Endoreduplication during soybean seed development

Soybean (Glycine max) is an important oil crop grown in the United States. A long term objective has been to improve the compositional quality of soybean. This goal requires a comprehensive understanding of the genetic mechanisms that control seed development. In this study we have focused on genetic events that control the onset of seed-fill and in particular events that accompany the establishment of endoreduplication in cotyledonary cells. Endoreduplication is an alternate version of the cell cycle in which cells shuttle between S and gap phases. Whereas DNA replication persists during endoreduplication, cell division by cotyledonary cells ceases. These results in large cotyledonary cells with increased ploidy, events considered to enhance the capacity for the synthesis and accumulation of proteins and oil. Endoreduplication is frequently encountered during differentiation and is often associated in cells that undergo intense protein synthesis (Nagl, 1976). By flow cytometric analysis of cotyledons harvested at different times during development, we demonstrated that an increase in the average ploidy of nuclei was associated with the increase in starch accumulation. In situ measurement of DNA content of nuclei in cotyledon sections from seeds before and after the onset of seed-fill revealed that polyploid nuclei were first observed in parenchyma cells that underlie the abaxial surface of cotyledons and that the distribution of highly endoreduplicated cells in this region increased rapidly. In situ RT-PCR revealed that β-conglycinin was localized in the same area as polyploidy nuclei, a result consistent with previous observations (Dhillon and Miksche,1983; Perez-Grau and Goldberg, 1989). Messenger RNAs encoding cell cycle components associated with mitosis rapidly decreased at the onset of seed-fill. mRNAs encoding CdkB2 were located in the primarily diploid parenchyma cells that subtend the adaxial side of the cotyledon. In contrast, mRNAs that encode ICKs were co-localized with highly endoreduplicated cells in the abaxial region of the cotyledon. The decrease in the hexose content occurred after the decrease in the transcripts of CDKB2 and CYCB1 was observed. The sucrose content increased markedly after seed-fill