Mechanism of Developmental Stagnancy of Rice Inferior Spikelets at Early Grain-Filling Stage as Revealed by Proteomic Analysis

Rice spikelets display different grain-filling patterns depending on their position on a panicle. The superior spikelets (SS) elongated soon after flowering, whereas the inferior spikelets (IS) hardly elongate and are morphologically stagnant at early grain-filling stage. The developmental stagnancy of IS often leads to slow grain-filling and a low grain weight. To better understand the mechanism of the stagnancy, we took a two-dimensional gel electrophoresis (2-DE)-based proteomic and phosphoproteomic approach to profile the proteins with expression abundance as well as the phosphorylation of SS and IS at the early grain-filling stage. A total of 81 protein spots were found to be significantly different in expression abundance, and 27 protein spots significantly differed in phosphorylation. Bioinformatic analyses implicated that the proteins might be involved in the diverse cellular processes including sugar conversion, starch synthesis, energy pathway, signal transduction, cell growth/division, and protein synthesis and destination. Moreover, using the pull-down assay, we identified 29 14-3-3 binding proteins, and altogether, these 14-3-3 protein was found the important scaffold in the signaling networks of IS development. Our findings provided new proteomic and phosphoproteomic insights on the developmental stagnancy of rice IS that could be highly useful for the improvement of the grain-filling of rice and other cereal crops.