Mapping quantitative trait loci associated with root penetration ability in rice (Oryza sativa L.).

Root penetration ability is an important factor for rice drought resistance in areas with soils subject to both compaction and periodic water deficits. However, breeding for root penetration ability is inhibited by the difficulties associated with measuring root traits. Our objective was to identify restriction fragment length polymorphisms (RFLPs) associated with root penetration ability. Using wax-petrolatum layers as a proxy for compacted soil, we counted the number of vertical root axes penetrating through the layer, the total number of vertical root axes and the number of tillers per plant of 202 recombinant inbred (RI) lines over three replications. As a measure of root penetration ability, we used a root penetration index defined as the percent of the total number of vertical root axes that penetrated through a wax-petrolatum layer. The RI population exhibited a wide range in the number of penetrating roots axes (10-115 roots), the total number of roots axes (74-226 roots), tillers per plant (6-18), and in the root penetration index (0.11-0.71). Single-marker and interval quantitative trait analyses were conducted to identify RFLP loci associated with the number of penetrating roots, total root number, root penetration index, and tiller number. Four quantitative trait loci (QTLs) were associated with the number of penetrated roots, 19 with the total root number, six QTLs with the root penetration index and ten with tiller number. Individually, these QTLs accounted for a maximum of 8% of the variation in the number of penetrating roots, 19% of the variation in total root number, 13% of the variation in root penetration index and 14% of the variation in tiller number as estimated from regressions. The multimarker regression model accounting for the greatest proportion of the variation in the root penetration index was a three-marker model that accounted for 34% of the variation. Two-marker models accounted for 13% of the variation in the number of penetrated roots, 25% of the variation in total root number, and 21% of the variation in tiller number. This is the first research paper to apply RFLP quantitative trait analysis to dissect genetic loci associated with the total number of roots, root penetration ability and tiller number.