Exogenous citrate and malate alleviate cadmium stress in Oryza sativa L.: Probing role of cadmium localization and iron nutrition

Organic acids play an important role in metal uptake and trafficking in plants. Therefore, the role of exogenous citrate and malate on Cd tolerance was studied in the seedlings of Oryza sativa L. cv MTU 7029. Seedlings were exposed to Cd plus organic acids in hydroponics and monitored changes in Cd accumulation, expression of metal transporters, chlorophyll fluorescence, and antioxidants. It found that organic acid supplements decrease Cd accumulation in leaf because of up-regulation of tonoplast localized heavy metal ATPase (OsHMA3) which allows vacuolar sequestration of Cd in the root. Malic acid helped Cd exclusion in the root too. A shift in Cd speciation from sulphhydryl to the carboxylic group also noticed in the roots of plants exposed to organic acids. Treatment of organic acids was effective to prevent Cd inducible Fe deficiency via up-regulation of the iron regulated transporter (OsIRT1), increase in ferric chelate reductase activity, and formation of Cd stabilized Fe3+ - organic acid complex respectively. Also, exposure to organic acids increased the accumulation of antioxidants such as anthocyanin and glutathione (GSH) under Cd stress. Above changes assisted in upholding of photosynthetic electron transport and biomass productivity during the course of Cd treatment with organic acid supplements.