Characterization of Rice Mutants Deficient in the Formation of Crown Roots

To investigate the genetic mechanism regulating crown root formation, we identified two recessive rice mutants (odm 202 and BRX334). The odm 202 mutant was detected in a MNU-mutagenized M 3 population of rice (cv. Taichung 65) and the BRX334 mutant in a γ-ray mutagenized M 2 population of rice (cv. Blue Rose). The number of crown roots of the odm 202 and BRX334 seedlings was significantly lower than that of the respective wild types and this mutation type was designated as crown rootless with the gene symbol crl. Allelism test indicated that the two mutant genes were not allelic and the CRL loci of odm 202 and BRX334 were designated as CRL1 and CRL2, respectively. Histological observations suggested that the initiation of crown root primordia was impaired in the crl1 mutant, while in the crl2 mutant both the initiation and subsequent growth of crown root primordia were impaired. In addition, the crl1 mutant grew normally until maturity except for the difference in the number of crown roots from the wild type, whereas the crl2 mutant showed many abnormal morphological characters besides the defect in the formation of crown roots. These facts suggest that CRL1 regulates specifically the initiation of crown root primordia but is not involved in the radicle and lateral root initiation or shoot development, while CRL2 regulates various processes of plant development.