Targeting antioxidant pathways for improved tolerance to chromium exposure

With advancement in functional genomics, it is now possible to engineer plants for heavy metal tolerance by altering the expression of proteins and genes involved in heavy metal stress resistance. The present study was carried out to evaluate the tomato transgenic plants overexpressing the genes associated with ascorbate-glutathione (AsA-GSH) pathway, namely, <i>APX</i> (ascorbate peroxidase), <i>MDHAR</i> (mono-dehydroascorbate reductase), <i>DHAR</i> (dehydroascorbate reductase), <i>GR</i> (glutathione reductase), and <i>SOD</i> (superoxide dismutase) for the response to chromium stress (Cr). Transgenic tomato plants with altered antioxidant pathway had higher levels of carotenoids (41.64%) and anthocyanins (34.60%) besides improved photosynthetic rate, transpiration, and stomatal conductance compared to untransformed wild type (WT) when supplied with potassium dichromate (K₂Cr₂O₇) at a concentration of 100 µM. Moreover, transgenic tomato showed increased osmolytes and phenolic concentrations with a simultaneous reduction in electrolyte leakage, malondialdehyde (MDA) and reactive oxygen species (ROS). Transcript analysis indicated higher expression of all the transgenes, whereas scanning electron microscopy (SEM) revealed lesser deformities in transgenic plants in response to Cr stress. Transgenic lines accumulated higher Cr in leaves (68%) and roots (56.89%) compared to the WT under Cr stress. We conclude that overexpressing the AsA-GSH pathway in tomato makes them suitable for use as phytoremediator.