via ROS mechanisms. We have also found that PHC, PAH, metals and salt activate many of the same acclimation processes via ROS signalling. To better understand mechanisms of phytotoxicity, we will examine the effects of PHC, PAH, salt and metals on Brassica napus, grasses and cereals by examining changes to photosynthesis, flavonoids, ROS scavenging enzymes and gene expression. We have recently found that during phytoremediation of soils, but prior to reaching predicted non-toxic levels, the contaminants in the soil become non-toxic. We will examine the mechanistic bases for loss of toxicity of the contaminants in the soil, which should point to the key mechanisms of contaminant phytotoxicity.