Integrated Proteomic and Cytological Study of Rice Endosperms at the Storage Phase

The endosperm at the storage phase undergoes a series of coordinated cellular and metabolic events, including starchy endosperm cell death, starch synthesis, and starch granule packaging, which leads to efficient accumulation of starch. However, the mechanism underlying the interconnections remains unknown. We used integrated proteomic and cytological approaches to probe the interconnections in rice (Oryza sativa) endosperm at the storage phase from 12 to 18 days after flowering (DAF). Starch granule packaging was completed first in the inner part of endosperm at 15 DAF and spread to almost the entire endosperm at 18 DAF. Programmed starchy endosperm cell death occurred after the starch granule packaging. Endogenous H2O2 was detectable in the inner part of endosperm at 12 DAF and the region beyond the inner part at 15 DAF, with an H2O2 burst at 15 DAF. Proteomics analysis with 2-D fluorescent difference gel electrophoresis and matrix-assisted laser-desorption ionization time-of-flight/ time-of-flight mass spectrometry revealed 317 proteins, including almost all known antioxidants, differentially expressed throughout the 3 stages of the developmental phase. More than two-thirds of the 317 proteins were potential thioredoxin targets, with a preferential skew toward central carbon metabolism, alcoholic fermentation, starch metabolism, amino acid metabolism, and protein synthesis or folding. These proteins implicated in starch synthesis and gluconeogenesis were upregulated