Genotypic Differences In Cadmium Tolerance And Accumulation In Rice And Its Mechanisms

Cadmium (Cd) is a harmful and nonessential heavy metal for plants and animals, and its entrance into food through uptake and accumulation in plants will threaten human's health. Cd tolerance and its accumulation and distribution in plants showed dramatic difference among plant species and genotypes within a species. Rice is one of the most important food crops in the world as well as in China and it has been shown to absorb Cd from soil and have high accumulation in plant tissues. Therefore, it is imperative to develop the cultivars with Cd tolerant and low Cd accumulation in grains. Meanwhile, Cd tolerance, absorption and accumulation in plants are greatly affected by many factors. So far the mechanisms involved in Cd tolerance and accumulation has been not cleared, thus posing the difficult in safe crop production. In the thesis, we investigated genotypic difference and genotypic and environmental variation in Cd accumulation and distribution between shoot and grain of rice, inhibition of Cd toxicity on plant growth and genotypic difference in terms of antioxidative enzyme and nitrogen metabolism in response to Cd stress. And we also determined the effect of Al and salt stress pretreatments and Fe nutrition on Cd tolerance and accumulation in rice.1. Genotypic and environmental effect on Cd accumulation anddistribution in rice plantsEleven japonica rice varieties were collected from different rice plant districts in Zhejiang province, and Cd concentration in grains and shoots was determined. The results revealed that Cd concentration in rice grains and shoot was closely associated with effective Cd level in soil, and there was a significant difference among genotypes. The ratio of Cd concentration in grain and shoot was negatively correlated with effective Cd level in soil, indicating that higher Cd in soil results in more Cd accumulation in shoot than in grain. Cd concentration in grain was mainly dependent on Cd allocation among tissues, thus the cultivars with high shoot Cd concentrationwas not essentially associated with high grain Cd concentration. Analysis of genotypic and environmental variations of Cd concentration in rice grain and shoot showed that genotypic and environmental effects and their interaction were all significant for grain Cd accumulation, while the interaction was most predominant in shoot Cd accumulation.2. Effects of cadmium stress on plant growth and antioxidativeenzyme systems in different rice genotypesThe effects of cadmium stress on growth, lipid peroxidation and antioxidant enzyme system were studied by using two rice cultivars, Bing 97252 with low and Xiushui 63 with high grain Cd accumulation. In a hydroponic experiment, four Cd levels, i.e. 0, 0.1, 1.0 and 5.0 nM were designed. The results showed that Cd stress reduced plant height and chlorophyll content, altered malondialdehyde (MDA) content and activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD). Roots and shoots responded differently to Cd in terms of antioxidant enzyme activity. Generally the activities of SOD, POD and CAT decreased with increase in Cd level, while MDA content in shoots and roots increased with Cd level, being more pronounced for Xiushui 63 than Bing 97252. At booting stage a decrease of 46-52 % in SOD activity was noted in the roots of plants exposed to 5u mol/L Cd, while at tillering stage the decrease was 13-19 % compared to the control. A significant decrease in chlorophyll content and plant height was noted in higher Cd treatment (1.0 and 5.0 u mol/L). The higher MDA and less chlorophyll content in Xiushui 63 indicate that it is more sensitive to Cd stress than Bing 97252.3. Different responses to cadmium stress in nitrogen metabolismbetween weedy-rice and cultivated rice (Oryza sativa)The effect of cadmimum stress on nitrogen uptake and some enzymes related to N metabolism was compared between weedy-rice and japanica rice cv. Koshihikari witha hydroponic experiment. There were four Cd levels, i.e. 0, 0. 1, 1.0 and 5.0 umol/L with ammonium nitrate as N source. The results showed that Cd concentration and accumulation in both shoots and roots of the two rices increased dramatically with increasing Cd level. However the extent of the increase varied with the rice genotypes in study. In high Cd levels (1.0 and 5.0 pmol/L), weedy rice had lower Cd concentration and fewer Cd accumulation in roots than Koshihikari, while for shoots the case was just opposite, with Koshihikari having smaller values for the two parameters. It was indicated that more Cd was translocated to shoots in weedy rice than in Koshihikari, and the shoots of weedy rice was more capable of detoxifying Cd than that of Koshihikari. N concentration, accumulation and the activities of nitrate reductase (NR), glutamicoxalacetic transaminase (GOT) and glutamatepyruvate transaminase (GPT) decreased markedly with increasing Cd level. There was a clear difference in the extent of the decrease between the two rices. In general, the inhibition of N uptake and N metabolism-related enzymes was relatively less for weedy rice than for Koshihikari. The present research proved that weedy rice had stronger Cd toxicity than japanica rice cv. Koshihicari and there is a close association between performance of these N metabolism related enzymes and Cd toxicity tolerance.4. Effect of Al3+ pretreatment on Cd tolerance and accumulation inriceAzucena, a japonica rice resistant to Al and IR1552, an indica rice sensitive to Al were used to investigate the response of antioxidative enzyme to Cd stress and its uptake and accumulation in rice after Al3+ pretreatment. The results revealed that Al3+ pretreatment increased MDA content in plants. Furthermore, MDA content was higher in Azuncena than in IR1552, indicating higher Al tolerance for Azuncena than IR1552. Al3+ pretreatment promoted SOD and POD activity in two rice plants under Cd stress. Results also showed that Al3+ pretreatment increased Cd concentration in two rice plant, and Cd concentration was lower in Azucena than in IR1552, which indicated that Al3+ pretreatment caused more severe damage of cell membrane to Azucena thanIR1552.5. Cadmium toxicity tolerance in rice as affected by betaine aldehydedehydrogenase gene and salt pretreatmentThe influence of betaine aldehyde dehydrogenase (BADH) and salt pretreatment on Cd stress tolerance in rice was investigated by using Xiushui 11 and its BADH transformant Bxiushui 11. The results showed that the plants treated previously with 0.5% for 5 d had higher Cd concentrations in both shoot and leaf of the two rice genotypes relative to the control (without NaCl). Meanwhile salt pretreatment resulted in a reduced malondialdehyde (MDA) content and an increased superoxide dismutase (SOD) and peroxidase (POD), and higher proline content. It may be assumed that salt pretreatment enhances the defense ability of plant or tissue against subsequent oxidative stress. In comparison with wild type cv. Xiushui 11, the transgenic Bxiushui 11 with BADH gene had lower Cd and MDA concentrations, higher SOD and POD activities, proline content. It is indicated that BADH expression may improve Cd stress tolerance through increasing anti-oxidative enzymatic activity and osmoprotectant.6. Cd toxicity to rice and its accumulation in plant is dependent oniron nutrition statusThe effect of Fe nutrition on Cd toxicity and its accumulation in rice plants was studied using a hydroponic experiment. At middle and high Fe levels (1.89 and 16.8 mg/L), cadmium stress reduced dramatically plant height and chlorophyll content, while at low Fe (0.171 mg/L) the reduction was much alleviated. MDA content in shoots and roots had less change with an increased Cd level at low Fe level of Fe3+, but showed a rapid increase when plants were exposed to middle and high Fe level. In comparison with low and middle Fe levels, high Fe treatment reduced markedly Cd concentration in both shoots and roots, and by contrast Fe concentration in the plants was lower at higher Cd level than at low Cd level. The results proved the theory thatiron transporter protein (carrier) for Fe uptake strategy II in cell membrane can also transport Cd, and indicated Fe-Cd competition for common carriers. Correlation analysis showed that there were significant correlations between MDA content and iron content in rice shoots and roots. It is suggested that occurrence of oxidative stress for the plants exposed to Cd stress is mediated by Fe nutrition disturbance due to Cd and Fe interaction. The present results also showed that Cd stress affect uptake of some macronutrients such as Ca, P and Mg and micronutrients such as Cu and Zn.