Cd Accumulation In Rice Grains Research Articles

Identification of QTLs and Validation of qCd-2 Associated with Grain Cadmium Concentrations in Rice

Cadmium (Cd) is one of heavy metals harmful to human health. As rice is the main staple food in Asia and Cd is easily contaminated in rice, the molecular regulation of Cd accumulation should be explored. In this study, a recombinant inbred population derived from Xiang 743/Katy was grown in Cd-polluted fields and used to map the quantitative trait loci (QTLs) for Cd accumulation in rice grains. We identified seven QTLs distributed on chromosomes 2, 3, 6, 7, 8 and 10. These QTLs displayed phenotypic variances of 58.50% and 40.59% in 2014 and 2015, respectively. Two QTLs, qCd-2 and qCd-7, were identified in both the two years. qCd-2 was detected on the interval of RM250–RM207 on chromosome 2, with an LOD of 2.51 and a phenotypic contribution of 13.75% in 2014, and an LOD of 3.35 and a phenotypic contribution of 14.16% in 2015. qCd-7 co-localized with the cloned qCdT7 on chromosome 7 and may represent the correct candidate. The other five QTLs were detected only in one year. To further confirm the effects of qCd-2, a residual heterozygous line designated as RHL945, with a heterozygous interval of RM263–RM207 on chromosome 2, was selected from the recombinant inbred population and used to develop an F2 population consisting of 155 individual plants. By incorporating further simple sequence repeat markers into the segmental linkage map of the target region, qCd-2 was delimited in the interval of RM5404–RM3774, with an LOD value of 4.38 and a phenotypic contribution of 15.52%. These results reflected the genetic regulation of grain Cd in rice and paved the way for the future cloning of qCd-2.

Decreasing cadmium uptake of rice (Oryza sativa L.) in the cadmium-contaminated paddy field through different cultivars coupling with appropriate soil amendments

In southern China, Cd-tainted rice produced in Cd-contaminated acidic paddy soils is a serious health concern. Thus, it is essential and urgent to remediate Cd-contaminated soils and reduce Cd accumulation in rice grains for food safety and human health. Four soil amendments (e.g., lime, biochar, soil conditioner, and seaweed organic fertilizer) combined with two rice cultivars (Yongyou17, YY-17; and Xiushui09, XS-09) with different Cd accumulation abilities were selected to evaluate the efficacy of safe utilization in a slightly moderately Cd-contaminated paddy soil. Results showed that soil amendments significantly increased soil pH and reduced Cd bioavailability compared with the control. Linear regression analysis showed that the decrease in Cd bioavailability was significantly and positively correlated with soil pH increase (P < 0.001). Soil amendments were efficient in reducing Cd uptake by rice root as well as translocation from root to aboveground plant parts. The Cd concentrations in brown rice of YY-17 and XS-09 were 0.34 and 0.22 mg kg−1 for control treatments, respectively, while the Cd concentrations of rice grain using were reduced by 45 to 88% and 55 to 88% soil amendments for two cultivars. Considering the costs and efficiency of soil amendments, the soil conditioner was the best amendment. In situ immobilization combined with low-Cd-accumulation rice cultivar is an effective approach for food safety in lightly moderately Cd-contaminated acidic paddy soils.

Effect of Particulate Organic Matter on Cadmium Uptake and Transport in Rice

A pot experiment was carried out to investigate the effects and mechanism of particulate organic matter (POM) on cadmium uptake of rice on purple paddy soil. Organic carbon content in soil and POM and Cd content in POM and rice were analyzed. The results showed that the contents of total soil organic carbon (SOC), dissolved organic carbon (DOC), organic carbon in POM (POM-C), Cd in POM (POM-Cd), and the Cd enrichment factor in POM increased with the application of POM. When POM was applied at a rate of 2.5 g·kg-1, the proportion of POM-C and POM-Cd in soil significantly increased. Application of POM also increased rice plant biomass and enhanced the accumulation of Cd in rice. The Cd content was reduced by 24%-42% in rice roots and increased by 9%-30% in rice shoots with POM application, whereas it was reduced by 17% and 36% in rice grains when the application of POM was 0.5 g·kg-1 and 1.0 g·kg-1, respectively, but increased by 39% when the application of POM was 2.5 g·kg-1. Application of POM had no significant effect on the distribution of Cd in rice roots and shoots, but significantly affected the distribution of Cd in rice grains. The transportation of Cd from shoots to grains was inhibited at low rates of POM, but was promoted at high rates of POM, and consequently increased the Cd content in rice grains. Correlation analyses showed that the content of soil available Cd was the main factor affecting Cd accumulation in rice shoots, and the total content of POM-Cd in soil was the main factor affecting Cd accumulation in rice grains. Therefore, the addition of POM to soil could affect the Cd uptake of rice by changing the content of SOC, DOC, POM-C, POM-Cd, and available Cd in paddy soil.

Inoculation of soil with cadmium-resistant bacterium Delftia sp. B9 reduces cadmium accumulation in rice (Oryza sativa L.) grains

Bioremediation of heavy metal polluted soil using metal-resistant bacteria has received increasing attentions. In the present study, we isolated a heavy metal-resistant bacterial strain from a Cd-contaminated soil, and conducted pot experiments to evaluate the effect of bacterial inoculation in soil on soil Cd speciation, rice grain biomass and Cd accumulation. We find that the isolated bacterial strain is a Gram-negative bacterium, and named as Delftia sp. B9 based on the 16S rDNA gene sequence analysis. TEM–EDS manifests that Cd can be bioaccumulated inside cell, resulting in intracellular dissolution. The Cd contents of rice grain in the two rice cultivars (early and late rice) are all below the standard limit for Food Safety of People's Republic of China (0.2 mg/kg) after the treatment of both living and non-living cells. Non-living cells are more applicable than the use of living cells for the short time bioremediation. The average content of soil exchangeable fraction of Cd decreases whereas the residual fraction increases with bacterial inoculation. All our results suggest Delftia sp. B9 is able to the stabilization of Cd in soil and reduce Cd accumulation in rice grain, therefore, this strain is potentially suitable for the bioremediation of Cd-contaminated paddy soils.

Biochar reduces cadmium accumulation in rice grains in a tungsten mining area-field experiment: effects of biochar type and dosage, rice variety, and pollution level.

Cadmium (Cd)-contaminated rice (Oryza sativa) in Southern China is a great threat to food security, and the paddy soil remediation is urgently needed to reduce Cd accumulation in rice. Application of biochar could effectively immobilize soil Cd and reduce Cd uptake by rice. Fields that were applied with soil treatments including control and 15 and 30tha-1 each hickory nut shell-derived biochar (KC) or maize straw-derived biochar (MC), and grown with two rice varieties (hybrid rice and late japonica rice) were selected for this study. The long-term effect of biochars on decreasing Cd bioavailability in paddy soils was evaluated. The results showed when MC was applied at 15tha-1, DTPA-Cd (soil cadmium extracted by diethylenetriamine pentaacetic acid) was reduced by 20.0 and 34.5% in Field A (slightly Cd pollution) and B (moderately Cd pollution), respectively. In Field B, soil DTPA-Cd concentrations with application of 30 t ha-1 biochars were all lower than that of 15tha-1 biochar, but there were no significant differences between the two types of biochars. Cd concentration in rice grains and straws of hybrid rice are two times more than those of late japonica rice. Cd bio-concentration factor both of grains and straw was significantly increased by biochar application, which in Field A was higher than that in Field B. Our results suggest that biochars reduce Cd accumulation in rice grains by immobilizing soil Cd. KC has a higher potential in lowering Cd bioavailability than MC. Hybrid rice should be prohibited to cultivate in these areas.

Effective methods to reduce cadmium accumulation in rice grain

Contamination of cadmium (Cd) in paddy soil is a serious environmental problem threatening food safety in some parts of southern China, where rice grain Cd concentration often exceeds the Chinese limit (0.2 mg kg−1). We tested the effect of CaCO3 liming combined with growing low Cd cultivars on Cd accumulation in rice grain in 2-year field trials. A liming model was used to predict the doses of lime required. Lime (2.25–7.5 t ha−1) was applied in 2016 only and the effect monitored in both 2016 and 2017. Soil pH was increased from the initial 5.5 to the target value 6.5 by 7.5 t ha−1 CaCO3. Liming greatly reduced CaCl2-extractable Cd in the rhizospheric soil. Grain Cd concentration in the control exceeded the limit by 2–5 times. Liming at 7.5 t ha−1 decreased grain Cd concentration by 70–80% in both seasons without affecting grain yield. Grain Cd concentration was below the limit in the 7.5 t ha−1 liming treatment in both seasons. Grain Cd concentration correlated closely with CaCl2-extractable Cd in rhizospheric soil at the grain maturing stage. Seasonal difference in grain Cd concentration was attributed to the soil water status at the grain maturing stage. Liming had no significant effect on grain arsenic concentration or speciation. A single application of CaCO3 to raise soil pH to 6.5, combined with low Cd cultivars and delayed drainage of paddy water during late grain filling stage, was highly effective at reducing Cd accumulation in rice grain.

Effects of root morphology and anatomy on cadmium uptake and translocation in rice (Oryza sativa L.)

A clear description of the certain mechanisms of cadmium (Cd) uptake and translocation in rice (Oryza sativa L.) may help to reduce Cd accumulation in rice grain. Hydroponic experiments were carried out to determine the effects of cultivation conditions (aerated and stagnant) on the uptake, translocation and subcellular distribution of Cd in relation to the morphology and anatomy of roots in two rice genotypes with different Cd accumulations in grains. Marked differences in morphology and anatomy were observed between these two genotypes under different cultivation conditions. Genotypes with low Cd accumulation in grains tended to develop fewer root tips per root surface area, larger root porosity and more mature apoplastic barriers. The stagnant cultivation condition decreased the number of root tips per root surface area but increased root porosity and accelerated apoplastic barrier formation in root tissues. Correlative Cd uptake studies revealed that rice plants with fewer number of root tips per root surface area reduced root Cd uptake ability, while mature apoplastic barriers increased root Cd retention in cell walls and the symplast. Thus, the fewer number of root tips per root surface area and the earlier formation of mature apoplastic barriers led to lower Cd uptake and translocation. The results indicated that the morphology and anatomy of roots could play important roles in Cd uptake and translocation in rice, and could be influenced by both genotype and cultivation conditions. The present results would be useful in screening and planting rice plants with low Cd accumulation.

Attenuation of inorganic arsenic and cadmium in rice grains using by-product iron materials from the casting industry combined with different water management practices

ABSTRACTWe examined the effect of two types of iron (Fe) material produced by the casting industry (spent steel shot [SSS] and residual iron material from steel shot production) on the mobility of arsenic (As) and cadmium (Cd) in soils. We also examined the uptake of these elements by rice plants (Oryza Sativa L.) under continuously flooded (CF) and water-saving (WS) cultivation. The application of both Fe materials (at 10 and 30 t ha‒1) strictly limited As mobilization in soils under CF cultivation. As a result, As uptake by rice plants declined, along with the total and inorganic As (iAs) concentration in rice grains. In comparison, As immobilization caused by the application of Fe material was less clear under WS cultivation. The rate of Fe material application was negatively correlated with As uptake by rice plants. It was also negatively correlated with total and iAs concentration in rice grains under both water management practices. The combination of applying Fe materials and WS cultivation decreased iAs concentration in rice grains to approximately one-fifth of that in rice grains produced from plants grown on soils without Fe material application under CF cultivation. CF cultivation strictly decreased dissolved Cd in soils, as well as Cd in rice grains with and without Fe material application. The application of Fe materials decreased Cd mobility and, hence, Cd uptake in rice plants, ultimately reducing the accumulation of Cd in rice grains under WS cultivation. Residual Fe material had a statistically greater effect at attenuating Cd accumulation in rice grains than SSS. The present study demonstrated the potential of combining by-product Fe material application and water management practices to attenuate iAs and/or Cd concentrations in rice grains. Practical countermeasures should be carefully adopted that consider the existing risks of iAs and Cd on each paddy field, and the combined effect of Fe material application and water management practices.

Quantitative Relationship Between Paddy Soil Properties and Cadmium Content in Rice Grains

Rice is a crop with the potential for high accumulation of Cd, which can be affected by many factors. Sixty pairs of soil and rice samples from different plots were collected and analyzed, in order to understand the quantitative relationships between the Cd content in soil and the properties of soil and the Cd content in rice grains under field conditions, by simple and multiple regression analyses. The results showed that the Cd contents in soil and rice grains ranged from 0.15-2.54 mg·kg-1 and 0.02-2.00 mg·kg-1, respectively. According to the result of simple regression analysis, there were significantly positive correlations (P<0.01) between the Cd contents in soil and rice grains (r=0.392); the pH, SOM, and CEC in soil also had certain effects on Cd accumulation in rice grains, which were not significant, however. When the soil pH was<6.5, the Cd content in rice grains increased with increasing soil pH, but decreased with increasing soil pH when the soil pH was>6.5. The Cd content, pH, SOM, and CEC in the soil and the Cd content in rice grains were analyzed by multiple regression analysis, and five equations, which all reached extremely significant levels (P<0.01), were obtained. The equation that included the four variables (Cd content, pH, SOM, and CEC in soil) had the most r, and it could predict the Cd content in rice grains better, given the conditions of the present study.

Effective reduction of cadmium accumulation in rice grain by expressing OsHMA3 under the control of the OsHMA2 promoter

Reducing cadmium (Cd) accumulation in rice grain is an important issue for human health. The aim of this study was to manipulate both expression and tissue localization of OsHMA3, a tonoplast-localized Cd transporter, in the roots by expressing it under the control of the OsHMA2 promoter, which shows high expression in different organs including roots, nodes, and shoots. In two independent transgenic lines, the expression of OsHMA3 was significantly enhanced in all organs compared with non-transgenic rice. Furthermore, OsHMA3 protein was detected in the root pericycle cells and phloem region of both the diffuse vascular bundle and the enlarged vascular bundle of the nodes. At the vegetative stage, the Cd concentration in the shoots and xylem sap of the transgenic rice was significantly decreased, but that of the whole roots and root cell sap was increased. At the reproductive stage, the concentration of Cd, but not other essential metals, in the brown rice of transgenic lines was decreased to less than one-tenth that of the non-transgenic rice. These results indicate that expression of OsHMA3 under the control of the OsHMA2 promoter can effectively reduce Cd accumulation in rice grain through sequestering more Cd into the vacuoles of various tissues.

Cadmium bioavailability well assessed by DGT and factors influencing cadmium accumulation in rice grains from paddy soils of three parent materials

High Cd resulted from natural background or agricultural activities in some of the paddy soils in South China is a concern. It is imperative to understand the relationship between Cd bioavailability in high Cd soils and Cd accumulation in rice grains. Therefore, our objectives are to explore the relationship between Cd accumulation in rice grains and Cd availability which was characterized with five different methods, and to investigate factors influencing Cd accumulation in rice. One hundred and twenty-two paired soil and rice plant samples were collected from paddy fields of three parent materials (sandshale, limestone, and Quaternary red earth) in Guangxi Province. Soil Cd availability was assessed using three chemical extractants (0.43 M HNO3, 0.01 M CaCl2, and 0.005 M DTPA) and compared with Cd in soil solution and that sampled by the diffusive gradients in thin films (DGT) method. A five-step sequential fractionation of Cd was also carried out. Random forest (RF) was employed for assessing factor relative importance in Cd accumulation in rice plants and grains in high Cd soils. All of the soil samples had a total Cd content exceeding the maximum permissible concentration (0.3 mg kg−1) for soils with the exchangeable fraction taking up 50, 56, and 39%, respectively. For the rice grains harvested from the soils derived from sandshale, limestone, and Quaternary red earth, 67, 95, and 28%, respectively, had a Cd content exceeding the maximum permissible concentration for grain (0.2 mg kg−1). The results demonstrated high Cd bioavailability in these areas and indicated potential health risk when people ingest the rice grains. Cd measured by DGT best reflected Cd bioavailability. The Cd in soil solution (Cdsoln), which is directly available, and CaCl2-extracted Cd were also significantly related to grain Cd content. Factor relative importance assessment based on random forest (RF) showed that soil pH was among the top two most important physicochemical factors of the three types of soils influencing Cd accumulation in rice grains. Besides, dissolved organic carbon (DOC), clay, and dithionite-citrate-bicarbonate-extracted Mn (MnDCB) was the other major influencing factor in the sandshale soils, limestone soils, and Quaternary red earths, respectively. The results indicated that DGT technique better predicted Cd bioavailability. Manipulating soil pH could be used to reduce Cd bioavailability and its accumulation in plants. However, manipulating soil DOC, clay and Fe/Mn content were soil-type-dependent.

A defensin-like protein drives cadmium efflux and allocation in rice

Pollution by heavy metals limits the area of land available for cultivation of food crops. A potential solution to this problem might lie in the molecular breeding of food crops for phytoremediation that accumulate toxic metals in straw while producing safe and nutritious grains. Here, we identify a rice quantitative trait locus we name cadmium (Cd) accumulation in leaf 1 (CAL1), which encodes a defensin-like protein. CAL1 is expressed preferentially in root exodermis and xylem parenchyma cells. We provide evidence that CAL1 acts by chelating Cd in the cytosol and facilitating Cd secretion to extracellular spaces, hence lowering cytosolic Cd concentration while driving long-distance Cd transport via xylem vessels. CAL1 does not appear to affect Cd accumulation in rice grains or the accumulation of other essential metals, thus providing an efficient molecular tool to breed dual-function rice varieties that produce safe grains while remediating paddy soils.

Effects of Straw Incorporation on Cadmium Accumulation and Subcellular Distribution in Rice

Cadmium (Cd) is classified as a Group-1 human carcinogen and rice consumption constitutes a major source of dietary intake of Cd for populations whose staple food is rice. Straw incorporation is widely performed in Cd-contaminated paddy fields, which may significantly affect the bioavailability of Cd in soil and the distribution of Cd in rice plants, consequently altering Cd accumulation in rice grains. In this study, both pot and field trials were conducted to investigate the effects of different amounts of straw incorporation (0.0%, 1.0%, 2.5%, and 5.0%) on Cd sub-cellular distribution in rice plants and Cd accumulation in rice grains. The results showed that Cd was mainly sequestered in cell wall, accounting for 86%-95% and 30%-51% of total cadmium in root and shoot cells, respectively. In shoot cells, about 35%-61% of Cd was distributed in cellular soluble fractions. When rice straw was incorporated at 1.0% and 2.5% levels, Cd sequestration in the cell wall significantly increased and Cd translocation from roots to shoots significantly decreased. However, when rice straw was incorporated at the 5% level, Cd sequestration in root cell walls significantly decreased and Cd translocation from roots to shoots significantly increased at the tillering stage. At the filling stage, 5% rice straw incorporation still significantly increased Cd sequestration in root cell walls and Cd translocation from roots to shoots did not significantly change. The rice straw and rape straw used for the field trail contained high concentrations of Cd (0.49 and 0.67 mg·kg-1, respectively). Rape straw incorporation alone or together with lime did not significantly affect Cd accumulation in brown rice or rice straw. Rice straw incorporation alone did not significantly affect Cd accumulation in brown rice or rice straw, while incorporation with lime significantly decreased Cd accumulation in both brown rice and rice straw. Biochar application can also significantly reduce Cd accumulation in rice and when biochar was added together with lime, the reduction in Cd accumulation in rice was more significant. Therefore, at Cd-contaminated paddy fields, rice straw or rape straw is not suggested to be returned directly; incorporation with lime would be better for reducing Cd accumulation in rice grains. The results of this study will provide theoretical and practical guidance for the safe production of rice and for straw recycling at Cd-polluted paddy fields.

Cadmium Accumulation Risk in Vegetables and Rice in Southern China: Insights from Solid-Solution Partitioning and Plant Uptake Factor.

Solid-solution partitioning coefficient (Kd) and plant uptake factor (PUF) largely determine the solubility and mobility of soil Cd to food crops. A four-year regional investigation was conducted in contaminated vegetable and paddy fields of southern China to quantify the variability in Kd and PUF. The distributions of Kd and PUF characterizing transfers of Cd from soil to vegetable and rice are probabilistic in nature. Dynamics in soil pH and soil Zn greatly affected the variations of Kd. In addition to soil pH, soil organic matter had a major influence on PUF variations in vegetables. Heavy leaching of soil Mn caused a higher Cd accumulation in rice grain. Dietary ingestion of 85.5% of the locally produced vegetable and rice would have adverse health risks, with rice consumption contributing 97.2% of the risk. A probabilistic risk analysis based on derived transfer function reveals the amorphous Mn oxide content exerts a major influence on Cd accumulation in rice in pH conditions below 5.5. Risk estimation and field experiments show that to limit the Cd concentration in rice grains, soil management strategies should include improving the pH and soil Mn concentration to around 6.0 and 345 mg kg-1, respectively. Our work illustrates that re-establishing a balance in trace elements in soils' labile pool provides an effective risk-based approach for safer crop practices.

Characterization of cadmium-resistant bacteria and their potential for reducing accumulation of cadmium in rice grains

Cadmium (Cd) pollution is a serious widespread environmental problem that not only destroys the microbial ecology of soil and decreases crop production, but also poses a serious risk to human health. Many methods have been used for the remediation of Cd pollution but none of these is totally satisfactory. Microbial remediation strategies have attracted increasing interest since they are environmentally friendly and cost-effective. In the present study, three Cd-resistant bacteria were isolated and evaluated for potential application in Cd bioremediation. Based on their morphological, physiological and biochemical characteristics, together with 16S rDNA gene sequence analyses, bacteria were identified as Stenotrophomonas acidaminiphila (2#), Pseudomonas aeruginosa (9#) and Delftia tsuruhatensis (12#). Pseudomonas aeruginosa showed very high tolerance to metals, especially Cd (2200mg/L), Zn (1800mg/L) and Pb (1200mg/L), and is thought to be a multi-metal-resistant bacterium. Pseudomonas aeruginosa was also sensitive to 13 different antibiotics. The effects of the bacterial strains on the growth of rice plants and their ability to reduce Cd accumulation from Cd-contaminated soils in pot experiments were also evaluated. For Oryza sativa L. A grown in contaminated soil (3mg/kg Cd), the accumulation of Cd was decreased by 31.2 and 25.5% in brown rice and polished rice, respectively, by strain 9#; Pseudomonas aeruginosa was more effective in reducing Cd accumulation in rice grains than a mixture of strains. For Oryza sativa L. B, a mixture of strains acting synergistically was more effective than a single strain in reducing Cd accumulation; treatment with mixed strains (strains+3mg/kg Cd) resulted in 41.3, 35.9, and 32.6% reductions in Cd accumulation in unhulled rice, brown rice and polished rice, respectively. Although different results were obtained for two rice varieties, it can still be concluded that Cd-resistant bacteria are suitable for reducing Cd accumulation in rice grains and show potential for bioremediation of Cd-contaminated soils.

Cadmium availability in rice paddy fields from a mining area: The effects of soil properties highlighting iron fractions and pH value

Cadmium (Cd) availability can be significantly affected by soil properties. The effect of pH value on Cd availability has been confirmed. Paddy soils in South China generally contain high contents of iron (Fe). Thus, it is hypothesized that Fe fractions, in addition to pH value, may play an important role in the Cd bioavailability in paddy soil and this requires further investigation. In this study, 73 paired soil and rice plant samples were collected from paddy fields those were contaminated by acid mine drainage containing Cd. The contents of Fe in the amorphous and DCB-extractable Fe oxides were significantly and negatively correlated with the Cd content in rice grain or straw (excluding DCB-extractable Fe vs Cd in straw). In addition, the concentration of HCl-extractable Fe(II) derived from Fe(III) reduction was positively correlated with the Cd content in rice grain or straw. These results suggest that soil Fe redox could affect the availability of Cd in rice plant. Contribution assessment of soil properties to Cd accumulation in rice grain based on random forest (RF) and stochastic gradient boosting (SGB) showed that pH value should be the most important factor and the content of Fe in the amorphous Fe oxides should be the second most important factor in affecting Cd content in rice grain. Overall, compared with the studies from temperate regions, such as Europe and northern China, Fe oxide exhibited its unique role in the bioavailability of Cd in the reddish paddy soil from our study area. The exploration of practical remediation strategies for Cd from the perspective of Fe oxide may be promising.

Comparative proteomic analysis provides new insights into cadmium accumulation in rice grain under cadmium stress

Rice is one of the most important staple crops. During the growth season, rice plants are inevitably subjected to numerous stresses, among which heavy metal stress represented by cadmium contamination not only hindering the yield of rice but also affecting the food safety by Cd accumulating in rice grains. The mechanism of Cd accumulation in rice grains has not been well elucidated. In this study, we compare the proteomic difference between two genotypes with different Cd accumulation ability in grains. Verification of differentially expressed protein-encoding genes was analyzing by quantitative PCR (QPCR) and reanalysis of microarray expression data. Forty-seven proteins in total were successfully identified through proteomic screening. GO and KEGG enrichment analysis showed Cd accumulation triggered stress-related pathways in the cells, and strongly affecting metabolic pathways. Many proteins associated with nutrient reservoir and starch-related enzyme were identified in this study suggesting that a considerably damage on grain quality was caused. The results also implied stress response was initiated by the abnormal cells and the transmission of signals may mediated by reactive oxygen species (ROS). Our research will provide new insights into Cd accumulation in rice grain under Cd stress.

A rice transporter involved in Cd accumulation in rice grains

A rice transporter involved in Cd accumulation in rice grains

Genotypic variation of cadmium accumulation and distribution in rice

An effective way to reduce the risk of cadmium (Cd) entering the food chain is to use low Cd-accumulation rice cultivars, particularly in Asia. The fundamental requirement for breeding low grain Cd-accumulation cultivars is to know the genotypic variation in Cd accumulation and the physiological processes and genetic basis governing the Cd accumulation in rice grain. In this experiment, genotypic variation in Cd accumulation and distribution among rice organs was studied using thirty-five rice varieties. They were grown with irrigation water containing 2 ppm Cd throughout rice growing season under field condition in 2007. At harvest, plants were sampled and analyzed for Cd concentration and accumulation in each rice organ. Significant variation of Cd concentration and accumulation in rice organs were found among thirty-five rice cultivars, revealing more than 8-fold varietal differences in grain Cd concentration and shoot Cd accumulation. Cd concentration and accumulation in grain were significantly different among cultivar groups, showing the highest in indica and the lowest in temperate japonica. Tongil-type and tropical japonica rice showed a Cd concentration intermediate to that of temperate japonica and indica rice. The higher Cd accumulation in grain of indica rice was attributable to the greater ability of Cd uptake. The greater ability of root-shoot translocation in tropical japonica and shoot-grain redistribution in tongil-type resulted in the significantly higher grain Cd concentration in these cultivar groups than in temperate japonica. For over 35 cultivars tested, grain Cd concentration revealed a significant positive correlation with root Cd concentration and shoot Cd concentration and accumulation while no significant correlation with root-shoot translocation factor and shoot-grain redistribution ratio. However, correlation analyses within each cultivar group showed that grain Cd concentration was significantly correlated with root-shoot translocation factor in indica, with root Cd concentration in tongil-type, with shoot Cd concentration and accumulation in tropical japonica, and with shoot Cd accumulation and shoot-grain redistribution ratio in temperate japonica. These results indicate that genotypic variation in grain Cd accumulation, in general, is controlled by all the three physiological processes but the major physiological process governing its genotypic variation within cultivar group is different depending on cultivar groups.

Absorption, translocation, and remobilization of cadmium supplied at different growth stages of rice

Cadmium (Cd) is absorbed by rice root and transferred into the other rice organs including grain. A solution-culture experiment was conducted to investigate the absorption and distribution of Cd supplied at different growth stages of rice. Two rice cultivars, a japonica ‘Chucheong’ and a tongil-type ‘Milyang23’ that exhibit high and low ability of Cd absorption by root and accumulation in grain were grown in culture solution and subjected to 2 ppm CdCl2 treatment for 2 weeks at four different growth stages: before panicle initiation stage (BPI), after panicle initiation stage (API), early ripening stage (ER), and mid-ripening stage (MR). Cd concentration and accumulation in rice organs were measured at harvest. The two rice cultivars accumulated two to three times greater amounts of Cd in grain in the two Cd treatments before heading (BPI and API treatments) than in the Cd treatment after heading (ER and MR treatment). The higher grain Cd accumulation in BPI and API treatments was not attributed to the higher Cd uptake but to the higher translocation from root to shoot and the higher redistribution from shoot to grain than ER and MR treatments These results imply that the remobilization of Cd through phloem during leaf senescence is the major process for Cd accumulation in rice grain rather than direct transport of absorbed Cd through the xylem-phloem transfer to grain. ‘Milyang23’ absorbed significantly smaller amount of Cd than ‘Chucheong’. However, ‘Milyang23’ accumulated more than a three times larger amount of Cd in grain compared to ‘Chucheong’ as the former exhibited the higher root-shoot translocation and shoot-grain remobilization as well. It indicates that the greater Cd translocation from root to shoot and subsequent higher Cd remobilization from shoot to grain, not the higher absorption ability, have led to the higher Cd accumulation and concentration in grain of ‘Milyang23’.