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. Unlike mitosis, the specific cell cycle regulation for meiotic progression control is elusive. As a known transcriptional repressor for floral meristem regulation, KNUCKLES (KNU) also regulates anther development. Loss-of-function of KNU leads to male sterility in Arabidopsis, but the detailed regulatory mechanism remains unknown. Here, we find that during anther development, KNU specifically localizes in meiocytes and mutating KNU disrupts the meiotic progression and behavior, thereby leading to apoptosis of microsporocytes. Transcriptome analysis shows that numerous genes related to meiosis are down-regulated in knu-2 meiocytes. Further, we demonstrate that KNU can directly repress the expression of two cell cycle inhibitors INTERACTOR/INHIBITOR OF CDK1/KIP-RELATED PROTEIN1 (ICK1/KRP1) and KRP3. Further knockout of either KRP1 or KRP3 in the null allele knu-2 background largely rescues the knu-2 defects in male meiosis and fertility. Consistently, overexpression of KRP1 driven by the native KNU promoter results in similar meiotic defects and the reduced expression of some meiosis-functional genes as in knu-2. Thus, our findings provide evidence that the transcription factor KNU can regulate the expression of meiotic cell cycle regulators and cohesins through suppression of KRP1/3, and this finding significantly broadens our understanding of plant meiosis.