Quantitative Proteomics Reveals the Role of Protein Phosphorylation in Rice Embryos during Early Stages of Germination

Seed germination begins with water uptake and ends with radicle emergence. A gel-free phosphoproteomic technique was used to investigate the role of protein phosphorylation events in the early stages of rice seed germination. Both seed weight and ATP content increased gradually during the first 24 h following imbibition. Proteomic analysis indicated that carbohydrate metabolism- and protein synthesis/degradation-related proteins were predominantly increased and displayed temporal patterns of expression. Analyses of cluster and protein-protein interactions indicated that the regulation of sucrose synthases and alpha-amylases was the central event controlling germination. Phosphoproteomic analysis identified several proteins involved in protein modification and transcriptional regulation that exhibited significantly temporal changes in phosphorylation levels during germination. Cluster analysis indicated that 12 protein modification-related proteins had a peak abundance of phosphoproteins at 12 h after imbibition. These results suggest that the first 12 h following imbibition is a potentially important signal transduction phase for the initiation of rice seed germination. Three core components involved in brassinosteroid signal transduction displayed significant increases in phosphoprotein abundance during the early stages of germination. Brassinolide treatment increased the rice seed germination rate but not the rate of embryonic axis elongation. These findings suggest that brassinosteroid signal transduction likely triggers seed germination.