The Anaphase-Promoting Complex Is a Dual Integrator That Regulates Both MicroRNA-Mediated Transcriptional Regulation ofCyclin B1and Degradation of Cyclin B1 duringArabidopsisMale Gametophyte Development

The anaphase-promoting complex/cyclosome (APC/C), an essential ubiquitin protein ligase, regulates mitotic progression and exit by enhancing degradation of cell cycle regulatory proteins, such as CYCB1;1, whose transcripts are upregulated by DUO POLLEN1 (DUO1). DUO1 is required for cell division in male gametophytes and is a target of microRNA 159 (miR159) in Arabidopsis thaliana. Whether APC/C is required for DUO1-dependent CYCB1;1 regulation is unknown. Mutants in both APC8 and APC13 had pleiotrophic phenotypes resembling those of mutants affecting microRNA biogenesis. We show that these apc/c mutants had reduced miR159 levels and increased DUO1 and CYCB1;1 transcript levels and that APC/C is required to recruit RNA polymerase II to MIR159 promoters. Thus, in addition to its role in degrading CYCB1;1, APC/C stimulates production of miR159, which downregulates DUO1 expression, leading to reduced CYCB1;1 transcription. Both MIR159 and APC8-yellow fluorescent protein accumulated in unicellular microspores and bicellular pollen but decreased in tricellular pollen, suggesting that spatial and temporal regulation of miR159 by APC/C ensures mitotic progression. Consistent with this, the percentage of mature pollen with no or single sperm-like cells increased in apc/c mutants and plants overexpressing APC8 partially mimicked the duo1 phenotype. Thus, APC/C is an integrator that regulates both microRNA-mediated transcriptional regulation of CYCB1;1 and degradation of CYCB1;1.