Regulation Of Egulation Of Nitric Oxide And Hydrogen Peroxide On Liginfication Of Root Tip Cell Wall Induced By Aluminum In Rice

Aluminum toxicity is one of the main factors limiting crop growth in acidic soils.Crops are liable to suffer from cell wall thickening and rigidity under short-term aluminum toxicity.Cell wall is the primary site for cell-to-Al contact.Maintaining the stability of cell wall structure and function is critical to Al tolerance for crops.How root tips perceive and adapt to Al toxicity needs further study.Nitric oxide（NO）and hydrogen peroxide（H₂O₂）are key signaling molecules in crops,which play a regulatory role in abiotic stress.Whether they play a role in Al-induced lignification of root tip cell wall and their interaction are not clear.The effects of NO and H₂O₂ on Al-induced lignification of root tip cell wall of rice were studied in this study,using indica rice Zhefu 802（Al-sensitive）and Japonica Rice Nipponbare（Al-tolerant）as materials.The results are as follows:（1）To study the responses of root exoplasts H₂O₂,NO and lignin to Al toxicity in rice.The results showed that the inhibition rate of root elongation and Al content in root tips of rice increased significantly with the increase of Al concentration after treatment with 0,50,100 and 200μmol·L^-1 Al for 24 h.Compared with the control group,the lignin of Zhefu 802 and Nipponbare increased by 31.12%,84.44%and33.82%,247.82%respectively under 50 and 100μmol·L^-1 Al treatment.The laccase activity increased by 22.19%compared with the control under 100μmol·L^-1 Al treatment.The enzyme activity of Zhefu 802 increased by 80.04%and 20.27%respectively under 50 and 100μmol·L^-1 Al treatment.Compared with the control,PAL activity of Nipponbare and Zhefu 802 increased by 19.48%,47.75%,80.64%and 8.03%,65.13%and 35.53%respectively when treated with 50,100 and 200μmol·L^-1 Al,indicating that Al toxicity could enhance the activity of lignin synthase and promote the massive synthesis of lignin.POD activity reached the maximum under 200μmol·L^-1 Al treatment for 24 h,producing the maximum amount of H₂O₂.After being treated with 50μmol·L^-1 Al for 8 h,12 h and 100μmol·L^-1 Al for 4 h,the NO content of Zhefu 802 increased significantly.When treated with 50μmol·L^-1 Al for 4 h,the NO content of Nipponbare also increased remarkably,which was 53.08%higher than that of the control,manifesting that Al toxicity enhanced POD activity and induced rapid synthesis of large amounts of H₂O₂,leading to the lignin deposition,in the meanwhile,it promoted NO accumulation in root system and regulated the lignin accumulation.（2）In order to study the role of NO in inducing the resistance of root tip cell walls to Al toxicity.The contents of lignin,synthetic lignin-related enzymes,callose and phenols were determined by exogenous addition of NO donor（SNP）and NO scavenger（cPTIO）.The results showed that compared with Al treatment,the growth inhibition rate of Zhefu 802 and Nipponbare roots decreased by 126.09%and58.25%respectively,the dry weight inhibition rate of shoots decreased by 10.93%and 18.53%respectively,and the Al content of rice root tips decreased by 165.35%and 120.46%,indicating that exogenous NO significantly alleviated the Al inhibition to rice root elongation and reduced Al accumulation in root tips.Relative callose content of Zhefu 802 and Nipponbare decreased by 13.28%and 21.13%respectively,and lignin content decreased by 180.68%and 2.20%in Al+SNP treatment.The activity of NADPH oxidase and oxalate oxidase decreased,while the content of H₂O₂decreased.With the addition of NO,the total phenol concentration of Zhefu 802 and Nipponbare decreased by 9.17%and 23.05%respectively.The results showed that NO could decrease the activities of NADPH oxidase and oxalate oxidase,thus reducing the lignin deposition.（3）The effect of H₂O₂ and H₂O₂ scavenger（CAT）on lignin and reactive oxygen species under Al stress was investigated.When Zhefu 802 and Nipponbare were treated with Al+H₂O₂,the relative elongation of roots increased by 43.55%and16.95%respectively,the relative dry weight of shoots increased by 79.51%and8.20%,the Al content of root tips decreased by 117.23%and 86.61%,and the MDA content decreased by 47.38%and 17.79%respectively,compared with Al treatment.The results showed that the treatment of exogenous H₂O₂ donor could alleviate the inhibition of Al toxicity on rice growth.The CAT activity of Zhefu 802 and Nipponbare increased by 7.67%and 54.03%respectively,POD activity increased by23.83%and 15.44%respectively,APX activity increased by 28.68%and 29.49%respectively,GR activity increased by 98.89%and 20.11%,respectively,manifesting that H₂O₂ enhanced antioxidant enzyme activity.The PAL activity decreased by 6.37%and 20.92%respectively,TAL activity decreased by 13.93%and 2.18%respectively,and CAD activity decreased by 1.23%and 56.47%respectively,indicating that the state of rapid lignin deposition could be reversed by H₂O₂ donors.The NO content of Zhefu 802 and Nipponbare root tips increased by101.02%and 34.38%respectively after adding H₂O₂ donor under Al stress.The results showed that H₂O₂ could decline the lignin production by inhibiting the activity of lignin synthetase,enhancing the activity of antioxidant enzymes and increasing the content of NO.It provides an important basis for slowing down the lignification of plant root tips and alleviating Al stress in acidic soils.It provides an important basis for slowing down the lignification of plant root tips in acid soils and alleviating Al stress in rice by exogenous addition of appropriate amount of H₂O₂ and NO.