Biological Responses and Proteomic Changes in Maize Seedlings under Nitrogen Deficiency

The influence of nitrogen (N) deficiency on tolerance mechanisms in seedlings of two maize hybrids (Xu178 × Huang-C and Xu178 × Zong3) and their parental inbred lines (Xu178, Huang-C and Zong3), which show different nitrogen use efficiency (NUE), was investigated using physiological measurements combined with global proteomics profiling. The root fresh weight and chlorophyll a/b ratio were reduced significantly in Huang-C (low NUE) under 0.002 mM nitrate treatment for 10 days, whereas no significant change in these two traits was observed in Xu178 (high NUE) under the same treatment compared with N-sufficient treatment. Fifty and 56 protein spots, which showed more than two-fold changes in abundance at P < 0.01 under low-N treatment compared with the control in the roots and leaves, respectively, were analyzed by protein mass spectrometry. Analysis of protein expression patterns revealed that proteins associated with carbohydrate metabolism, nucleotide metabolism, amino acid metabolism, disease/defense, and photosynthesis may be involved in N-deficiency responses. Low-N treatment led to an increased abundance of glutamine synthetase and transcripts in the root to improve the efficiency of N assimilation in the inbred line with HNE, and affected photosynthetic carbon fixation and starch metabolism in the leaves and consequently seedling growth.