Minisymposium 8: Cell division

Abs # 21003: Temporal and Spatial Regulation of Cyclins in Developing Maize Endosperm

Presenter:	Nguyen, Hong N, hnguyen@email.arizona.edu
Authors	Nguyen, Hong N (A)   Dante, Ricardo A (A)   Sabelli, Paolo A (A)   Larkins, Brian A (A)
Affiliations:	(A): University of Arizona
Web Site:	http:// http://www.ag.arizona.edu/research/larkinslab/

In developing maize endosperm, three types of cell cycle occur successively. During the first three days after pollination (DAP), nuclei divide without cytokinesis to form a syncytium. This is followed by a mitotic stage that occurs after the cellularization of the syncytium and persists until 8-10 DAP. Finally, there is a period of endoreduplication that coincides with the terminal differentiation of starchy endosperm cells. The mitotic stage comprises the four classic cell cycle phases (G1, S, G2, M), while the endoreduplication stage consists of alternating G and S phases resulting in polyploid cells. Progress through the cell cycle phases is controlled by cyclin-dependent kinases, which are regulated by association with variable cyclin subunits. Among these, D- and B-type cyclins regulate the G1-S transition and M-phase progression, respectively. The nature and role of these cyclins during the mitotic and endoreduplication periods of developing endosperm are unknown. To investigate these questions, we characterized cyclins B1;3, D1;1, and D2;1 and generated polyclonal antibodies to determine their accumulation pattern, associated kinase activity, and spatial localization in developing endosperm. Cyclin B1;3 and D1;1 mRNA levels sharply decreased as endosperm transitioned from the mitotic stage to the endoreduplication stage, whereas cyclin D2;1 mRNA levels decreased gradually. Despite the decrease in mRNA accumulation, protein levels remained nearly constant for all three cyclins throughout development. Kinase activity associated with cyclin B1;3 was markedly lower at 15 DAP than at 7 DAP, while that of cyclins D1;1 and D2;1 was only slightly lower. Cyclins B1;3 and D2;1 were found throughout the endosperm while D1;1 was mostly in aleurone and subaleurone cells. Cyclin B1;3 formed aggregates in the cytoplasm and punctate foci in nuclei of interphase cells, while it localized to the spindle at metaphase, spindle poles at anaphase and the phragmoplast at telophase of mitotic cells. Cyclin D2;1 was restricted to nuclei, whereas D1;1 formed aggregates in the cytoplasm of aleurone cells at 7 DAP and continued to accumulate so that by 15 DAP, it was observed in the aleurone and two subaleurone layers.