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Specific cell cycle event occurs during the early phase of rice seed development. Three different types of cell cycles occur during endosperm development: (i) mitosis without cytokinesis, resulting in syncytium formation; (ii) mitosis coupled with cell division; and (iii) endoreduplication. The primary endosperm nucleus undergoes a series of divisions without cytokinesis, producing a multinucleate cell known as a syncytium. The syncytial nuclei are evenly arranged in the peripheral cytoplasm surrounding a large central vacuole. At a specific time point in seed development, mitosis in the peripheral syncytium ceases. Thereafter, the syncytium is partitioned into individual cells by a specific type of cytokinesis called cellularization (Mizutani et al., 2010). Although the latter two cell cycle types have been characterized during the later stages of endosperm development, the molecular factors that control the cell cycle in the syncytium are scarcely known (Mizutani et al., 2010). Endoreduplication, occurring during a modified cell cycle, duplicates the entire genome without being followed by M-phase. Endoreduplication occurs when CDK/Cyclin complex low or inactive caused by ubiquitin-mediated degradation by APC and their activators (Su?udi et al., 2012). Cyclin-dependent kinase (CDK) inhibitors (CKIs) bind CDKs and inhibit cell cycle progression. OsKRP1 and OsKRP3 are CKIs and controls cell cycle by inhibiting CDKs. Suppressed expression of OsCycB1;1, a B type cyclin, result in abnormal endosperm cellularization (Guo et al., 2013). OsCCS52A, an APC activator, plays important role in maintaining endoreduplication during endosperm development (Su?udi et al., 2012).
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