ZmDLR9 is required for lateral root development via pre-rRNA processing in maize

Pumilio (PUM) proteins comprise an evolutionarily conserved family of sequence-specific RNA-binding proteins that act as key post-transcriptional regulators in eukaryotes. In this study, we identified a maize PUM protein, named ZmDLR9 (Defective in Lateral Root 9), as a positive factor involved in lateral root development. Loss-of-function mutation in the ZmDLR9 (dlr9-1) severely impaired lateral root formation. Functional analysis revealed that ZmDLR9 localizes in the nucleolus and interacts with pre-rRNA processing-associated proteins, including BRX1-1 (BIOGENESIS OF RIBOSOMES IN XENOPUS 1-1), BRX1-2, and GRC3 (a subunit of the ITS2 processing complex). The dlr9-1 mutation resulted in aberrant accumulation of pre-rRNA processing intermediates, as well as defects in ribosome assembly and translation. Heterologous expression of ZmDLR9 partially rescued the growth defects of an Arabidopsis PUM loss-of-function mutant (apum24-2), demonstrating functional conservation of the PUM protein family across the plant kingdom. Furthermore, the dlr9-1 mutation disrupted nucleolar morphology, thereby inhibiting cell proliferation and delaying lateral root primordia emergence. Together, our results demonstrate that ZmDLR9, a Pumilio protein, is required for lateral root development by mediating pre-rRNA processing-dependent cell proliferation.