GENETIC DISSECTION OF ROOT-GROWTH IN RICE (ORYZA-SATIVA L) .2. MAPPING QUANTITATIVE TRAIT LOCI USING MOLECULAR MARKERS

Drought is a major abiotic stress of upland rice, and good root growth has been associated with drought avoidance. We report on the genetic mapping of root growth traits in an F-2 population derived from two dr ought-resistant rice varieties, 'Bala' and 'Azucena'. Restriction frag ment length polymorphism (RFLP) between the parents was 32%, and a mol ecular map with 71 marker loci and 17 linkage groups covering 1280 cM was produced. Quantitative trait loci (QTLs) for eight root growth cha racteristics were mapped using phenotype data obtained in a hydroponic screen previously described in a companion paper. Using a significanc e threshold of LOD 2.4, we observed one QTL for maximum root length af ter 28 days growth on chromosome II. It had a LOD score of 6.9, explai ned nearly 30% of the variation and appeared to be largely additive in effect. QTLs for maximum root length after 3, 7, 14 and 21 days of gr owth were also revealed. Some root-length QTLs, including that on chro mosome II, varied greatly with developmental stage. One QTL for root v olume and two QTLs for adventitious root thickness were detected. No Q TLs were detected for the length of cells in the mature (fully expande d) zone of adventitious root tips. The results obtained are discussed in the context of previous reports on mapping root growth parameters i n rice.