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 functions are well characterized in Arabidopsis, their role in other plant species remains unclear. Here, we show that in rice, the JKD ortholog NUTCRACKER (NUC) restricts cell division in the root meristem. nuc 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. NUC associates with SHR and SCR and regulates the expression of SCR and QUIESCENT CENTER-SPECIFIC HOMEOBOX (QHB/WOX5). NUC is expressed in the ground tissue and exodermis and complements the nuc mutant when driven by its native promoter. These findings reveal a role for the BIRD protein NUC in controlling cell division and maintaining the stem cell niche in the rice root meristem.