Regulation of cadmium toxicity in roots of tomato by indole acetic acid with special emphasis on reactive oxygen species production and their scavenging

Toxic impact of cadmium (Cd) on plants is well known which affects their productivity. To mitigate toxic impact of metals such as Cd, exogenous application of phytohormones like indole acetic acid (IAA) has been well recognized in the recent past. But, mechanisms related to the IAA-mediated mitigation of metal toxicity remain elusive. Therefore, in this study, effect of IAA on growth and photosynthetic attributes, nitric oxide, cell viability, reactive oxygen species (ROS) and ascorbate-glutathione cycle (AsA-GSH cycle) was investigated in tomato roots exposed to Cd stress. Cd declined growth and photosynthetic attributes which were accompanied by the excess accumulation of Cd and decreased level of nitric oxide (NO). Among photosynthetic attributes, quantum yield parameters were more sensitive to Cd and these results were in parallel of photosynthetic pigments. However, exogenously applied IAA together with Cd significantly improved level of NO, growth and photosynthetic attributes together with reduced accumulation of Cd. Cd enhanced level of superoxide radical and hydrogen peroxide leading to severe damage to lipids and membranes as indicated by increased level of lipid peroxidation and electrolyte leakage which collectively reduced cell viability of roots. Moreover, components of the AsA-GSH cycle i.e. enzymes (ascorbate peroxidase, monodehydroascorbate reducatse, dehydroascorbate reducatse and glutathione reductase) and metabolites (ascorbate and glutathione) were declined by the Cd. However, addition of IAA with Cd had up-regulated components of the AsA-GSH cycle. Interestingly, application of 2,4,6-triiodo-benzoic acid (TIBA, a polar auxin transport inhibitor) diminished growth attributes and its combination with Cd worsened its toxicity and these events were in parallel with decline in NO content and enhancement in Cd accumulation. The results also showed that IAA was also able in mitigating Cd toxicity in tomato roots even in the presence of TIBA. Overall results show the essentiality of IAA in mitigating Cd stress in tomato roots through NO that up-regulates components of the AsA-GSH cycle for balancing ROS and their associated damages and hence much improved growth and photosynthetic attributes were noticed.