| As a kind of transition metal element, Chromium (Cr) is widely located in soil, water and air. It has several oxidation states with+3and+6being the most stable forms. Recently, Cr concentration in agricultural land increase dramatically for the aggravation of environment pollution. Therefore, current experiments were carried out to find the quantitative trait loci (QTLs) related to the Cr tolerance by using a rice DH population developed a cross of ZYQ8/JX17with difference in Cr-tolerance, and to identify the lines with significantly different Cr-tolerance. Further more, alleviating effect and mechanism of exogenous GSH to the Cr toxicity on antioxidative system, cell viability and ultra-structure, sulfhydryl compound concentration, water-uptake, nutrition balance and organic acid excretion were also studied by using the Cr-tolerant DH line117and Cr-sensitive line41. The major results are summarized as follows:1QTL mapping of growth traits and Cr-tolerance of a rice DH population under Cr stressThe results showed the differences caused by Cr stress on the growth and the Cr concentration among the lines of a rice DH population as well as the two parents. For the parents ZYQ8and JX17, plant height (PH), root length (RL), chlorophyll content (SPAD value) and shoot dry weight (SDW) were all inhibited under Cr stress, while root dry weight (RDW) was slightly increased. For the DH population, PH, RL and SDW were also decreased, while RDW and SPAD values were weakly affected. Cr concentration in roots was much higher than in shoot and varied significantly among DH lines. QTL analysis identified several loci related to the growth traits under Cr stress, while each one explained less than10%of the phenotypic variation. Two QTLs linked to the Cr concentration and translocation was also found, each one explained more than10%of the variation. Additionally, positive correlation existed between Zn and Cr concentrations, so QTL mapping was also conducted. 2Identification of different Cr-tolerance lines in a rice DH populationCurrent experiment studied the growth variation of all DH lines by using the weighted analysis under Cr tress, then identified the lines with high and low Cr-tolerance. Among the observed growth traits, SDW was most significantly affected by Cr stress, while SPAD value was less affected. The integrated scores of the lines117and101were the highest, while the lines41and49were the lowest. Cr concentration in roots of the line117was significantly higher than that of the line41, but Cr accumulation in root (RCC) was just opposite for the two lines. Cr stress affected the zinc concentration among DH lines and parents, with shoot Zn concentration (SZC) being reduced much more than root Zn concentration (RZC) for the two parents. Hence, it may be suggested that the lines117and101were high Cr tolerant, while the lines41and49were Cr sensitive.3Alleviating effect of exogenous GSH on Cr toxicity to rice plants and the identification of the optimal GSH concentrationThe two experiments were conducted to investigate the alleviating effect of exogenous GSH on Cr toxicity. The results showed that addition of GSH into the culture solution, without Cr addition, did not affect growth and chlorophyll content of rice seedlings, while GSH addition could greatly alleviate the growth inhibition caused by Cr exposure. The greatest alleviating effect occurred when the concentration ratio of Cr6+and GSH was1:3or1:5, while root elongation was significantly inhibited when the ratio was up to1:10. GSH addition increased Cr concentration in roots and reduced it in shoots, while Cr concentration in both roots and shoots was reduced when the ratio was1:10. It may be suggested that suitable GSH addition in culture solution may alleviate Cr toxicity to rice.4The influence of Cr stress on anti-oxidative system, cell viability and ultra structure, and its alleviation by GSHThe response of rice DH lines117and41to Cr stress and its alleviation by exogenous GSH were examined. The results showed that Cr stress reduced the concentration of photosynthetic pigment, and increased MDA content, affected activities of anti-oxidative enzymes. The difference could be found in the extent of the influence between lines, with117having higher activities of oxidative enzymes under Cr stress. Exogenous GSH alleviated Cr toxicity, being characterized by higher anti-oxidative capacity In addition, GSH improved cell viability in rice roots relative to the Cr treatment alone. Cr stress caused chloroplast deformation and chloroplast lamellae loose in mesophyll cells, and cell structure disintegration, matrix thinning, vacuole larger and nucleolus disappearing in root tip cells, while GSH addition alleviated these changes of cell ultra-structure.5Alleviation of exogenous GSH on Cr toxicity to thiol-compound synthesis, water and nutrient uptake, and organic acid secretion in rice seedlingsThe present study showed Cr toxicity to thiol-compound synthesis (concentration), water and nutrient uptake, and organic acid secretion in the two rice DH lines117and41, while addition of exogenous GSH in the culture solution could alleviate these toxicity caused by Cr stress. GSH ameliorated Cr inhibition on the uptake of water, Fe and Mn. Cr affected the pattern of organic acid secretion in rice roots, however the difference existed between lines117and41. Addition of GSH dramatically increased level of acetic acid and malic acid, with line117having more increase than line41, and there was a close association between the two organic acid contents and alleviation of GSH to Cr toxicity. Although no PCs was found in this study, it was clearly showed that Cr stress affected GSH and Cys levels in rice seedlings. Moreover, the compound with the similar retention time as PC3was identified and waits for further identification.6Effect of GSH on Cr form in rice rhizosphere and its absorption, translocation and accumulationThis experiment determined the effect of GSH on Cr form in rice rhizosphere and its distribution in rice tissular, cellular and subcellular levels. Addition of GSH caused a dramatic decrease of Cr6+in the cultural solution. Meanwhile, GSH addition increased Cr concentration in roots, but reduced it in shoots, suggesting that most Cr was not absorbed in the form of Cr6+. In the treatment of100pM Cr6+, Cr was mainly located in epidermis, cortices and vascular bundles of rice roots, but in the treatment of100μM Cr6+and300μM GSH, Cr was mainly found in epidermis, indicating that epidermic cells can alleviate Cr toxicity to the plants. The results at sub-cellular level showed that Cr was mainly presented on cell walls, followed by soluble composition. Exogenous GSH reduced Cr concentration in all subcellular components of shoots, and in the soluble composition of roots, while no effect was found on cell walls. It may be suggested that GSH inhibited Cr translocation, but did not affect Cr uptake. |
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