Development of the endosperm of Sorghum bicolor during the endoreduplication-associated growth phase

Spatial occurrence of endoreduplication, a variant of the cell cycle resulting in endopolyploidy, was investigated in the developing sorghum caryopsis between 5 and 16 DAP (days after pollination). This was a period of intense mitotic and endoreduplication-associated endosperm growth. Endopolyploidy was quantitatively analyzed on median caryopsis sections using image densitometry that provided in situ cytometrical data. In the endosperm, the first endopolyploid nuclei with a nuclear DNA content of 12C (where 1C represents the nuclear DNA content of a non-replicated haploid genome) were detected at 5 DAP. In subsequent days progressively higher levels of endopolyploidy occurred, and nuclei with the highest amount of DNA (96C) were first observed at 10 DAP. The highly endopolyploid nuclei were located only in the central region of the endosperm and their occurrence coincided with the onset of starch deposition in the endosperm. Cells with non-endopolyploid nuclei (3C and 6C) were found exclusively in the peripheral layers of the endosperm. No starch was observed in the basal part of the endosperm where the highest level of endopolyploidy was 24C. The volume of endosperm nucleai and cells showed a positive correlation with the level of endopolyploidy. Endoreduplication was also prominent in the pericarp, where the highest level of endopolyploidy was 16C.