To investigate the molecular and physiological responses in rice roots to cadmium (Cd) stress, a set of hydroponic culture experiments treated with Cd²⁺ at 0, 50, and 100 μmol/L in the solutions were conducted to analyze the differentially expressed proteins in the roots of two rice (Oryza sativa L.) cultivars (Cd-tolerant rice PI312777 and Cd-sensitive rice IR24) 7 days after the treatments. The results showed that 18 proteins were differentially expressed in the roots of PI312777 and IR24 under Cd stress; 12 of them were identified by the MALDI-TOF/MS technique. The biological functions of these identified proteins were involved in four metabolic pathways, including (1) reactive oxygen species (ROS) stress: peroxidase (POD), methionine adenosyltransferase (MAT), and germin-like protein precursor; (2) glutathione (GSH) synthesis: S-adenosylmethionine synthetase (SAMS), glutamine synthetase (GS), and glutamate dehydrogenase (GDH); (3) stress response induced by abscisic acid (ABA): HVA22 and abscisic acid-stress-ripening-inducible 5 protein (ASR5); and (4) cell division regulation: GTP-binding nuclear protein Ran-2.
Under Cd stress conditions, the expression of SAMS and GTP-binding nuclear protein Ran-2 were up-regulated and MAT, POD, the germin-like protein precursor, and GS were down-regulated in the roots of the two rice cultivars. The expression of NADP-GDH, GDH and phosphoglycerate mutase were down-regulated in the roots of rice IR24, but they were down-regulated in PI312777 only upon the treatment with 100 μmol/L Cd²⁺. The expression of protein with HVA22 domain was up-regulated in PI312777, but it was downregulated reversed in IR24. ASR5 was up-regulated in rice PI312777, but no significant change was found in IR24. The 60s acidic ribosomal protein P0 was down-regulated in PI312777, but up-regulated in IR24 at 100 μmol/L Cd²⁺condition. Our results suggest that Cd stress increases ROS and produces oxidative stress in rice roots, which lead to Cd toxicity in rice roots. To alleviate Cd toxicity, rice roots increase the GSH synthesis by up-regulation of SAMS and GS. Different expression patterns of stress-related proteins such as ASR5 and HVA22 are important in understanding the differences in Cd tolerance across rice cultivars.