Grain protein content comparison and proteomic analysis of foxtail millet (Setaria italica L.) seed response to different drought stress levels

As one of the main indicators of nutritional values of foxtail millet, grain protein contents (GPCs) of seeds can be affected by soil water conditions. Studies in this report indicated that intensified drought in filling stage increased the GPCs. To gain better insight into the interaction mechanism between the GPCs and soil water deficits, the proteomic profiles of mature seeds under three drought stress levels (moderate drought-W2; combined drought-W1; severe drought-W0) were conducted on two varieties, Longgu6 (LG6) and Huangjinmiao (HJM) by two-dimensional electrophoresis, with acidic (pH 4-7) gels. There were 83 protein spots in total identified successfully in two comparison groups (W1 vs W2 and W0 vs W2), associated with four major biochemical and metabolic pathways, including storage protein, amino acid and protein metabolisms, energy metabolism, response to stress and defense, and chaperones. Further analysis revealed that the increase of GPCs in LG6 under both W0 and W1 may be owing to the up-regulated storage proteins and the up-regulated protein response to drought stress. The levels of DEPs related to protein and amino acid biosynthesis and starch metabolism increased greatly in LG6 under W1, and this may also result in the high GPC level of seeds in LG6 under W1. Protein biosynthesis was up-regulated and drought stress-related pathways were enhanced in HJM under W1, thereby resulting in the increase of GPC in HJM under W1. These results will contribute to the existing knowledge on the complexity of protein changes leading to the GPC changes in response to drought.