Mechanisms Of Arsenic Transformation And Detoxification In The Rhizosphere Soil-Microorganism-Pteris Vittata System

Arsenic?As?is a toxic metalloid widely distributed in the environments and has been classified as a type I carcinogen to humans by the International Agency for Research on Cancer.The finding of As-hyperaccumulator Pteris vittata provides a reliable alternative to remediate As-contaminated soils,so far the fern has transitioned from labaroty to field application due to its cost effectiveness and environmentally-friendly advantage.Although P.vittata displays staggering As tolerance and accumulation abilities,the mechanisms and influence factors behind these activities are still unclear.To look insights into As transformation in the rhizosphere of and As uptake and detoxification in P.vittata,it is important to understand the roles of microorganisms in these processes.However,current studies mostly focus on some culture-dependent rhizobacteria and endophytic bacteria of P.vittata and their As resistance,while little is known about their roles in As transformation in the rhizosphere of and As uptake by the fern.In this context,this Ph.D.program mainly aimed to study As transformation and detoxification mechanisms and the factors involved in these processes in the rhizosphere soil-microorganism-P.vittata system,emphatically investigated microbial As^? oxidation,As^?reduction and As^? methylation and consequent influences on plant As uptake and detoxification in P.vittata.The specific work in this program is followed as:1)The impacts of 50mg kg^-1 As^? and/or 1.5%PR amendment on bacterial communities,functional gene diversities and their relationships with environmental variables in the rhizosphere of P.vittata were investigated.qPCR analysis showed that As stress significantly decreased 16S rRNA gene abundance,but it significantly incrased both aroA-like and arsC gene abundances.Moreover,PR amendment alleviated As inhibitory effects on 16S rRNA gene,while the gene frequencies of aroA-like and arsC also decreased to the levels in controls.In addition,the aroA-like genes in the rhizosphere were 50 times more abundant than arsC genes,the former showed a higher biodiversity than the latter.Redundancy analysis indicated that Asv and DOC concentration significantly influenced gene abundances,while soil pH,available Ca and P.and As^? concentration were key factors driving diverse compositions in aroA-like gene community.2)The impacts of P.vittata tissue extracts with or without surface sterilization on As transformation in the solutions containing 100 ?g L^-1 As^? or As^? were investigated.The results showed that the presence of sterilized and unsterilized root extracts resulted in 45%and 73%oxidation of the total As^?,respectively.Unlike to As^? oxidation,As^? reduction was detected in all tissure extracts of P.vittata,being 3.7%-24%of the total Asv.However,both As^? oxidation and Asv reduction were not observed in the solutions after filtrating or boiling treatments.by filtration or boiling,we hypothesized that only bacteria transformed As in the solutions.It can be concluded that 1)only bacteria transformed As in the solutions,2)As^? oxidation in the solutions mixed with root extracts might be mediated by As^? oxidizers,but As^? reduction in the solutions mixed with root,rhizome and frond extracts might be associated with Asv reducers,and 3)both rhizobacteria and endophytic bacteria were involved in As transformation in the rhizosphere and tissues of P.vittata.3)Bacteria-mediated As transformation and their impacts on As behaviours in the rhizosphere of and As uptake by P.vittata were investigated under sterile condition and high As stress at 37.5 mg kg^-1.The results showed that bacterial inoculation enhanced As and P uptake by up to 47%and 69%,and P.vittata growth by 20%-74%.In addition,95%of the As in the bacteria-free media was As^?,which was up to 100%with the inoculation of As^? reducers.In contrast,the detected As in P.vittata rhizosphere predominated as As^? at 95%when As^?oxidizer was inoculated.It can be concluded that 1)similar to typical plants,P.vittata also effluxed amounts of As^? to the external environment to aid As detoxification under high As stress,and 2)As^? oxidizers aided efficient As accumulation in P.vittata.4)The impacts of soil As levels at 0,50 and 200 mg kg^-1 on plant growth and As accumulation in P.vittata were investigated.After 3 and 6 months of plant growth,soil As decreased by 7.9%-58%,which was also observed in the controls by 4.0%-31%.Moreover,soil As loss well correlated to arsM copies?r>0.78,p = 0.0045-0.2115?.In addition,while 50 mg kg^-1 As promoted P.vittata growth as compared to controls,a high As level at 200 mg kg^-1 induced necrotic symptoms in the margin of pinnae,where the As concentration was 2.26 times as that in the internal tissues.The SEM-EDS analysis showed that As was translocated from the veins to the margin of pinnae.Besides,the line scanning model of SEM-EDS revealed that the relative content of Si was well correlated with As in the pinnae at r = 0.49?p<0.05?.It can be concluded that 1)water leaching,plant uptake and bacterial methylation might contribute to As loss in the soils,2)As translocation from the veins to the margin of pinnae was a possible pathway to release As burden in P.vittata,and 3)As translocation in P.vittata fronds was likely to be associated with Si transporters or Si was involved in As detoxification.