Development of upland rice introgression lines and identification of QTLs for basal root thickness under different water regimes.

Introgression lines (ILs) are valuable materials for identifying quantitative trait loci (QTLs), evaluating genetic interactions, and marker assisted breeding. A set of 430 ILs (BC(5)F(3)) containing segments from upland tropical japonica cultivar IRAT109 in a lowland temperate japonica cultivar Yuefu background were developed. One hundred and seventy-six polymorphic markers were used to identify introgressed segments. No segment from IRAT109 was found in 160 lines. Introgressed segments of the other 270 lines covered 99.1% of the donor genome. The mean number of introgressed donor segments per individual was 3.3 with an average length of 14.4 cM. QTL analysis was conducted on basal root thickness (BRT) of the 270 ILs grown under irrigated lowland, upland and hydroponic conditions. A total of 22 QTLs affecting BRT were identified, six QTLs (qBRT3.1, qBRT3.2, qBRT6.1, qBRT8.2, qBRT9.1, and qBRT9.2) were consistently expressed under at least two environments (location and water regime), and qBRT7.2 was a new BRT QTL identified under lowland conditions. IL255 containing qBRT9.1 showed an increase of 10.09% and 7.07% BRT over cultivar Yuefu when grown under upland and lowland conditions, respectively. Using a population of 304 F(2:3) lines derived from the cross IL255×Yuefu, qBRT9.1 was validated and mapped to a 1.2 cM interval between RM24271 and RM566. The presence of qBRT9.1 explained 12% of BRT variation. The results provide upland rice ILs and BRT QTLs for analyzing the genetic basis of drought resistance, detecting favorable genes from upland rice, and rice drought resistance breeding.