Variations of endoreduplication and its potential contribution to endosperm development in rice (<i>Oryza sativa</i> L.)

Endoreduplication is the phenomenon by which cells increase their ploidy. Endoreduplication is initiated by the transition from the mitotic cell cycle to the endocycle, in which DNA replication occurs without a subsequent chromosome separation and cytokinesis, and is enhanced by endocycle reiteration. This process appears to play an important role in endosperm development, but the characteristics of endoreduplication in the endosperm of rice (Oryza sativa) remain unclear. To elucidate the features and variations of endoreduplication in rice endosperm, endoreduplication progression in the developing endosperm was compared among 10 cultivars based on flow cytometry and fluorescence microscopy. The flow cytometric analysis detected significant differences among 10 cultivars in the following three parameters: mean ploidy of all nuclei, the proportion of nuclei ≥6C (%E, an estimate of the initiation of the endocycle), and the mean ploidy of nuclei ≥6C (E6P, an estimate of the reiteration of the endocycle). However, no significant correlation between %E and E6P was observed, suggesting that the initiation and reiteration of the endocycle are independently regulated. Fluorescence microscopy revealed that the ploidy of the nuclei was higher in the intermediate region than in the central and peripheral regions of the endosperm. Cells with a higher ploidy were larger in the developing endosperm. Furthermore, the mean ploidy in the developing endosperm was significantly correlated with the mean cell size in the mature endosperm. These results indicate that endoreduplication progression in the endosperm differed significantly among the 10 rice cultivars and such differences may influence endosperm cell size. Abbreviations: Ak: Akitakomachi; DAP: days after pollination; DAPI: 4′,6-diamidino-2-phenylindole; E6P: mean ploidy of nuclei ≥ 6C; Ha: Habataki; Ho: Hokuriku193; IR: IR64; Ka: Kasalath; Ki: Kinuhikari; Ko: Koshihikari; Ni: Nipponbare; PEG: polyethylene glycol; Sa: Sasanishiki; Ta: Takanari; %E: proportion of nuclei ≥ 6C.