Fine mapping and characterization of two novel quantitative trait loci for early seedling growth in rice

Two novel QTLs for early seedling growth in rice were fine mapped, with one of which to a 4-kb identical to the known GW6a gene, and another one to a 43-kb region that contains six candidate genes. Leaves are extremely important for plant photosynthesis: the size and shape of which determine the rate of transpiration, carbon fixation and light interception, and their robust growth at seedling stage endow crops with the ability to compete with weeds. So far, many genes for the traits have been cloned with mutants; however, identification of those quantitative trait loci (QTLs) that control early seedling growth has seldom been reported. In this study, we report the identification of two QTLs, qLBL1 and qLBL2 on the rice chromosome 6 for leaf blade length at early seedling stage. Fine mapping revealed that qLBL1 was placed into a 4-kb, and qLBL2 was delimited to a 43-kb genomic interval. We further found that LBL1 was equivalent to the known grain-size gene GW6a and the qLBL2 region contains 6 candidate genes. Genetic analysis using nearly isogenic lines and transgenic rice plants revealed that both genetic factors were positive regulators. The genetic effects were mainly due to alterations of cell division by cytological observations. RT-qPCR results showed that LBL1 was preferentially expressed in leaf blades, and consistently, histochemical staining of pGW6a::GUS plants showed that GUS signal was strong in the vascular tissues of leaf blade of seedlings. Thus, we fine mapped and characterized two QTLs for early seedling growth and provided useful information to improve crop breeding.