Isolation and characterization of rl ( t ) , a gene that controls leaf rolling in rice

Moderate leaf rolling is one of the most important morphological traits in rice breeding for plant ideotype. Previous studies have shown that the rl ( t ) gene has a high breeding potential for developing hybrid-rice varieties with an ideal ideotype, because it leads to an appropriate leaf rolling index (LRI) of about 30 % in the heterozygous state, and had a positive effect on grain yield. In this study, we isolated rl ( t ) and performed a preliminary investigation of its function in regulating leaf rolling in rice. DNA sequencing identified a single base change (G to T) in the finely mapped region (11 kb) containing rl ( t ) , and this is located in 3′-untranslated region (3′-UTR) of the only predicted gene, Roc5 (Rice outermost cell-specific). The expression level of Roc5 is significantly higher in the rl ( t ) mutant than in the wild-type. Using RNAi and overexpression analysis, we found that the expression level of Roc5 correlated with LRI and leaf bulliform area, and was also associated with leaf abaxial or adaxial rolling. These results confirmed that Roc5 controls leaf rolling in a dosage-dependent manner. Bioinformatics analysis revealed a conserved 17-nt sequence (called the GU-rich element) in the 3′-UTR of HD-GL2 (Homeodomain-Glabra2) family genes including Roc5 . Based on the model of this element in regulating mRNA stability in mammals, we speculate that the single nucleotide change in this element accounts for the higher expression level of Roc5 in the rl ( t ) mutant compared to the wild-type, which ultimately leads to adaxial rolling of the leaf. This discovery further enhances our knowledge of the molecular mechanisms underlying leaf rolling in rice.