Polyethyleneimine Coated Mxene Quantum Dots Improved Cotton And Rapeseed Salt Tolerance Via Maintaining Ros And Na~+/K~+ Homeostasis

Soil salinization is an important factor limiting efficient crop production worldwide.Saline-alkali land is mainly distributed in northwest China,such as Xinjiang.Cotton and rapeseed are widely cultivated in Xinjiang,but the salinization of cultivated land seriously affects the efficient production of cotton and rapeseed.At present,soil salinization improvement is mainly achieved through physical,chemical and biological measures.Plant nanotechnology is a new interdisciplinary subject,which has a great application prospect in efficient agricultural production and sustainable development.Although there are a lot of reports about nanomaterials improving plant salt resistance,the potential environmental pollution and biosafety problems should not be ignored.Therefore,We designed and synthesized polyethyleneimine coated MXene quantum dots（PEI-MQDs）which is a potential environmentally friendly nanomaterial with degradation ability and reactive oxygen scavenging property.In this study,we screened the optimal concentration and application method of PEI-MQDs to improve the salt resistance of cotton and rapeseed,and further explored the possible mechanism of PEI-MQDs to improve the salt resistance of cotton and rapeseed from the reactive oxygen species（ROS）homeostasis and sodium（Na⁺）-potassium（K⁺）homeostasis.Our results can not only provide new ideas for how to improve the salt tolerance of crops,but also provide new technical solutions for expanding the cultivation of cotton and rapeseed in saline-alkali land.The main results of this study are as follows:（1）We synthesized and characterized PEI-MQDs.PEI-MQDs were degraded in vitro with 100μmol/L hydrogen peroxide（H₂O₂）at room temperature.Fluorescence spectrum results showed that PEI-MQDs were degraded gradually within 3 days.The content of titanium（Ti）in the degradation solution of dialysis bag was determined by ICP-MS,and the results further confirmed that 100μmol/L H₂O₂ really degraded it.In addition,we also used 100 mg/L,200 mg/L and 300 mg/L PEI-MQDs to carry out in vitro scavenging experiments of hydrogen peroxide（H₂O₂）,superoxide anion（^·O₂^—）and hydroxyl radical（^·OH）.The results showed that the scavenging ability of PEI-MQDs to hydrogen peroxide（H₂O₂）,superoxide anion（^·O₂^—）and hydroxyl radical（^·OH）increased with the increase of concentration within a certain range.（2）The growth,biomass,chlorophyll content and soluble protein content of cotton treated with 0.5 mg/L PEI-MQDs were significantly better than those of the control group under 10 days of 250 mmol/L sodium chloride（Na Cl）stress.Foliar application of 0.5 mg/L PEI-MQDs can not only significantly reduce the contents of H₂O₂ and^·O₂^—,but also alleviate the damage of reactive oxygen species by increasing the activities of catalase（CAT）and ascorbate peroxidase（APX）.In addition,leaf application of 0.5 mg/L PEI-MQDs significantly reduced the Na⁺content in cotton leaves and the number of dead cells in cotton leaves.The subcellular localization of Na⁺and K⁺in cotton leaf mesophyll cells showed that 0.5 mg/L PEI-MQDs significantly reduced the content of Na⁺in cotton cytoplasm and vacuole under salt stress.In general,0.5 mg/L PEI-MQDs can improve the salt tolerance of cotton by removing ROS,alleviating plasma membrane damage and maintaining intracellular ion homeostasis.（3）Under soil culture condition,200 mg/L PEI-MQDs significantly increased the biomass,leaf width,chlorophyll content,soluble protein content and peroxidase（POD）activity of rapeseed under 25 days of 200 mmol/L Na Cl stress.The content of H₂O₂ and^·O₂^—in leaves was significantly reduced,and the oxidative damage of rapeseed seedlings was alleviated by ROS.In addition,application of 200 mg/L PEI-MQDs under salt stress significantly reduced Na⁺content and the number of dead cells in leaves.The subcellular localization of Na⁺and K⁺in leaf mesophyll cells showed that 200 mg/L PEI-MQDs decreased the content of Na⁺in the cytoplasm and vacuole of rapeseed under salt stress,and increased the content of K⁺in the cytoplasm and vacuole of rapeseed under salt stress,so as to maintain a high K⁺/Na⁺ratio.In general,200 mg/L PEI-MQDs improved the salt tolerance of rapeseed by removing ROS,alleviating plasma membrane damage and maintaining intracellular ion homeostasis.In summary,our results showed that PEI-MQDs improved the salt tolerance of cotton and rapeseed by removing reactive oxygen species and regulating sodium and potassium homeostasis under salt stress.