Investigation of the nitric oxide-induced cadmium tolerance in barley (Hordeum vulgare L.) seedlings

Cadmium (Cd) contamination in agricultural soils is a prevalent environmental issue and poses potential threats to food security. Nitric oxide (NO) is a gaseous bioactive signaling molecule regulates environmental stress responses. Although NO is reported to alleviate the Cd toxicity, limited information is existing about the NO-induced Cd tolerance at molecular level in barley seedlings. In this study, therefore, the potential role of NO in response to Cd stress was examined by physiological, biochemical, and proteomic analyses in leaves of barley (Hordeum vulgare L.) seedlings. The results showed that 50 M Cd stress dramatically reduced plant growth and chlorophyll content, and increased lipid peroxidation and H2O2 content. However, exogenous application of 100 M sodium nitroprusside (SNP; NO donor) significantly alleviated Cd-induced growth inhibition and reductions in chlorophyll content, and dramatically reduced leaf malondialdehyde (MDA) accumulation. Cd stress significantly increased the activities of superoxide dismutase (SOD), guaiacol peroxidase (POD) and ascorbate peroxidase (APX). On the other hand, application of exogenous SNP further increased the activity of APX, and significantly decreased the levels of SOD and POD activity as compared to Cd stress. The leaf proteins exhibiting significant abundance changes (2.0-fold change) were separated by two-dimensional gel electrophoresis (2-DE). Thirty-four protein spots were successfully identified by MALDI TOF/TOF (matrix-assisted laser desorption/ionization time-of-flight/ time-of-flight) mass spectrometry. When compared with control, 28 proteins were up-regulated and 6 proteins were up-regulated by SNP and/or Cd stress. Meanwhile, exogenous SNP treatment differentially regulated the levels of proteins. Differentially-expressed proteins were functionally classified into six groups including photosynthesis and carbohydrate biosynthesis (47.1%), protein metabolism (23.5%), primer metabolism (11.8%), energy metabolism (11.8%), stress defense (5.9%), and signal transduction (2.9%). Metabolic pathway analysis indicated that several Cd-disruptive pathways were markedly reversed by SNP treatments, including photosynthesis, protein synthesis, redox homeostasis, and signal transduction. Exogenous SNP up-regulated proteins involved in energy metabolism implying an enhancement in energy generation. Proteins involved in protein metabolism were differently regulated by SNP which indicated the roles of NO in stress resistance and proteome rearrangement. These findings suggest that NO may alleviate Cd-induced growth inhibition through the promotion of photosynthesis to generate more energy, protection of proteins, and activation of stress defense.