Plant Root Exudates And Its Contribution To Biodegradation Of Phenanthrene-contaminated Soils

Polycyclic aromatic hydrocarbons(PAHs)are ubiquitous and persistent pollutants in environments,which poses a significant risk to the ecological environment and human health.Remediation of PAHs-contaminated environment has become one of the serious problems in ecology and environment worldwide.While a number of studies have shown that root exudates enhance biodegradation of PAHs in soils,the response of root exudates to PAHs stress is still unknown and the mechanisms involved remain poorly understood.To our knowledge,few studies were carried out to examine how PAHs and root exudates affect microbial community structure in a polluted rhizosphere.In this study,over ten common greening tree species grown in southern cities of Chinese were used as experimental materials,and phenanthrene(PHE)was selected as the representative of PAHs pollutants.By using GC-MS,transmission electron microscopy and high-throughput sequencing,the responses of plant root exudates under PHE stress were investigated;the physiological adaptations of ultrastructural level of root cells to PHE stress were observed;the biological 1 degradation processes of PHE were examined under natural condition and under additions of living and artificial plant root exudates were analyzed;and the effects of both living and artificial plant root exudates on soil microbial community structure in PHE pollutanted soils were studied.The main results were as following:(1)Under the concentrations of 200 mg·kg-1(low stress,PHEL)and 2000 mg·kg-1(high stress,PHEH)of PHE treatments,the chemical compositions of root exudates of all tested tree species(Lorop etalum chinense var.rubrum,Gardenia Ellis var fortuniana Lindl,Photinia × fraseri Dress,Ligustrum japonicum 'Howardii',Rhododendron simsii Planch.,Osmanthus fragrans(Thunb.)Lour.,Gardenia jasminoides Ellis,Buxus sinica(Rehd.et Wils.)Cheng ex M.Cheng subsp.sinica var.parvifolia M.Cheng,Camellia sasanqua Thunb.,Euonymus japonicus Thunb.var.aurea-marginatus Hort.)were remarkably altered compared to the control.The influences of PHE on chemical compositions in root exudates were more serious in PHEH treatments than in PHEL treatment.With the increase of PHE stress,the secretion processes of plant root exudates were changed from the controllable state of physiological adaptation to the uncontrollable state.According to the response characteristics of plant root exudates under PHE stress,the Osmanthus fragrans(Thunb.)Lour.,Camellia sasanqua Thunb.and Rhododendron simsii Planch.were high resistant tree species to PAHs stress.Among them,Osmanthus fragrans(Thunb.)Lour.was the best.(2)Through the observation of transmission electron microscope,it was found the ultrastructures of plant root cells were changed significantly due to PHE stress.PHE stress significantly altered the morphology of root cells of K.paniculata from a long-spindle form to an irregular form.The PHE stress also resulted in cell plasmolysis,deformation or rupture of cell nucleus,vacuole enlargement and organelle disappearance within cells for K.paniculata.PHE stress caused a change in morphology of root cells for C.camphora from a long shuttle shape to an irregular shape,companying with cell plasmolysis,nucleus collapses and fragments,and raising salt content in cytoplasm.From the perspective of ultrastructural change of root cells,K.paniculata was more sensitive to PHE stresses than C.camphora.(3)The biodegradation of PHE was examined under the treatments of planting K.Paniculata and adding artificial root exudates containing different compounds,as well as the control.After 180 days,PHE was naturally degraded to 265.39 mg·kg-1 in the control treatment.The degradation percentage was 46.44%when compared to the PHE concentation pre-treatment and an average daily degradation rate was 1.47 mg·kg-1.Under treatment of planting K.Paniculata,PHE was degraded to 426.84 mg.kg-1,which indicated additional 161.45 mg·kg-1 of PHE was degraded compared with the control.Addition of low-molecular-weight organic acids significantly promoted PHE degradation in soil,of which addition of acetic acid had the most significant effect on PHE degradation.Moreover,artificial root exudates composed of multiple substances had better biodegradation effect on PHE in soil than artificial root exudates composed of single substances.(4)Both adding artificial root exudates and planting K.Paniculata significantly changed the soil bacterial community structure.Compared to the control,adding root exudates and planting K.Paniculata reduced OTU number and diversity of bacterial flora,but increased the abundance of PAHs degrading bacteria.With the passage of time,the total OTU number of bacteria showed a downward trend,and the number of unique species showed a trend of first increasing and then decreasing.The results showed that the quantity and diversity of soil bacteria were not the main factors leading to soil PHE degradation,and the abundance of specific PHE-degrading bacteria was the key factor to determine soil PHE degradation.(5)After 180 days,both adding artificial root exudates and planting K.Paniculata had no significant effect on the number and abundance of fungi,but reduced the diversity of fungus community.Compared with CK,addition of root exudates significantly promoted the relative abundance of Pseudeurotium.With the passage of time,the total OTU number of fungi showed a downward trend,and the number of unique species showed a trend of first increasing and then decreasing.There was no significant difference in total number of OTU and biodiversity between different treatments.In summary,the current study revealed teratogenic phenomena occurred at the plant root cell level under PHE stress-changes in cell morphology,cell plasmolysis,rupture or fragmentation of the nucleus.The physiological responses were manifested by the significant alteration of the chemical components of the root exudates.Both live plant root exudates and artificial root exudates significantly promoted the biodegradation of soil PHE.The biological mechanisms were that root exudates stimulated the abundance of specific PHE-degrading microbal organisms in the soil.The results provide a theoretical basis for phytoremediation of PAHs polluted environments and a scientific reference for the industrial production of biodegradable PAHs materials.