Developing Rice Embryo Proteomics Reveals Essential Role for Embryonic Proteins in Regulation of Seed Germination

Two-dimensional gel electrophoresis (2-DGE) in conjunction with mass spectrometry was utilized to unravel the changes in embryo proteins of geminating rice seeds. For this purpose, the embryos were dissected at 1 and 2 days after imbibition and germination of mature dry seeds, Proteins were extracted and their expression patterns were analyzed by large-format 2-DGE. A total of 642 silver nitrate stained protein spots were detected on 2-D gels, subjected to image analysis using the ImageMaster 6.0 2D Platinum software, resulting in the identification of 109 differentially expressed protein spots compared with imbibed seeds. MALDI-TOF-MS analysis resulted in the identification of 60 proteins, including 6 redundant and 54 nonredundant proteins. The identified proteins were classified according to their functional groups: metabolism (15 spots), oxygen-detoxifying (11 spots), protein processing/degradation (8 spots), stress/defense (5 spots), and energy (3 spots). Northern blot analysis demonstrated a good correlation between the mRNA expression profile and 2-DGE results for 27 proteins. Furthermore, Western blot analysis was used to confirm the high expression patterns of ascorbate peroxidase b (OsAPxb, spot 12/13) and L-ascorbate peroxidase 1 (OsAPx1, spot 17/27) in the embryo as compared with the endosperm of dry seeds. Present results suggest that seed germination is related with multiple regulations of functional proteins. For example, proteins related to metabolism (i.e., glycolysis and TCA cycle) are essential for the energy supply. On the other hand, oxygen-detoxifying proteins and stress/defense related proteins may take part in an important role in adaptation to environmental conditions during seed germination.