| Aluminum (Al) stress has become one of the main constraints of plant growth in acidic soils. Cell wall plays an important role in mechanism of response to Al toxic%, and lignin is an important component in cell wall. So metabolism of lignin in rice is also closely related to Al tolerance. Nitric oxide (NO) as the key signal molecules in plants participates in a variety of defense mechanism. So this research pays attentiont to lignin metabolism in rice root tips mediated by nitric oxide under Al stress. In this study, two rice cultivars with different aluminum (Al) tolerance were used, we studies the effect of NO on lignin and active oxygen metabolism under Al stress, then analyse the cell wall components and signaling pathways mediated by NO. The experiment results are performed as follows:(1) To investigate the lignin and active oxygen metabolism in two rice cultivars of Nipponbare and Zhefu 802 under Al stress, lignin and active oxygen content and related enzyme were determined. Lignin content in Nipponbare and Zhefu 802 increased by 37.5% and 47.9% under 50μmol/L Al for 24h compared to control. H2O2, O2- and MDA content also enhanced by 46.7% and 57.1%,15.4% and 36.8%,50.4% and 38.6% respectively. There was a significant rise in antioxidant enzyme activity, the results showed that antioxidant system in rice root were activated. The ROS scavenger DTT effectively alleviated the accumulation of lignin induced by Al, and the lignin content decreased by 20.7% and 41.0% in rice of Nipponbare and Zhefu 802. The results suggested that lignin accumulation in root tips caused oxidation stess.(2) In order to study the impact of NO on lignin and reactive oxygen metabolism induced by Al stress, the root elongation inhibition rate of Nipponbare and Zhefu 802 was reduced by 21.4% and 42.2% with SNP+Al treatment compared with Al treatment, Al content was reduced by44.5% and 45.3%, lignin content decreased by 43.3% and 25.3%, lignin synthase was also significantly reduced. SNP+Al treatment decreased root H2O2、O2- and MDA content compared with Al treatment, and increased leaf superoxide dismutase (SOD), peroxidase (POD) activities by 78.9% and 34.0%,25.5% and 70.9% in Nipponbare and Zhefu 802. These results provided strong evidence that disruption of antioxidant enzymes activities and drcrease active oxygen content meditaed by NO was associated with Al tolerance in rice.(3) The role of cell wall component in Al tolerance and NO function in regulating cell wall formation were studied by treateated BFA and SNP. Results showed that in the presence of Al and BFA, the relative root elongation in Nipponbare and Zhefu 802 decreased by 46.4% and 50.6%, respectively, root tips Al increased by 145.7% and 114.0%, while Al content in root cell wall decreased. Pectin content in root cell wall decreased by about 35.7% and 48.3% in Nipponbare and Zhefu 802 after treatment with SNP+Al than that treatment with Al, whereas thg hemicellulose 1 and hemicellulose 2 content had no significant derfference between SNP+Al and Al treatment. Functional group in cell wall were at lower levels under treatment of BFA+Al, while adding SNP, active functional groups such as hydroxide radical, carbonyl and amino had risen sharply. These active functional groups are susceptible to oxidation, so can reduce the degree of cell of oxidative stress. So that NO can remit Al stress and oxidative stress through regulating the cell walls of cell wall components and functional group.(4) Protein inhibitors of CDPK and MAPK were added to to explore the transduction system of NO and H2O2 signalling in Al tolerance of rice root cell walls. With the addition of W7 and PD98059, the root relative elongation of Nipponbare were reduced by 39.4% and 54.3%, the root relative elongation of Zhefu802 were reduced by 20.5% and 34.1%, respectively; Al content in Nipponbare root tips increased by 341.7% and 150.7%, Al content in root of Zhefu802 increased by 296.1% and 113.1%. The results suggested that production inhibition of CDPK and MAPK protein aggravated Al toxicity. NO content decreased by 21.7% and 6.59% in Nipponbare and Zhefu 802 roots treated by A1+W7, and NO content increased by 8.21% and 7.30% treated by Al+PD98059. H2O2 content in Nipponbare and Zhefu802 were reduced by 47.3% and 45.6% in Al+W7 treatment, and H2O2 content in Nipponbare and Zhefu802 were reduced by 26.8% and 40.7% in Al+PD98059 treatment. The results suggested that CDPK positively regulated NO and H2O2 synthesis, MAPK protein positively regulated H2O2 synthesis. Therefore we concluded that Al activated the CDPK system firstly, upregulated the NO production, then downstream MAPK proteins and H2O2 activated the antioxidant system intracellular. These responses reduced lignin accumulation. |
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