Interaction of plant genomic DNA with arsenic and cadmium: Double trouble or protective pair

Arsenic (As^III) and cadmium (Cd^II) are among the most toxic environmental pollutants that frequently co-occur in contaminated soils, due to industrial and agricultural activities. While their individual toxic effects are well-documented, their combined effects remain poorly understood. The present study deals with UV-Vis-NIR spectroscopy-based interaction studies of As^III, Cd^II and As^III + Cd^II, independently with plant genomic DNA under aqueous conditions. While the DNA spectral pattern at 260 nm remained unchanged with As^III or Cd^II, significant hyperchromic shift was observed under As^III + Cd^II, suggesting a synergistic response in terms of base pair unstacking and helix destabilization. The cleavage pattern obtained with restriction enzyme HindIII revealed that DNA accessibility under As^III + Cd^II was significantly different than As^III and Cd^II and comparable to those of control, indicating either the competitive binding or As^III + Cd^II complexation. The As^III + Cd^II interaction was further supported by bimodal DNA surface-charge distribution. The post-germination phenotyping highlighted antagonistic growth responses under As^III + Cd^II, wherein the whole seedling biomass in the model plant Arabidopsis thaliana was comparable to those of As^III-alone, while maximal growth reduction was seen under Cd^II treatment. Interestingly, the accumulation of total As and Cd remained unchanged, suggesting that As^III and Cd^II interaction did not affect their uptake potential by the plants. Further, the As^III + Cd^II growth antagonism was limited to only 1:1 ratio till 50 μM concentration, beyond which the co-exposure led to synergistic toxicity. Taken together, the study provided evidence about 'effect modification' under As^III + Cd^II in terms of altered DNA interaction and weakened accessibility, thereby explaining As^III + Cd^II antagonism in plants.