Maize endonuclease NOB1 couples ribosome biogenesis with translational regulation during kernel development

NOB1-mediated cleavage of 20S pre-rRNA represents a critical quality-control step during cytoplasmic maturation of the eukaryotic 40S ribosomal subunit, yet its developmental significance in crops remains largely unexplored. Here, we identified two highly conserved maize NOB1 paralogs, ZmNOB1a and ZmNOB1b, both exhibiting RNA-binding and endonuclease activities and associating with pre-40S ribosomal particles. Genetic analyses demonstrated that the two paralogs function redundantly, but with unequal contributions, to support gametophyte viability and kernel development. Simultaneous disruption of ZmNOB1 activity resulted in severely impaired male gametophyte transmission, markeldly reduced female gamete viability, and profound defects in embryo and endosperm development. Molecular analyses further revealed widespread disruption of pre-rRNA maturation, including defective D-site cleavage, accompanied by impaired ribosome assembly and reduced translational capacity. Consistent with these molecular defects, mutant kernels exhibited decreased starch and storage protein accumulation, together with elevated levels of free ribosomal subunits and reduced 80S monosomes and polysomes. Moreover, ZmNOB1 exhibited co-sedimentation and directly interacted with the translation initiation factor eIF4A, implying a potential functional coordination between late ribosome maturation and translation initiation. Collectively, our findings establish ZmNOB1-mediated pre-rRNA processing as a key determinant of translational competence, reproductive fitness, and seed filling in maize.