KNUCKLES promotes meiotic cell-cycle progression by directly repressing the expression of KRP1 and KRP3 to ensure male fertility.

Meiosis produces haploid gametes that are essential for sexual reproduction in most eukaryotes, but the specific cell-cycle regulation that controls meiotic progression in plants remains to be fully characterized. KNUCKLES (KNU), a known transcriptional repressor in floral meristem regulation, also regulates anther development. Loss of KNU function leads to male sterility in Arabidopsis, but its detailed regulatory mechanism is unknown. Here, we find that KNU is specifically localized in meiocytes during anther development, and mutation of KNU disrupts meiotic progression and behavior, leading to apoptosis of microsporocytes. Transcriptome analysis shows that numerous genes related to meiosis are downregulated in knu-2 meiocytes. We demonstrate that KNU can directly repress the expression of two cell-cycle inhibitors, INTERACTOR/INHIBITOR OF CDK 1/KIP-RELATED PROTEIN 1 (ICK1/KRP1) and KRP3, and knockout of KRP1 or KRP3 in the null allele knu-2 background largely rescues the knu-2 defects in male meiosis and fertility. Consistent with these results, overexpression of KRP1 driven by the native KNU promoter results in meiotic defects and reduced expression of some meiosis-related genes, similar to the phenotypes of knu-2. Thus, our findings provide evidence that the transcription factor KNU regulates the expression of meiotic cell-cycle regulators and cohesins through suppression of KRP1/3, significantly broadening our understanding of plant meiosis.