During maize endosperm development, cells switch from mitotic into endoreduplication cell cycles, concomitant with the differentiation of starchy endosperm cells. The identity and role of cyclins and cyclin-dependent kinases (CDKs) that control these cell cycles are poorly understood. I identified and characterized maize cyclins of the D- (CYCD2;1 and CYCD5;1) and A1-types (CYCA1;3) that are expressed in endosperm. CYCA1;3 RNA decreased sharply as the endosperm transitioned from the mitotic to the endoreduplication stage, whereas CYCD2;1 and CYCD5;1 transcripts declined gradually. Polyclonal antibodies against CYCA1;3, CYCD5;1, and CYCD2;1 and a B-type homologue, CYCB1;3, were used to characterize cyclin protein accumulation, the associated CDK activity, and spatial localization of cyclins in endosperm. Except for CYCA1;3, cyclin protein levels were nearly constant or decreased slightly throughout development. CDK activity associated with CYCA1;3 was most abundant 7 days after pollination (DAP), while that associated with CYCB1;3, CYCD2;1 and CYCD5;1 peaked at 11 DAP. CYCA1;3 appeared to be most abundant in the cytoplasm of non-endoreduplicating cells. CYCB1;3 and CYCD2;1 were found throughout the endosperm, while CYCD5;1 was mostly localized in aleurone and subaleurone cells. Stability assays using in vitro-synthesized cyclins showed that CYCA1;3, CYCB1;3, CYCD2;1 and CYCD5;1 were degraded to a large extent via the proteasome in mitotic but not in endoreduplicating endosperm extracts. The results suggest that cyclin proteolysis may not play a major role in controlling endoreduplication cell cycles in maize endosperm. I identified and characterized maize CDKs of the A- (CDKA;3) and B- (CDKB1;1 and CDKB2;1) types that are expressed in developing endosperm. Their transcripts were more abundant during the mitotic than the endoreduplication stage of development. Polyclonal antibodies that specifically recognized CDKA;3 and CDKA;1, and CDKB1;1, were obtained. A-type CDK protein levels were nearly constant, while those of CDKB1;1 decreased abruptly during endosperm development. CDKB1;1 extracted from 9-DAP endosperm bound p13suc1 in vitro, suggesting that CDKB1;1 contributes to mitotic CDK activity at early stages of endosperm development. In co-expression experiments in Drosophila S2 cells, CDKA;1 and CDKA;1 formed catalytically active complexes with CYCD2;1 and inactive complexes with CYCB1;3 and CYCD5;1.

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