| NO is an important signaling molecules in plants, which participates in a variety of responses to stress.Cell wall is the first site of Al accumulation in root growth, and the rapid root growth inhibition after exposure indicates that the Al quickly damages the cell wall structure and function. So Al resistance can be clarified by maintaining the stability of cell wall and structure. Rice (Oryza sativa) is the most aluminum (Al)-resistant crop species among the small-grain cereals, and the exclusion of Al from the root tips mediated by cell wall has been suggested one of a key mechanism in Al tolerance. While the signal trancduction knowledge of cell wall in acclimating and defending Al toxicity is quite limited. We will analyse the endogenous nitric oxide (NO) production and the cell wall components with graminaceous plants which have significant difference in Al tolerance under Al toxicity. On this basis, we analyze the dynamic changes of endogenous NO and the way of NO production in rice root tips under Al toxicity, then confirmed the role of NO in Al resistance. Further experiments will be carried out to investigate the acclimation changes about cell-wall polysaccharides components and the main components in binding Al in response to Al toxicity under NO donor or scavenger pretreatment, which will further clarify the relationship between NO and the regulating effect on cell wall synthesis and modification. The main results are as follows:(1) A solution culture was conducted to study the capability in Al resistance ability of8varieties of4gramineous plants, including rice, maize, sorghum and wheat, and to determin the main cell wall polysaccharides component for Al accumulation. Under5～200μmol·L-1Al treatment, root growth inhibition rate of the rice cultivar Zhefu802and Nipponbare was19.8%and34.7%, respectively,27.0%and45.5%in maize cultivar Jidan7and Nongda108,49.3%and59.5%in sorghum cultivar Fengyou6and Wannongliangnuo1,58.2%and67.8%in wheat cultivar Yang16and Yang18, and their capabiliy of Al resistance is rice> corn>sorghum> wheat. so rice and wheat are selected as materials to further study. The content of pectin and hemi cellulose in root tips is higher in wheat than that of in rice treated with50μmol·L-1Al, and Al content in Al-sensitive genotypes were higher than the Al-tolerant genotypes in rice and wheat. While the content of hemicellulose2of rice is higher than wheat. Al accumulation in pectin accounted for63.6%and30.2%, and34.8%and68.3%in hemicellulose1of cell wall Al in the rice cultivar Zhefu802and Nipponbare. Al accumulation in pectic accounted for59.9%and39.3%, and38.3%and61.1%in hemicellulose1in the wheat cultivar. Adsorption and desorption Al in cell wall showed that more Al is adsorbed and bound in cell wallof wheat than in rice. NO content in root tips significantly increased under Al treatment, and NO in rice is significantly higher than that in wheat. These results indicated that polysaccharides plays an important role in external exclusion of Al in gramineous plants, and the Al-sensitive varieties tend to remove Al with pertin while the Al-tolerante varieties exclude Al mainly with himicellulose1.(2) To verify the effects of extraneous NO on root growth, NO production and Al resistance of cell wall, Zhefu802(Al-sensitive) and Nipponbare (Al-tolerant) were used. Results showed that root growth inhibition and root apical Al content in t Zhefu802was higher than that in Nipponbare, and Al accumulation in cell wall of root tips accounted for90.4%and70.1%of the total Al in root apex in Zhefu802and Nipponbare, respectively. The endogenous NO content in root tips increased with the processing time and maximized at12h supplied by50μmol·L-1Al stress, and NO content in ’Nipponbare’significantly higher than in Zhefu802. Nitrate reductase (NR) activity increased significantly while nitric oxide synthase (NOS) activity had no change with Al treatment, and the activity of NR and NOS in root tips of Nipponbare was higher than that in Zhefu802, suggesting that NR may regulate NO release in rice root under Al toxicity, and high Al tolerance of Nipponbare may be related to the more NO release in root tips. Root relative elongation in Zhefu802and ’Nipponbare’ with SNP+Al treatment increased by32.3%and91.7%, and Al content in root tips decreased by68.6%and60.9%, and Al content in cell wall reduced by39.4%and44.3%, compared with Al treatment only, indicating that NO can partly alleviate Al toxicity by mediating the reduce Al accumulation in cell wall. Exogenous NO decreases the content of cell wall polysaccharides (pectin, hemicellulose1, hemicellulose2), and decreased the activity of the apical pectin methyl enzyme (PME). Furthermore, NO ignificantly reduced adsorption and binding of Al in rice cell wall in Al adsorption-desorption experiments, suggesting the regulating role of NO in synthesis of cell wall polysaccharides. This effect of NO was reversed by application of75μmol·L-1NO scavanger (carboxy-PTIO, cPTIO) and5μmol·L-1NO endogenous ihibitor (NaN3). These results suggested that NO decreases the Al accumulation in cell wall by regulating component of root apical cell wall.(3) To verify the NO production in rice root tips under Al toxicity, Zhefu802is used under NO donor or scavenger pretreatment. The results showed that50μmol·L-1Al significantly inhibited root growth, increased Al content in root apex, and induced NR activity and caused NO release in root. NR inhibitors with2μmol·L-1Na2WO4and5μmol·L-1NaN3pre-treatment aggravated root growth inhibition and Al accumulation in root tips, NR activity and NO production were also inhibited. The effect of NO scavenger was reversed by application of100μmol·L-1SNP. While NOS inhibitors including L-NAA and L-NAME showed little effect NOS activity and NO production. These results suggested that NR-dependent NO producton is involved in Al tolerance in(4) To study the effect of endogenous NO on root growth and reactive oxygen species (ROS) production, the Al-sensitive genotype Zhefu802and Al-tolerant genotype Nipponbare were cultivated in hydroponic solutions, and then studied the regulating mechanism of NO mediated by reactive oxygen species to improve rice resistance to Al. The results indicated that5μmol·L-1Al significantly inhibited root elongation, increased Al content in root tips,1hydrogen peroxide (H2O2) and superoxide anion freebase (O2-.) content, and this effect performed more obviously in Zhefu802than that in ’Nipponbare’. Root relative elongation of rice seedings Zhefu802and Nipponbare increased by91.7%and32.3%with100pmol·L-1SNP pre-treatment, meanwhile Al content in root apex decreased by68.6%and60.9%, H2O2content decreased by15.4%and16.9%, O2-. decreased by12.9%and22.9%, respectively, indicating that NO partly alleviated Al toxicity and this effect is related to ROS. Endogenous NO increasee apical NO level in Zhefu802and Nipponbare, meanwhile significantly decrease NR activity but make no influence on NOS activity. In contrary, the ameliorating effect was reversed by the addition of100μmol·L-1NO scavenger (carboxy-PTIO, cPTIO) and intensified by5μmol·L-1NaN3. It is suggested that endogenous NO plays an important role in regulating reactive oxygen metabolism and reducing apical Al accumulation by promoting apical NO content. |
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