Role of Metal Tolerant Rhizobacteria on Phytoremediation of Cd and Ni Contaminated Soils by Catharanthus Roseus L. G. Don

Heavy metal contaminated soils are considered as hazardous for our environment and human health as these cause toxification of food commodities and ground water reserves. Numerous physico-chemical and biological techniques can be employed for the decontamination of metal polluted soils. However, most of the physico-chemical approaches are not cost effective and eco-friendly. While, the biological technologies being economical and ecologically green are most suitable option for remediation of metal affected regions. Among biological techniques, phytoremediation which involves the application of green plants for heavy metals sequestering from contaminated sites, has got much attention now a days. Since, heavy metal toxicity severely affects growth and biomass production of plants and subsequently slows down the phytoremediation process. Therefore, it becomes essential to find out some ecofriendly phenomenon which sustains phytoextraction potential of plants. It has been observed that certain Plant Growth Promoting Rhizobacteria may modulate innate stress resistance in plants against a number of biotic and abiotic stresses. The metal resistant growth promoting rhizobacteria possess the ability to improve growth, biomass production, and mitigate stress in plants growing in heavy metal infected soils. Consequently, it was assumed that native metal tolerant rhizospheric bacterial strains may manage metal toxicity and perhaps augment the phytoremediation capability of Catharanthus roseus. In this study, the symbiotic role of native metal tolerant rhizobacteria and C. roseus plants was analyzed regarding metal stress alleviation and phytoremediation of cadmium and nickel contaminated soils. In the first phase of study, survey was performed to assess the main metal pollutants and their respective levels in the agricultural soil irrigated by industrial effluents contaminated water of Nullah Daik, district Sheikhupura, Pakistan. The composite soil samples of these sites revealed presence of Cd and Ni in a level which cause phytotoxicity and perhaps result food contamination. From soil samples and bacterial conservatory, 11 Cd-tolerant and 14 Ni-tolerant rhizobacterial strains were screened and identified. Here two strains, viz: Cd-tolerant Burkholderia cepacia CS8 and Ni-tolerant Bacillus megaterium MCR-8 provided most significant results in the terms of phosphate solubilization, auxin, gibberellic acid, siderophore and ACCD synthesis. ii The Cd-tolerant B. cepacia CS8 and Ni-tolerant B. megaterium MCR-8 exhibited pronounced results for metal bioavailability, biosorption and MIC, attributes assisting in phytoremediation. The assessment of in vitro growth biomarkers of inoculated C. roseus seedlings including germination percentage, plant growth and vigor index, once again revealed supremacy of B. cepacia CS8 and B. megaterium MCR-8 on rest of the tested bacterial strains. Therefore, B. cepacia CS8 and B. megaterium MCR-8 were evaluated for downstream phytoremediation experimentation by C. roseus under green house conditions. However, B. cepacia CS8 inoculation improved growth, biomass, gas exchange, chlorophyll contents and declined malondialdehyde and hydrogen peroxide in C. roseus plants exposed to Cd stress. Moreover, B. cepacia CS8 diluted the Cd stress by augmenting protein, proline, phenols, flavonoids contents and improving activity of antioxidant enzymes including peroxidase (POD), superoxide dismutase (SOD), ascorbate peroxidase (APX) and catalase (CAT) in C. roseus plants. The inoculated C. roseus plants demonstrated increased metal bioavailability, succeeding higher uptake, bio-concentration factor (BCF), translocation factor (TF), tolerance index (TI) and extraction amount of Cd in case of Cd-contaminated soil. The Ni stress mitigation in B. megaterium MCR-8 inoculated plants was attributed to the reduced levels of MDA and H2O2, enhanced synthesis of protein, proline, phenols, flavonoides in conjunction with enhanced activity of antioxidant enzymes (SOD, CAT, POD, and APX). Furthermore, B. megaterium MCR-8 supplementation improved water extractable metal concentration, BCF, TF, TI and subsequently increased phytoextraction of Ni by C. roseus plants. Both of these bacterial strains are capable to enhance innate metal resistance and phytoremediation potential of C. roseus plants along with growth promotion under heavy metal stress, indicating a great potential for future field applications.