Calcium deficiency increases Cd toxicity and Ca is required for heat-shock induced Cd tolerance in rice seedlings

While growing in the field, plants may encounter several different forms of abiotic stress simultaneously, rather than a single stress. In this study, we investigated the effects of calcium (Ca) deficiency on cadmium (Cd) toxicity in rice seedlings. Calcium deficiency alone decreased the length, fresh and dry weight, and the Ca concentration in shoots and roots. Also, the content of glutathione (GSH), the ratio of GSH/oxidized glutathione, and the activity of catalase were lower in Ca-deficient leaves compared to control leaves. Exogenous Cd caused a decrease in the contents of chlorophyll and protein, and induced oxidative stress. Based on these stress indicators, we found that Ca deficiency enhanced Cd toxicity in rice seedlings. Under exogenous Cd application, internal Cd concentrations were higher in Ca-deficient shoots and roots than in the respective controls. Moreover, we observed that Ca deficiency decreased heat-shock (HS) induced expression of HS protein genes Oshsp17.3, Oshsp17.7, and Oshsp18.0 in leaves thereby weakening the protection system and increasing Cd stress. In conclusion, Ca deficiency enhances Cd toxicity, and Ca may be required for HS response in rice seedlings.