No abstract

This study was conducted to identify and map the quantitative trait locus (QTL) controlling Al tolerance in rice using molecular markers. A population of 171 F(6) recombinant inbred lines (RILs) derived from the cross of Oryza sativa (IR64), the Al susceptible parent, and Oryza rufipogon, the Al tolerant parent, was evaluated for Al tolerance using a nutrient solution with and without 40 ppm of active Al(+3). A genetic map, consisting of 151 molecular markers covering 1,755 cM with an average distance of 11.6 cM between loci, was constructed. Nine QTLs were dentified including one for root length under non-stress conditions (CRL), three for root length under Al stress (SRL) and five for relative root length (RRL). O. rufipogon contributed favorable alleles for each of the five QTLs for RRL, which is a primary parameter for Al tolerance, and individually they explained 9.0-24.9% of the phenotypic variation. Epistatic analysis revealed that CRL was conditioned by an epistatic effect, whereas SRL and RRL were controlled by additive effects. Comparative genetic analysis showed that QTLs for RRL, which mapped on chromosomes 1 and 9, appear to be consistent among different rice populations. Interestingly, a major QTL for RRL, which explained 24.9% of the phenotypic variation, was found on chromosome 3 of rice, which is conserved across cereal species. These results indicate the possibilities to use marker-assisted selection and pyramiding QTLs for enhancing Al tolerance in rice. Positional cloning of such QTLs introgressed from O. rufipogon will provide a better understanding of the Al tolerance mechanism in rice and the evolutionary genetics of plant adaptation to acid-soil conditions across cereal species.

Drought stress is the major constraint to rice (Oryza sativa L.) production and yield stability in rainfed ecosystems. Identifying genomic regions contributing to drought resistance will help develop rice cultivars suitable for rainfed regions through molecular marker assisted breeding. Quantitative trait loci (QTLs) linked to plant water stress indicators, phenology and production traits under irrigated and drought stress conditions were mapped by means of a doubled‐haploid (DH) population of 154 rice lines from the cross CT9993‐5‐10‐1‐M/IR62266‐42‐6‐2. The DH lines were subjected to water stress before anthesis in three field experiments at two locations. The DH lines showed significant variation for plant water stress indicators, phenology, plant biomass, yield and yield components under irrigated control and water stress. A total of 47 QTLs were identified for various plant water stress indicators, phenology, and production traits under control and water stress conditions in the field, which individually explained 5 to 59% of the phenotype variation. A region was identified on chromosome 4 that harbored major QTLs for plant height, grain yield, and number of grains per panicle under drought stress. By comparing the coincidence of QTLs with specific traits, we also genetically dissected the nature of association of root traits and capacity for osmotic adjustment with rice production under drought. Root traits had positive correlations with yield and yield components under drought stress. This study demonstrated that the region RG939‐RG476‐RG214 on chromosome 4 identified for root‐related drought resistance component QTLs also had pleiotropic effects on yield traits under stress. Consistent QTLs for drought resistance traits and yield under stress were detected and might be useful for marker‐assisted selection for rainfed rice improvement.

Aluminum toxicity is the main factor limiting the productivity of crop plants in acid soils, particularly in the tropics and subtropics. In this study, a doubled-haploid population derived from the rice ( Oryza sativa L.) breeding lines CT9993 and IR62266 was used to map genes controlling Al tolerance. A genetic linkage map consisting of 280 DNA markers (RFLP, AFLP and SSR) was constructed to determine the position and nature of quantitative trait loci (QTLs) affecting Al tolerance. Three characters - control root length (CRL), Al-stressed root length (SRL) and root length ratio (RR) - were evaluated for the DH lines and the parents at the seedling stage in nutrient solution. A total of 20 QTLs controlling root growth under Al stress and control conditions were detected and distributed over 10 of the 12 rice chromosomes, reflecting multigenic control of these traits. The two QTLs of largest effect, qALRR-1-1 and qALRR-8 for root length ratio (a measurement of Al tolerance) were localized on chromosomes 1 and 8, respectively. Three other QTLs in addition to qALRR-8 were apparently unique in the CT9993 x IR62266 mapping population, which may explain the high level of Al tolerance in CT9993. Comparative mapping identified a conserved genomic region on chromosome 1 associated with Al tolerance across three rice genetic backgrounds. This region provides an important starting point for isolating genes responsible for different mechanisms of aluminum tolerance and understanding the genetic nature of this trait in rice and other cereals.