The Mechanism Of Hydrogen Sulfide To Improve The Cd Resistance Of Isatis Indigotica Seedlings

Cadmium(Cd)is a non-essential element for the growth of animals and plants.The heavy metals which enter the environment will enter soil microorganisms and crops,finally enriched in the food chain,endangering the growth and development of organisms at all levels,and eventually causing harm to human body.Heavy metals will directly contact with plants through the atmosphere,water,soil and other environments,and enter the plant through leaves and roots to interfere with various physiological and biochemical processes in the plant.Previous studies have shown that heavy metals can induce the outbreak of reactive oxygen species(ROS),cause imbalance in the absorption of plant nutrients,and destroy the lipid membrane and the photosynthesis process of plants,leading to changes in chloroplast structure and lower photosynthetic rate.Heavy metals entering plant cells can cause structural damage to proteins,nucleic acids,cell membranes,and lipids,ultimately affecting the normal physiological metabolic processes.Studies have shown that the gas signal molecule hydrogen sulfide(H2S)may participate in the process of plants resisting heavy metal stress.However,the mechanism of H2S in mitigating Cd stress in Isatis indigotica is unclear.In this study,Isatis indigotica was used as the research object,and the H2S exogenous donor NaHS was added at the same time as Cd stress.Investigate the physiological indexes,cell wall components,and Cd distribution in Isatis indigotica seedlings under NaHS treatment.Effect of indigo and indorubin synthesis precursors on the expression of MEP and MVA pathway-related enzyme synthesis genes.We clarified the role of H2S in Isatis indigotica seedlings in response to Cd stress from the plant growth,antioxidant system and Cd enrichment,and the key role of cell walls in blocking Cd from entering the cytoplasm.The main conclusions are as follows:(1)Defense mechanism of Isatis indigotica seedlings to Cd stress under H2S treatment.The results showed that Isatis indigotica seedlings response to Cd stress though activating the antioxidant system,increasing antioxidant enzyme activity and GSH content in seedlings,and inducing ROS outbroken in root tips.The addition of exogenous H2S alleviated the oxidative damage caused by Cd through enhancing the activities of various oxidases and GSH content in the seedlings,which resulted in the reducing of ROS accumulation in root tips;exogenous H2S also enhanced the seedlings'antioxidant capacity by elevating the activities of various oxidases,GSH content and reducing the accumulation of ROS in plant cells,finally alleviated the oxidative damage caused by Cd.Therefore,we speculated that H2S alleviated seedlings growth inhibition caused by heavy metal stress through increasing the antioxidant level of Isatis indigotica seedlings.(2)Tolerance mechanism of Isatis indigotica seedlings to Cd stress underH2S treatment.The results showed that the Cd content in the leaves and roots of Isatis indigotica seedlings,the Cd content in the sub-cells,and the Cd content in the basic polysaccharide component of the cell wall all increased with the increase of the Cd concentration.Under different concentrations of Cd treatment,H2S significantly reduced the Cd content in above-ground tissues,enriched Cd in seedling roots,and reduced the transport rate of Cd in seedlings.Compared with different concentrations of Cd treatment,the Cd content in the above-ground tissues was significantly reduced by 50.89%,46.74%,43.24%,the Cd content in roots were significantly increased by 34.30%,25.69%,and 29.48%,respectively,and the Cd transport coefficients in the seedlings were significantly reduced by 63.49%.At the same time,H2S enhanced the Cd enrichment in the leaf and root cell walls of the seedlings,reduced the content of Cd entering the cells,and finally reduced the toxicity of Cd to the seedlings.Cd stress significantly reduced the glucose content in the cell wall of the seedlings and affected the cellulose synthesis.H2S alleviated the decrease of glucose in the cell wall under Cd stress,and attenuated the effect of Cd stress on cell wall cellulose synthesis in seedlings.Finally,the cellulose content in the cell wall of Isatis indigotica seedlings was increased to enhance the binding capacity of the seedling cell wall,and the tolerance of Isatis indigotica to Cd.Therefore,H2S enhanced the Cd fixation and enrichment by increasing the cellulose content in the cell wall,thereby reduced the toxicity to Isatis indigotica seedlings.(3)Different concentrations of Cd treatments mainly up-regulated BcSK,BcDXR,BcASA1,BcASA2,and BcASB genes.Cd treatment induced the expression of related genes in Isatis indigotica and affected the synthesis of related metabolites.H2S treatment significantly upregulated the relative expression of BcSK,BcDXR,BcASA1,BcASA2,and BcASB genes in seedlings under low concentration Cd treatment(0.5 mg/L Cd,1.5 mg/L Cd).The relative expressions of H2S were not consistent,and the effect of H2S treatment on BcASB gene in leaves showed a significant down-regulation effect.