| Plastic pollution in soil ecosystems has attracted attention in recent years due to the long degradation cycle of plastics and their ability to degrade into smallersized nanoplastics(NPs)under external environmental forces.It has been confirmed that NPs can negatively affect crops and be absorbed by plant roots.NPs migrate to the aboveground through transpiration,posing potential risks as they are transmitted to humans through the food chain.Therefore,there is an urgent need to develop a method that can effectively reduce plant uptake of NPs and reduce the health risks and adverse effects on agricultural production caused by NPs.Arbuscular mycorrhizal fungi(AM fungi)can form symbiotic relationships with over 80%of terrestrial plants and occupy important ecological niches in soil ecosystems.They can assist host plants in resisting external stress and promoting growth through direct and indirect effects.In this experiment,AM fungi(Rhizophagus irregularis)were taken as the research object,and PS NPs with different surface charges were synthesized by emulsion polymerization.Through pot experiments,the response mechanism of AM fungi to plant physiology and biochemistry(photosynthetic system,oxidative stress antioxidant system)and molecular level(transcriptome,critical gene expression)under the stress of NPs was revealed.A two-compartment root-organ culture system constructed based on Ri T-DNA transformed hairy roots was used to elucidate AM fungi’s structural and functional changes under NPs stress.High-throughput sequencing technology was used to study the cycling mechanism of soil phosphorus in key micro-regions(rhizosphere and hyphosphere)of the symbiotic system under NPs stress.The effects of NPs addition on the organic phosphorus mineralization ability mediated by AM fungi in different micro regions and the structure of soil microbial communities were studied.The main results were the following:(1)Adding 5 g kg-1 NPs with different surface charges to a soil medium:sand=1:1(v/v)will reduce the aboveground biomass of lettuce.When exposed to positively charged PS-NH2 and negatively charged PS-SO3H NPs(?=～30 nm),AM fungi can increase the aboveground biomass of lettuce by 25%to 100%and reduce the content of positively charged NPs in the aboveground.Inoculation with AM fungi increased the content of ascorbic acid(ASA)in the aboveground part of lettuce under charged NPs stress.It reduced the content of superoxide anion free radicals(O2·-)and hydrogen peroxide(H2O2).Under PS-SO3H stress,AM fungi also reduced the content of malondialdehyde(MDA)and increased the content of glutathione(GSH)and photosynthetic rate(Pn),indicating that inoculation with AM fungi can improve the scavenging ability of active oxygen species(ROS)in lettuce and alleviate oxidative stress,enhanced plant tolerance.Transcriptome analysis shows that stress relief is mainly attributed to the upregulation of defence-related genes involved in plant hormone signalling,cell wall metabolism,and oxidant clearance.In summary,under NPs stress,mycorrhizal symbiosis improves the growth status of lettuce by altering the KEGG pathway in roots and buds,including reduced oxidative stress,increased aboveground biomass,increased antioxidant activity,and photosynthesis.A sterile pure culture system constructed from Ri T-DNA transformed hairy roots and fluorescently labeled NPs was used to study the absorption and transport of NPs with different charges in the extracellular hyphae.The results showed that the extracellular hyphae could actively capture NPs,and NPs with different surface charges could be passively transported into the extracellular hyphae and further transported to the roots.The exudate from the hyphae aggregates positively charged NPs,thereby reducing their absorption.The positively charged PS-NH2 that enters plants through the hyphal pathway is fixed in cortical cells,while the electrically neutral PS and negatively charged PS-COOH can enter the pericycle,potentially leading to transport to the aboveground.(2)Nanoplastics widely distributed in the soil ecosystem inevitably come into contact with AM fungi dispersed in the soil.However,the interactions between these small-sized nanoplastics and AM fungi are unknown.In previous studies,we found that NPs can be delivered to the root system through the extra-root hyphae,so whether the transmembrane transport of NPs in the hyphae affects the structure and function of AM fungi needs further investigation.In this chapter,A twocompartment root organ culture system was constructed using Ri T-DNA transformed hairy roots,and the effects of NPs on AM fungal spore recognition symbiotic signals,hyphal branching development,and fungal structure and function during the symbiotic stage were studied.The colonisation capacity of AM fungal spores after NPs treatment and changes in phosphorus uptake capacity after mycorrhizal symbiosis were investigated in a pot experiment.The results showed that elevated pH exacerbated the attachment of NPs to AM fungal spores,and a small number of NPs were able to enter the interior of the spores.NPs on the surface of AM fungal spores inhibit the ability to recognise chemical signals released by the plant.After germination,spores cannot establish symbiotic relationships with the host plant at long distances through hyphal elongation.NPs were able to enter the extraradical hyphae,disrupting the cell membrane permeability and organelle structure of the hyphae by inducing the production of reactive oxygen species,decreasing the activity of succinate dehydrogenase(SDH),which is associated with ATP synthesis,and inhibiting hyphal elongation and branching formation.In the pot experiment,we observed a significant reduction in the total and arbuscular colonization rates of AM fungal spores in roots treated with NPs and a significant decrease in hyphal length density in the soil,consistent with the results of the hairy root experiment.In addition,NPs impeded organophosphorus mineralization by decreasing soil phosphatase activity and expression of bacterial genes related to the phosphorus cycle,including ppk,phnK and gcd,which are involved in phosphorus solubilization and uptake by soil bacteria.Our findings highlight the significant negative impact of NPs on the overall AM fungal symbiosis process.NPs alter the lifestyle of AM fungi and soil phosphorus cycling processes,which urgently requires an assessment of the systemic impact of NPs on the pervasive mycorrhizal symbiosis.(3)In this chapter,a two-compartment pot experience was used to construct a soil environment containing hyphae only to investigate the effects of NPs on the soil microbial community and the mechanism of action of plant phosphorus uptake.The results showed that inoculation with AM fungi in the rhizosphere increased the relative abundance of functional genes,gcd and ppk,related to inorganic phosphate uptake.By reducing the Chao1 and Shannon indices and increasing the relative abundance of Actinobacteria,while decreasing the relative abundance of Acidobacteria a specific soil microbial community was formed,enhancing soil phosphorus activation capacity and promoting phosphorus uptake.Adding NPs led to a decrease in maize root biomass,an increase in the relative abundance of Gammaproteobacteria,and a decrease in soil available phosphorus content.In the mycelial zone,inoculation with AM fungi reduced the Shannon and Pielou_e indices,increased mycelial density,and increased the connectivity between microbe-symbiont networks.Additionally,by increasing the relative abundance of Gammaproteobacteria and Alphaproteobacteria,the symbiotic organism facilitated the conversion of organic phosphorus to inorganic phosphorus.NPs inhibited the relative abundance of the phoD gene and decreased phosphatase activity,affecting the interaction between soil microorganisms and inhibiting soil phosphorus activation and turnover processes.This study focused on the AM fungi-plant symbiosis and nanomaterial particles(NPs),investigating the absorption and migration processes of NPs in the symbiotic system and elucidating the mechanisms through which NPs hinder spore germination and infection,as well as the potential ecological risks involved.These findings provide scientific evidence for using AM fungi to mitigate NPs environmental risks in terrestrial ecosystems. |
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