RRL1, RRL2 and crl2 loci regulating root elongation in rice

Abstract

To analyze the genetic mechanism regulating root elongation, we characterized five recessive mutants in rice (BRX65, BRX117, BRX430, BRX448 and crl2 mutant). In fifteen-day-old seedlings, the root length of BRX65 was 36% of that of the wild type root, while the length of the roots of BRX117, BRX430 and BRX448 was about 45% of that of the wild type root. These types of mutation were designated as reduced root length with the gene symbol rrl. In contrast, the length of the crl2 mutant root was 114% of that of the wild type root. Allelism test indicated that rrl mutations occurred in two complementation groups and that these mutant genes were not allelic to crl2. The first RRL locus was designated as RRL1 (BRX65) and the other locus as RRL2, and the alleles were designated as rrl2-1 (BRX117), rrl2-2 (BRX430) and rrl2-3 (BRX448). In the rrl1 mutant, only the mature cortical cell length was significantly shorter than that of the wild type, whereas, the values of the cell length, apical meristem size and cell flux of the rrl2-1 mutant root were all clearly lower than those of the wild type root. On the other hand, the values of the mature cortical cell length, apical meristem size and cell flux of the crl2 mutant root were all clearly higher than those of the wild type. Mature cortical cell length of the rrll crl2 double mutant was intermediate between that of the parental single mutants, while the cell length, meristem size and cell flux of the rrl2-1 crl2 double mutant were all intermediate between those of the parental single mutants, respectively. These results suggest that there are genes that inhibit or promote the maintenance of root apical meristem and cell elongation, and the opposite effects between these individual genes determine the extent of root elongation in rice.