Integrated Transcriptome and Proteome Analyses of Maize Inbred lines in Response to Salt Stress

To better understand the resistance of maize (Zea mays L.) to salt stress, maize inbred lines 8723 and P138, which are salt-tolerant and salt-sensitive, respectively, were investigated using the transcriptional and proteomic profiling of seedling roots under normal conditions and 180 mM NaCl stress. The screening criteria for differentially expressed proteins (DEPs) were a fold change (FC) >= 1.20 (up-regulated) or <= 0.83 (down-regulated). Additionally, the screening criteria for differentially expressed genes (DEGs) were FC >2 or <0.5. We analyzed the correlation between the protein and mRNA levels of two maize inbred lines under salt stress and found that a total of 3152 associated genes/proteins were identified in line 8723 under salt stress. However, only 14 DEGs were also identified by their corresponding DEPs, with a correlation coefficient of 0.07. A similar comparison of the 3159 genes/proteins affected by salt stress in line P138 identified just 8 DEGs with corresponding DEPs, with a correlation coefficient of 0.05. This indicates major differences in the regulation of transcriptional and translational processes in response to salt stress. In addition, in line 8723, we identified just eight DEGs with the same expression trend as their corresponding DEPs and six DEGs that behaved in contrast to their DEPs under salt stress. Compared to P138, the root response to salt stress in line 8723 involved the following processes. First, the up-regulation of lipid transporters and the lipid transfer-like protein VAS resulted in an increased lipid metabolism. Next, the increased expression of CAD6, as well as PRP1 and PRP10 under salt stress, promoted lignin synthesis and activated the abscisic acid signal pathway, respectively. In addition, the up-regulation of ADK2 and adenylate kinase expression regulated the concentration of purine ribonucleoside to help maintain dynamic energy balance in the maize cells. Furthermore, reactive oxygen species (ROS) scavenging and protective mechanisms were effectively enhanced by the up-regulation of peroxidase 12, peroxidase 67, glutathione transferase 9 and the putative laccase family protein, and the down-regulation of peroxidase 72. Therefore, maize enhances its salt tolerance by enhancing its lipid metabolism, promoting lignin biosynthesis, activating the abscisic acid signaling pathway, maintaining the dynamic energy balance of the maize cells, and enhancing the ROS clearance and protection mechanisms. Our study identified some genes and proteins related to salt tolerance in maize, and has thus provided new and important clues to better understand the resistance of maize to salt stress.