The Mechanism Of Nitric Oxide Alleviate Aluminum Toxicity In Rice Root

Aluminium (Al) in acidic soil is the main limiting factor affected the crop production by inhibiting plant growth. The decreasing of aluminium ions to the physiological toxicity in crops and the increasing crop yields has important practical significance. In this paper, the rice seedlings were used to study the physiological response to Al stress and nitric oxide (NO) on the alleviation of Al toxicity. NO as a key signaling molecule plays an important role in plant growth. In order to study the role of NO mitigation of Al toxicity, we pretreated rice seedlings with 25μM SNP for 24 h, and then treated with 75μM Al for 24 h, through the changes of rice root length, Al accumulation in root tip, lipid oxidation level and antioxidant enzymes activities in the rice roots, as well as the cell membrane permeability and cell wall pectin hemicellulose to evaluate the Al toxicity.The main results are summarized as follows:1. The Al tolerance in rice roots pretreated with SNP got a significant increase compared to the control (without SNP pretreated). The application of NO promoted rice root elongation under Al stress. The root length of rice seedling with SNP pretreated in Al stress could generally return to normal growth levels. By determining Al accumulation in rice root and rice root cell wall, we found that NO could reduce the Al accumulation in rice root and root cell wall. Under Al stress, the content of Al in rice seedlings root with SNP pretreated showed two times to the normal control group (nothing treated) and 1/3 times the Al treated only. It showed that Al could accumulate in rice root tip and inhibit rice root elongation at very low concentration (75uM) of Al. NO also had significant alleviation in decreasing the inhibition of rice root elongation and the Al accumulation in root tip and root cell wall induced by Al stress.2. By studying the production of reactive oxygen species (ROS), lipid peroxidation and the activity of antioxidant enzymes in the rice root under Al stress, we found that a large amount of ROS (including peroxide hydrogen, superoxide anion) emerged. The lipid peroxidation was induced by excessive ROS. In order to alleviate the oxidation of ROS, the rice itself increased the activities of antioxidant enzymes including SOD, POD, CAT and APX. The generation of ROS and lipid peroxidation in the rice root at the presence of SNP was significantly lower than that of in the rice root with Al treated only. And the activities of antioxidant enzymes in the rice root at the presence of SNP were significantly higher than that of in the rice root with A1 treated only.The application of butylated hydroxyanisole (BHA, an antioxidant) could only alleviate the oxidation induced by Al stress and had little effect on the rice root elongation. Thus, NO could mitigate the Al-induced oxidation and enhance the antioxidant capacity of the rice root. But the role of NO in mitigating Al-induced oxidation and the promotion of root elongation in rice seemed to no direct correlation.3. The content of hemicellulose and pectin in rice root cell wall markedly increased and the degree of pectin methyl esterification significantly decreased compared with the control (nothing treated). The content of hemicellulose and pectin reduced and the degree of pectin methyl esterification increased compared with the Al treatment only in the pretreatment with SNP. Further study on the absorption and desorption of Al in root cell wall, the results showed that NO could significantly reduce the capacity of cell wall to the Al absorption and increase Al desorption in the rice root. These data suggested that NO can change the components in root tip cell wall and reduce the degree of pectin methyl esterification of rice cell walls to reduce the capability of cell wall binding Al.4. In order to study on the alleviated role of endogenous NO of Al toxicity in rice, two inhibitors of NO biosynthesis in plant were used respectively. They are sodium tungstate (an inhibitor of nitrate reductase (NR)) and LNNA (N-nitro-L-arginine) (an inhibitor of nitrous oxide synthesase, NOS). We found that sodium tungstate could significantly increase the inhibition of Al stress to the rice root growth and pectin content in rice roots cell wall, as well as decrease the degree of pectin methyl esterification. Wheras the roles of LNNA on them seemed to not significant compared with sodium tungstate. Therefore, endogenous NO can also alleviate the Al toxicity in rice root and the production of endogenous NO may depend on NR neither on NOS in rice root under al stress.