Green Guardians: Unveiling the Arsenic Shielding Powers of Endophytes Pseudomonas PD9R and Rahnella laticis PD12R in Phytoremediation

Arsenic contamination in soils is a significant environmental and public health hazard. It can be released into the soil through human activities such as mining, industrial processes, and the use of certain pesticides and herbicides. It is toxic to humans and with long-term exposure, even at low levels, can lead to serious health problems. This includes skin lesions, cardiovascular diseases, and an increased risk of several types of cancers. The arsenic in soil can be taken up by plants where it enters the food chain and potentially affecting human health through food consumption. Moreover, high levels of arsenic in soil can inhibit plant growth and reduce crop yields. Furthermore, arsenic in soil can lead to the contamination of groundwater and this is a significant concern in areas where groundwater is used for drinking, irrigation, and other purposes. Traditional remediation techniques, though effective, are costly and environmentally invasive, underscoring the need for alternative solutions like phytoremediation. Phytoremediation offers a promising, sustainable approach for the remediation of arsenic-contaminated soils, leveraging the symbiotic relationship between plants and their microbiome.  This green technology employs plants and their associated microorganisms, including endophytes like Pseudomonas sp. PD9R and Rahnella laticis PD12R, to stabilize or extract contaminants. Understanding the response of these endophytes to arsenic is critical for enhancing phytoremediation efficiency. In a new study published in the journal Environmental Science & Technology led by Robert Tournay, Dr. Andrea Firrincieli, Shruti Parikh, Dominic Sivitilli, and Professor Sharon Doty from the School of Environmental and Forest Sciences at University of Washington, the authors investigated two endophytes, Pseudomonas sp. PD9R and Rahnella laticis PD12R, exploring their arsenic resistance, EPS (Extracellular Polymeric Substances) synthesis, biofilm formation, and plant growth-promoting capabilities. PD12R demonstrates remarkable tolerance to arsenic, exhibiting significant EPS production and biofilm formation under arsenic stress, which is potentially advantageous for phytoremediation processes.

The authors employed a multifaceted approach, combining in vitro and in silico methods. Endophytes were isolated from plants in arsenic-contaminated soils and subjected to various arsenic concentrations to assess their tolerance and response. Using Murashige & Skoog Basal Medium, endophytes were isolated from native plants in arsenic-contaminated soils. The endophytes were inoculated in Arabidopsis thaliana to evaluate their effect on plant growth under arsenic stress. EPS synthesis and biofilm formation were quantified under varying arsenic concentrations to understand the bacterial response. The researchers conducted comparative genomic analysis  to understand the genetic basis of arsenic resistance and EPS synthesis.

The team showed that PD12R displayed a superior tolerance to arsenic compared to PD9R, surviving at concentrations five times higher. Interestingly, while both strains synthesized EPS in response to arsenic, PD12R demonstrated a more pronounced ability to form biofilms at high arsenic concentrations. This capacity to sequester arsenic in biofilms could be key in mitigating phytotoxicity.  The inoculation of A. thaliana with these endophytes yielded varied results. PD9R enhanced plant growth at lower arsenic levels, whereas PD12R’s impact was less pronounced. This suggests a complex interplay between endophyte arsenic resistance mechanisms and plant growth promotion under stress. Moreover, both PD9R and PD12R increased EPS synthesis in response to arsenic, a defense mechanism against heavy metal stress. However, PD12R’s ability to form resilient biofilms even at high arsenic concentrations sets it apart, potentially offering a buffer against arsenic toxicity. The genomic analysis revealed unique arsenic resistance mechanisms in both strains, divergent from typical plant-associated bacteria. While PD9R’s genome harbored diverse arsenic resistance genes, PD12R’s simpler genetic makeup was equally effective in arsenic tolerance. These findings underscore the genetic variability and adaptability among endophytes in arsenic-contaminated environments.

The study by Robert Tournay and colleagues highlights the significant role of endophytes in arsenic phytoremediation. PD12R, with its superior arsenic tolerance, EPS synthesis, and biofilm formation abilities, emerges as a potential candidate for enhancing phytoremediation efficiency. The interaction between these endophytes and plants under arsenic stress warrants further investigation, particularly focusing on the in planta dynamics of biofilm formation and EPS production. Ultimately, leveraging the unique capabilities of endophytes like PD9R and PD12R could significantly advance the field of phytoremediation, offering a sustainable solution to the global challenge of arsenic contamination.