2008 Volume 2 Pages 19-28
Adventitious (crown) roots account for the majority of the root system of monocots. It is reported that auxin plays an important role in the formation of crown roots, but the underlying molecular mechanisms are still unknown. We characterized a rice (Oryza sativa L.) mutant crown rootless4 (crl4) that was found to have defective crown root formation. Besides reduced crown root number, the crl4 mutant showed auxin-related abnormal phenotypical character-istics such as reduced lateral root number and impaired root gravitropism. CRL4 encodes a protein highly homologous with Arabidopsis GNOM, which mediates auxin-dependent plant growth by coordinating the polar localization of auxin efflux carrier PIN1. In the crl4 mutant, auxin transport was impaired in shoots and roots. Besides, the GUS staining controlled by DR5 promoter in the node of crl4 mutant was fainter and spread wider than that of wild-type. These results indicate that maintaining an appropriate auxin accumulation and gradient through CRL4 in the basal portion of shoots is essential for crown root formation in rice.