NUTCRACKER orchestrates cortical cell divisions and sustains stem cell niche integrity in the rice root meristem

In Arabidopsis thaliana, BIRD nuclear factors, also known as the INDETERMINATE DOMAIN (IDD) protein family, regulate asymmetric cell division and tissue patterning in the root meristem. The BIRD protein JACKDAW (JKD) forms a regulatory complex with the GRAS transcription factors SHORT-ROOT (SHR) and SCARECROW (SCR) to maintain the stem cell niche and stabilize tissue boundaries. While BIRD protein function is well characterized in Arabidopsis, their role in other plant species remains unclear. Here, we show that in rice, the JKD ortholog OsNUTCRACKER (OsNUC) restricts cell division in the root meristem. Osnuc knockout mutants display ectopic divisions in the ground tissue and vasculature, a loss of quiescence in the quiescent center, and premature differentiation of columella stem cells. OsNUC associates with OsSHR and OsSCR and regulates the expression of OsSCR and QUIESCENT CENTER-SPECIFIC HOMEOBOX (QHB/WOX5). OsNUC is expressed in the ground tissue and exodermis and complements the Osnuc mutant when driven by its native promoter. These findings reveal a role for the BIRD protein OsNUC in controlling cell division and maintaining the stem cell niche in the rice root meristem.