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Crown roots are main components of the fibrous root system and important for crops to anchor and absorb water and nutrition. To understand the molecular mechanisms of crown root formation,we isolated a rice mutant defective in crown root emergence designated as Oscand1 (named after the Arabidopsis homologous gene AtCAND1). The defect of visible crown root in the Oscand1 mutant is the result of cessation of the G2/M cell cycle transition in the crown root meristem. Map-based cloning revealed that OsCAND1 is a homolog of Arabidopsis CAND1. During crown root primordium development,the expression of OsCAND1 is confined to the root cap after the establishment of fundamental organization. The transgenic plants harboring DR5::GUS showed that auxin signaling in crown root tip is abnormal in the mutant. Exogenous auxin application can partially rescue the defect of crown root development in Oscand1. Taken together,these data show that OsCAND1 is involved in auxin signaling to maintain the G2/M cell cycle transition in crown root meristem and,consequently,the emergence of crown root. Our findings provide new information about the molecular regulation of the emergence of crown root in rice.