Effect of cadmium on proline accumulation and ribonuclease activity in rice seedlings: Role of proline as a possible enzyme protectant

Abstract

When seedlings of two rice (Oryza sativa L.) cultivars Ratna and Jaya were raised under 100 and 500 μM cadmium nitrate in the medium, a high proline content was noted in Cd2+ stressed seedlings compared to controls. Seedlings grown under 500 μM Cd(NO3)2 maintained increased proline level compared to non-stressed seedlings. Kinetic properties of RNase extracted from control grown and Cd2+ stressed seedlings showed a marked alteration in Km due to Cd2+ treatment. The RNase isoforms were purified from 15-d-old rice seedlings with a total purification of 22.25 fold and 74.75 % yield using conventional biochemical techniques. Three RNase isoforms, namely I, II and III were eluted from DEAE-Sephacel column. The isoform RNase II had Km value of 3.2 mg(RNA) cm-3. The in vitro osmotic stress created by incorporation of PEG in the enzyme assay medium led to decreased affinity of enzyme towards its substrate with increase in Km. This loss in affinity was partially restored by the addition of 1 M proline in the assay medium, suggesting the possible protective role of proline on RNase under osmotic stress.