CRD1, an Xpo1 domain protein, regulates miRNA accumulation and crown root development in rice

Crown root (CR) is the main component of the fibrous root system in cereal crops, but the molecular mechanism underlying CR development is still unclear. Here, we isolated the crown root defect 1 (crd1) mutant from ethyl methane sulfonate-mutated mutant library, which significantly inhibited CR development. The CRD1 was identified through genome resequencing and complementation analysis, which encodes an Xpo1 domain protein: the rice ortholog of Arabidopsis HASTY (HST) and human exportin-5 (XPO5). CRD1 is ubiquitously expressed, with the highest expression levels in the CR primordium at the stem base. CRD1 is a nucleocytoplasmic protein. The crd1 mutant contains significantly reduced miRNA levels in the cytoplasm and nucleus, suggesting that CRD1 is essential for maintaining normal miRNA levels in plant cells. The altered CR phenotype of crd1 was simulated by target mimicry of miR156, suggesting that this defect is due to the disruption of miR156 regulatory pathways. Our analysis of CRD1, the HST ortholog identified in monocots, expands our understanding of the molecular mechanisms underlying miRNA level and CR development.