A plant root system consists of various components including seminal root, crown root, and their lateral roots. Root structure is defined by a combination of some parameters representing these root components, that is, length, number and their configuration. Such root structure determines the overall function of the root system (Lynch 1995). Despite the importance of root for plant growth, the mechanism of root development has not yet been clarified. To investigate the genetic mechanism for crown root initiation, we isolated the rice CROWN ROOTLESS1 gene, CRL1, by positional cloning. crl1 mutant has been identified and characterized (Inukai et al. 2001). This mutant fails to form crown roots, but does not have any defects in the seminal and lateral root formation or shoot development (Fig. 1). Histological observations showed that the initiation of crown root primordia was impaired in crl1 mutant (Fig. 2). These observations suggest that CRL1 specifically regulates the initiation of crown root primordia.

For mapping, we crossed japonica rice cv. Taichung 65, homozygous plants for crl1, with an indica rice cv. Kasalath, homozygous for the wild-type allele (CRL1/CRL1). Then, F₁ plants were cultivated and self-pollinated to obtain F₂ seeds. In the F₂ seedlings (CRL1/CRL1, CRL1/crl1, and crl1/crl1), we screened homozygous plants for crl1 by counting their number of crown roots and isolated the genomic DNAs for PCR analysis. We used 42 F₂ crl1 homozygous plants. The CRL1 locus was roughly mapped on the short arm of chromosome 3. For further analysis, we screened about 2,500 F₂ crl1 homozygous plants, and investigated

the linkage between CRL1 locus and three markers, C725, R3131 and R1811, located near the CRL1 locus. Linkage analysis revealed that CRL1 was located between C725 and R3131, with distance of 0.76 cM and 0.60 cM, respectively (Fig. 3). We are now analyzing these F₂ crl1 homozygous plants using several new markers located between C725 and R3131.

References

Lynch J, 1995. Root architecture and plant productivity. Plant Physiol. 109: 7-13.

Inukai Y., Miwa M, Nagato Y, Kitano H, Yamauchi A, 2001. Characterization of rice mutants deficient in the formation of crown roots. Breed. Sci. 51: 123-129.