Physio-Biochemical And Molecular Mechanisms Of Rapeseed Under The Combination Stress Of Microplastics And Arsenic

Microplastics’small size and low density make migration easy,and because they’re slow to degrade naturally,they readily accumulate in various circles throughout the environment.Plant growth,development,and physiological activities are adversely affected by arsenic,which is a non-essential element that can be absorbed by plants and ingested by humans through the food chain.Facility vegetable plots are the noteworthy land-use types of agricultural microplastics and arsenic pollution,and vegetables are at significant risk of joint pollution by microplastics and arsenic.There is no proper study on the toxicity and mechanism of microplastics and arsenic combined in vegetables,especially pentavalent arsenic.This paper uses polymethyl methacrylate（PMMA）microplastics and rapeseed（Brassica campestris L.）as experimental materials.In this study,we used germination and hydroponics experiments to examine rapeseed germination,physiology,biochemistry,and accumulation of pollution,as well as resistance mechanisms.It explained and discussed the toxicity and mechanism of pollution by PMMANPs and As⁵⁺on rapeseed.The results are:（1）Micron-sized PMMA microplastics were less toxic than nano-sized PMMA microplastics in terms of germination.According to the dose-additive model（CA）,PMMANPs and As⁵⁺synergistically inhibited rapeseed germination.PMMANPs reduced arsenic accumulation in rape shoots at moderate levels（20 mg/L）of As⁵⁺but increased it at high levels（40-60 mg/L）of As⁵⁺.Varieties of rapeseed had different susceptibility to the combined stress of PMMANPs and As⁵⁺.According to the SSD curve fitting results based on the germination index,Jinglv No.1 had the highest sensitivity to the combined stress of PMMANPs and As⁵⁺,and Jingguan No.4 had the highest resistance.（2）Single stress of PMMANPs with different exposure pathways caused oxidative stress,leaf senescence,and cell damage in rapeseed.Root exposure to single PMMANPs caused more oxidative stress and cell damage in rapeseed.Combined with As⁵⁺,foliar exposure of PMMANPs had a profound effect on the oxidase system.（3）This paper studied the internalization of PMMANPs and As⁵⁺in rapeseed under different exposure pathways of PMMANPs and As⁵⁺under hydroponic conditions.Both leaf and root-exposed PMMANPs promoted As accumulation in rapeseed roots and shoots,with foliar exposure contributing more.PMMANPs promoted As accumulation in rapeseed mainly by increasing the arsenic content in the cell wall.The roots of rapeseed exposed to PMMANPs absorbed more As to increase As content in roots and shoots.Rapeseed under leaf exposure treatment transferred more As from roots to shoots.PMMANPs were internalized by roots of rapeseed under single stress.As⁵⁺promoted the entry of PMMANPs into rapeseed through root and foliar exposure pathways.（4）Laser confocal microscopy observations revealed that with the enhancement of combined stress,ROS burst changes in rapeseed taproot and stem tissues from external to internal,from highly differentiated to less differentiated cells.Rapeseed stem lipids were more damaged by PMMANPs stress,and PMMANPs increased lipid peroxidation in rapeseed under As⁵⁺stress.PMMANPs caused DNA damage in rapeseed,while As⁵⁺did not cause DNA damage.The DNA damage under combined stress was more severe than that caused by PMMANPs alone.（5）Through the detection and analysis of the eukaryotic transcriptome,the reasons for the high resistance of Jingguan No.4 to the combined stress of As⁵⁺and PMMANPs include up-regulation of lipid anabolism and cellulose synthesis,repaired of damaged biofilm and cell walls,up-regulated thiamine synthesis,enhanced regulation of nutrition and energy metabolism,enhanced antioxidant capacity,stimulated the synthesis of condensing to maintain normal DNA function.Jingguan No.1 had low resistance to the combined stress of As⁵⁺and PMMANPs for various reasons,including DNA synthesis and transcription,metal cation transport,and immobilization-related genes.（6）In rapeseed,PMMANPs stimulated transcription of fungal disease and DNA damage-associated genes,while As⁵⁺induced differential expression of cell wall-associated genes.Under the combined stress of As⁵⁺and PMMANPs,the transcriptional regulation induced by the two pollutants alone was involved,and regulating photosynthesis-related genes in rapeseed was jointly affected.PMMANPs allowed rapeseed to accumulate As by regulating phosphate transportase activity at the m RNA level and boosted its ability to respond to oxidative stress by regulating its oxidoreductase genes.