Morph-physiological Traits Of Drought Tolerance Rice Lines And Genetic Overlap Of Drought And Salinity Tolerance

Abiotic stresses such as drought and salinity are very important factor limiting rice productivity in the rainfed and costal areas of Asia. Development and adoption of high-yielding and stress tolerant rice varieties are the best solution for solving the problems. However, it is well known that rice tolerances to both drought and salinity are controlled by large numbers of genes or quantitative trait loci (QTLs), making it difficult to breed superior rice varieties with good tolerances to both stresses. In past 10 years, many sets of introgression lines (ILs) and pyramiding lines (PLs) with significantly improved drought tolerance (DT) and/or salt tolerance (ST) have been developed using backcross breeding and molecular pyramiding at IRRI. The objectives of this study were:(1) to characterize the phenotypic and physiological traits associated with DT PLs and (2) to understand the genetic basis of DT and ST using ILs and molecular markers.Two series experiments were performed in this study. In the first set of experiments,48 PLs, their parental ILs and recurrent parent, IR64 (check) were evaluated for their yield performances and related traits under severe drought stress at the reproductive stage (RS) and vegetative stage (VS) and irrigated control in three years to understand the relationship between drought tolerance (DT) and yield potential (YP) in rice and their underlying mechanisms. When compared to IR64, all PLs had significantly improved DT to RS and 36 PLs also had significantly improved DT to VS (p<0.05). In addition,23 PLs had higher YP than IR64 (p<0.05) and the remaining 25 PLs had similar YP as IR64 under the irrigated control. Detailed characterization of the PLs revealed 3 mechanisms that functioned together to contribute to their improved DT and/or YP. The most important mechanism was drought avoidance (DA) characterized with significantly higher biomass and harvest index (HI) of the PLs than IR64 under stress, suggesting the PLs' better abilities to maintain higher water status and function under stress. Because most PLs did not show any reduction in biomass under control, this type of DA resulted most likely from improved dehydration resistance. The second mechanism was efficient partitioning characterized by improved HI in all PLs as compared to IR64, resulting primarily from higher GW and/or SF under control and was the major constituent of the improved YP in the 17 PLs. Drought escape (DE) by accelerated heading under drought was the third mechanism that contributed to DT of the PLs to RS. The considerable variation in the measured traits among the PLs with similar levels of DT and YP implies the complex genetic control of the mechanisms for DT/YP and offers opportunities to further improve DT and/or YP by fine-tuning of QTLs/genes among the PLs using MAS. Finally, our results indicate that selection for yield plus some secondary traits under appropriate type(s) of stress and non-stress conditions similar to target environments are critically important for improving both DT and YP in rice.A selected set of promising PLs was further analyzed for the dynamic responses of plant growth and water use to progressive water deficit at different growth stages in lysimeters under controlled phytotron conditions and rainfed lowland condition. In vegetative stage dry-down experiment, the PLs that performed best under drought were those that had the lowest threshold of transpiration response to soil moisture decrease (P<0.01), i.e. lines that kept stomata open longer or had large root system during the soil drying, and hence extracted more soil water, which resulted in higher shoot and root biomass, and an overall improvement of transpiration efficiency. ABA-dependant and ABA-independent signal pathway co-regulated the different response to the FTSW among the genotypes. In intermittent water stress, the sharp dry-down stress resulted in the similar performance between lines in the threshold of FTSW. The lines with higher water use efficiency (WUE) and large biomass at the end of stress showed good drought recovery ability, and thus produced higher spikelet fertility and grain yield. However, those lines produced large root biomass did not show superior drought recovery, probably due to the specific root anonomy structure. Under the lowland progressive water stress, the PLs produced high yield potential as the result of large biomass production (more tillers and panicles), while under drought, they improved WUE by balancing the transpiration and tiller and leaf area production, and performed higher yield, harvest index and spikelet fertility, but lower above ground shoot production. As well, the lines with large root system also conferred to drought tolerance in lowland condition, to some extent.In the second set of experiments, two BC1F2 populations (1500plants) in the genetic background of a high-yielding indica variety, Huang-Hua-Zan, were screened under drought and salinity conditions at IRRI and China, resulting in development of 36 DT ILs and 30 salt tolerant (ST) ILs. These ILs were analyzed using X2 and multilocus probability tests to identify functional genetic units (FGUs) located in the donor segments associated with DT and ST. Together, a total of 53 FGUs associated with HY, DT and/or ST were identified in the ILs, forming 3 genetic networks underlying HY, DT and ST in rice. The first one detected in the HHZ/OM1723 population contained 28 FGUs, including 9 FGUs contributing to HY, DT, and ST,14 FGUs affecting 2 traits, and 5 FGUs that contributed to one trait only. Of these,3 loci (RM24, RM6 and RM130) appeared to be important as they had high frequencies of introgression and were placed in the upstream of the network based on the new molecular-quantitative genetics theory. Similarly, the genetic network identified in the HHZ/Teqing population had 25 FGUs, including 19 FGUs contributing to HY, DT, and ST,4 FGUs affecting 2 traits, and 2 FGUs that contributed to one trait only. Again, these FGUs included 2 important ones in the upstream of the genetic network. In contrast to the high genetic overlap among the three traits, especially between DT and ST, however, there were few shared loci identified in the HHZ/OM1723 and those in the HHZ/Teqing population, indicating the 2 donors contributed different sets of loci to the DT and/or ST in the selected ILs. Our results were in consistent with our current knowledge that DT and ST in rice are under complex genetic control, and provided useful information and materials for further improvement of DT and ST in rice by pathway pyramiding, and for detailed molecular dissection of DT and ST in rice using high throughput genomic tools.