Screening Of Rice Varieties With Cadmium-Tolerance And Mapping Of QTLS Related To Cadmium-Stress In Rice

As one of the most important grain crops, rice is a staple food for urban and rural residents in china. Rice absorbs heavy metals easily from polluted soil, special under cadmium stress. To investigate the uptake and accumulation capability of heavy metals in rice cultivars from polluted soils is extremely necessary.In this study, the traits were analyzed with 10 rice cultivars under different cadmium concentration (2,5 and 10 mg/kg) stress and the control conditions, such as germination vigor, germination rate, germination index, seedling height, seedling maximum root length, seedling fresh weight per plant and seedling dry weight per plant. The cadmium levels in different plant organs were analyzed with 9 rice cultivars under 5mg/kg cadmium stress and the control conditions. Under 5mg/kg cadmium stress, quantitative trait loci (QTL) for seedling tolerance to cadmium toxicity and controlling cadmium concentration in brown rice were mapped using a recombinant inbred lines (RILs) population derived from the cross of Jiucaiqing (japonica)×IR26(indica), based on relative seedling height, relative maximum root length, relative fresh weight per plant, relative dry weight per plant and cadmium concentration in brown rice.The objectives of this study were to provide insights for screening and breeding of rice varieties which was low uptake or accumulation capability in grains by MRS approach and to provide genetic basis for genetics studies of cadmium-tolerance in rice. The main results indicated as follows:1 The effects on seed germination, seedling growth under cadmium stress and the control conditions were studied through exposure to a series of cadmium concentrations (2, 5 and 10 mg/kg) by solution culture. There were no significant difference in seed germination vigor, seed germination rate and seed germination index between the control conditions and different cadmium concentration treatment. However, seedling maximum root length, seedling height were significantly decreased under cadmium stress, and the restraining effects were enhanced with the increase of cadmium concentration. Seedling fresh weight per plant was significantly decreased under cadmium stress, but there was no significant difference among various concentrations of cadmium treatment. Seedling dry weight per plant of most rice varieties were increased under cadmium stress. In summary, seed germination vigor, seed germination rate and seed germination index might not be regarded as the indicators reflected cadmium-tolerance capability of rice seedling, while using seedling maximum root length, seedling height and seedling fresh weight per plant together might evaluate the seedling capability to cadmium-tolerance.2 Under cadmium (5mg/kg) stress and the control conditions, a pot experiments were conducted to investigate the difference of cadmium concentration in different plant organs. The uptake and accumulation capability of cadmium in indica cultivars was stronger than that in japonica cultivars under cadmium stress, and there was significant difference for uptake and accumulation capability in different cultivars. The cultivar, IR28, accumulated higher cadmium level in its plant organs than other varieties under cadmium stress. Nanjing 41 accumulated lower cadmium level in its plant organs than other varieties, indicating that Nanjing 41 was a cultivar which had lower uptake and accumulation capability of cadmium. Under cadmium stress, the cadmium concentration in roots was 6.414 mg/kg, range from 3.357 to 12.723 mg/kg, as 4.9 times and 29.8 times as that of cadmium concentration in shoots and brown rice respectively. Roots were the main organ accumulated cadmium in rice plants. Significant and positive correlations were existed for cadmium concentration between roots and shoots, shoots and brown rice under cadmium stress. There was no significant connection between cadmium concentration in plant organs and cadmium-tolerance capability of rice seedling.3 According to the relative traits of seedling growth, the QTL controlling cadmium-tolerance capability for seedling were mapped using a RILs population derived from a cross of Jiucaiqing (japonica)×IR26 (indica) under cadmium stress (5mg/kg) by solution culture. Three QTL for cadmium-tolerance capability were detected on chromosomes 7 and 11 in 2007, qRSH-7 controlled the loci of relative seedling height, qRDW-11a and qRDW-11b controlled the loci of relative seedling dry weight per plant. The additive effects were 5.09,4.77 and 4.37, respectively. Six QTL for cadmium-tolerance capability were detected in 2008. Three out of the six QTL, qRDW-1, qRDW-2 and qRDW-7, controlled the loci of relative seedling dry weight per plant were on chromosomes 1,2 and 7. qRSH-2, controlled the loci of relative seedling height was on chromosomes 2. qRFW-2 and qRFW-7, controlled the loci of relative seedling fresh weight per plant were on chromosomes 2 and 7. Among 9 detected QTL, both qRDW-7 and qRFW-7 were located between markers RM6872 and RM11 on chromosomes 7,and the additive effects of qRDW-7 and qRFW-7 were 4.89 and 5.44, explaining over 15.0% of the total phenotypic variation respectively. The additive effects of qRSH-7 and qRDW-1 were 5.09 and 3.64, explaining 13.48% and 10.06% of the total phenotypic variation, respectively. The others QTL were explained below 10%, respectively, of the total phenotypic variation.4 Under cadmium stress (5 mg/kg), QTL determining cadmium concentration in brown rice was mapped using a RILs population, derived from the cross of Jiucaiqing (japonica)×IR26 (indica). Two QTL determining cadmium concentration in brown rice were detected on chromosomes 11, named as qCCBR-11a and qCCBR-11b. Their additive effects were 0.089 and 0.075 respectively, explaining 11.17% and 7.66% of total phenotypic variation. The useful alleles determining low uptake and accumulation capability in brown rice was derived from Jiucaiqing. There was no significant correlation between the cadmium concentration in brown rice and plant height, spikelets per panicle, filled grains per panicle, seed setting rate and 1000-grains weight, indicating that cadmium concentration in brown rice was a relative independences trait, controlled by genetic genes.