Comparative metabolomic profiling in the roots of salt-tolerant and salt-intolerant maize cultivars treated with NaCl stress

Maize crops are sensitive to NaCl stress, which is one of the most harmful abiotic stresses affecting agricultural productivity. To gain further insights into the differential metabolic responses to NaCl stress, we employed metabolomics and physiological approaches to understand the response of salt-tolerant (PH6WC) and sensitive (PH4CV) cultivars of maize. Salt stress caused a significant reduction in root growth, lower root numbers, softened roots, leaf etiolation, inhibition of leaf formation, and decreased shoot height and stem width in both the tolerant and sensitive genotypes compared with the control. These morphological characteristics increased with the progression of the NaCl concentration, however, they were less prominent in the salt-tolerant genotype. Evans blue staining demonstrated that NaCl-induced root cell death, and the root cells of 'PH4CV' were almost completely dead following 9 d of exposure to 100 mM NaCl. Under treatment with 100 mM NaCl, 79 compounds in the roots of 'PH4CV' were identified as being significant metabolites, and 85 compounds were identified as being significant metabolites in the roots of 'PH6WC'. The NaCl-induced changes in the metabolomes of these two maize cultivars indicate that 80 root-based compounds were different between NaCl-treated plants and controls. Among these metabolites, 30 were found in both maize cultivars when responding to NaCl stress. These compounds were associated with the metabolism of some basic compounds such as cis-9-palmitoleic acid, L-pyroglutamic acid, galactinol, deoxyadenosine, and adenine. The changing abundance of the 30 metabolites was not completely consistent in 'PH4CV' and 'PH6WC'. Glucose metabolism was exclusively induced by NaCl in the 'PH4CV' maize seedlings whereas nucleic acid metabolism was more significant in the 'PH6WC' maize seedlings in response to NaCl stress. Overall, 'PH6WC' and 'PH4CV' responded differently to NaCl stress, and this information is helpful in understanding how maize seedlings respond to this type of abiotic stress.