Localization of QTL for basal root thickness in <Emphasis Type="Italic">japonica</Emphasis> rice and effect of marker-assisted selection for a major QTL

Root traits are key components of plant adaptation to drought environment. By using a 120 recombined inbred lines (RILs) rice population derived from a cross between IRAT109, a japonica upland rice cultivar and Yuefu, a japonica lowland rice cultivar, a complete genetic linkage map with 201 molecular markers covering 1,833.8 cM was constructed and quantitative trait loci (QTLs) associated with basal root thickness (BRT) were identified. A major QTL, conferring thicker BRT, located on chromosome 4, designated brt4, explained phenotypic variance of 20.6%, was selected as target QTL to study the effects of marker-assisted selection (MAS) using two early segregating populations derived from crosses between IRAT109 and two lowland rice cultivars. The results showed that the flanking markers of brt4 were genetically stable in populations with different genetic backgrounds. In the two populations under upland conditions, the difference between the means of BRT of plants carrying positive and negative favorable alleles at brt4 flanking markers loci was significant. Phenotypic effects of BRT QTL brt4 were 5.05–8.16%. When selected plants for two generations were planted at Beijing and Hainan locations under upland conditions, MAS effects for BRT QTL brt4 were 4.56–18.56% and 15.46–26.52% respectively. The means of BRT for the homozygous plants were greater than that of heterozygous plants. This major QTL might be useful for rice drought tolerance breeding. L. Liu and P. Mu are contributed equally to this work.