| Cadmium (Cd) has in general been thought of as a very toxic heavy metal to bothplants and animals without beneficial biological function. Cd is also one of theelements strictly controlled discharge into the environment. Considering large scalemedium/low heavy metal contaminated farmlands, decreasing farmlands quantitiesand increasing population and the extending agricultural products demand, thephytoremediation with the non-food crops is not a good choice in China. Based on thewell understanding of crops Cd absorption, transport and accumulation processes, thedevelopment of agricultural biological and chemical measures to control food heavymetal pollution risk is instructive and meaningful. Under heavy metal stress, plantgrowth promoting rhizobacteria are able to induce plant system resistance (ISR),enhance the nutrients utilization efficiency of plants and stimulate plant growth byproducing auxin. In this study, the effects of a heavy metal resistance PGPR strainD54on the growth and development, Cd absorption, transport and subcellular fractionaccumulation were investigated. The main results were showed as below：1. Isolate and screening of heavy metal resistance PGPRTwelve Cd-tolerant bacteria strains were isolated from heavy metal multipolluted soils by gradient dilution plate methods. Strain D54exhibited good plantgrowth promotion and Cd-tolerant traits. Strain D54is Gram-negative, aerobic,mesophilic (optimum30°C) within a pH range of2-11(optimum7.0). On the basis ofthe morphological, physiological and16S rRNA gene sequence analysis, strain D54was recognized as a species of the genus Burkholderia sp.(GenBank/EMBL/DDBJaccession no. HM467915).2. Plant growth-promoting features of the Cd-resistance PGPR strain D54Strain D54exhibited capacity to produce indole-3-acetic acid (IAA),1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, siderophore and tosolubilize tricalcium phosphate. The results of Cd MIC tests showed that strain D54had the MIC value for Pb (800mg l-1), Cd (1500mg l-1), Cu (150mg l-1) and Zn (2500mg l-1). The toxicity order of the heavy metals for strain D54is Cu> Pb> Cd>Zn.3. The test of Cd-resistance PGPR strain D54on Cd absorption/desorption actionThe Cd absorption rate of strain D54live cells was higher than the dead cellsunder low Cd concentrations. However, the dead cells absorption rate of strain D54was higher than the live cell under high Cd concentrations. Freundlich and Langmuir adsorption isotherms were used to model the equilibrium adsorption of Cd bylive/dead bacteria cells. It is interesting to note that the Cd adsorption of Cd by livebacteria cells of strain D54obeys Langmuir adsorption isotherms, different from theresults of dead bacteria cells which obey Freundlich adsorption isotherms.Furthermore, the results of the organic acid determination of strain D54liquid culturesshowed that strain D54is able to produce9low molecular weight organic acids (e.g.oxalic acid, acetic acid and citric acid). Strain D54had ability to solubilize slightlysolubility Cd minerals such as CdCO3. There were negative correlations between thepH values and the watersoluble Cd.4. Screening of Cd-tolerant Solanum lycopersicum L. cultivar speciesAccording to the germination rate (GR), germination potential (GP), germinationindex (GI) and seeding fresh weight, three selfing line Solanum lycopersicum L.cultivar MF, Liger and MFG exhibit more Cd-tolerant abilities than TA-BD andROMVFP. The biomasses and Cd concentrations of5tomato cultivars changeddifferently. Furthermore, the hydroponic experiment results showed that the Solanumlycopersicum L. cultivar Liger exhibit good growth and tolerance abilities under bothhigh and low Cd concentrations. The biomass of the Liger was biger than othertomato cultivars under Cd stress. Moreove, the root Cd concentrations of the Ligerwere increased, while the leaf Cd concentrations were decreased significantly.5. Effects of Cd-resistance PGPR strain D54on the tomato seedling growth, Cduptake and accumulation.The chlorophyll a (Chl a), chlorophyll b (Chl b) contents of leaves of tomatoplants (S. lycopersicum L. cultivar Liger) which were inoculated with strain D54weresignificantly increased under Cd stress contrast with non-inoculation controls.However, the carotenoid (Car) contents and Ch a/Ch b ratio decreased significantly.The results of biomass showed that the root and leaf dry weights of tomato plantswhich were inoculated with strain D54increased significantly, no significantdifferences were detected on stem dry weights. Inoculation with strain D54significantly increased the root Cd concentrations of tomato plants. However, thestem Cd concentrations were significantly decreased. There were no significantdifferences observed on leaf Cd concentrations of tomato plants. The subcellularfraction Cd accumulation results indicated that inoculation with strain D54significantly increased the root cell wall and cytosol Cd concentrations of tomatoplants, contrast with the non-inoculation controls. The root cell nucleus and chondriosome Cd concentrations decreased significantly after inoculation with strainD54. Inoculation with strain D54significantly increased the cell nucleus Cdconcentrations, decreased the chondriosome Cd concentrations and no significantdifferences were detected in cell wall and cytosol Cd concentrations of stems.Moreover, the leaf cell wall and nucleus Cd concentrations increased significantlywhile the chloroplast and chondriosome Cd concentrations decreased significantly.There were no significant changes on cytosol Cd concentrations. The results ofantioxidant enzyme system of tomato plants indicated that inoculation with strain D54significantly increased the root SOD and GR activities, decreased APX and CATactivities strongly, contrast with non-inoculation controls. There were no significantlychanged on POD activities and the soluble protein contents. Moreover, inoculationwith strain D54significantly increased leaf POD, CAT activities and soluble proteincontents, decreased APX and GR activities significantly. There were no significantlychanges on SOD activities in leaves of tomato plants. The activities of antioxidantenzymes and the Cd subcellular distribution of tomato seedlings were influnced bystrain D54. The decreased Cd transport might alleviate the Cd toxity of tomatoseedilings.6. Effects of Cd-resistance PGPR strain D54on the tomato seedling geneexpression profiling under Cd stressTo well understand the effects of heavy metal resistance PGPR strain D54on Cduptake, transport and accumulation of tomato seedlings, the Affymetrix TomatoGenome Array GeneChips were used to analyze the transcriptional response of tomatoseedlings inoculated with strain D54under Cd stress. The results showed thatinoculation with strain D54induce systemic resistance (ISR) of tomato seedlings.Inoculation with strain D54influenced the transcription of many Genes involved inseveral gene families under Cd stress, such as photosynthetic system, carbohydratemetabolism, stress response, system defense, antioxidase system and metal ionstransport. The up-regulated gene LOC54359(Iron-regulated transporter) related toroot Cd uptake of tomato seedlings which inoculated with strain D54was observedunder Cd stress. The gene LeXET2(Xyloglucan endotransglycosylase) mightparticipate in Cd sequestration in root cell walls of tomato seedlings which inoculatedwith strain D54was up-regulated under Cd stress. The gene LOC543981(Superoxidedismutase, SOD) up-regulated while the TPX2(Peroxidase, POD), P4, PR1b1, PR-P2(Pathogenesis-related protein, PRs) and HSP17.4(Heat shock protein) down-regulated in root of tomato seedlings which inoculated with strain D54underCd stress. The genes ERF1and IAA8which involve in ethylene response wereup-regulated in leaves of tomato seedlings which inoculated with strain D54under Cdstress. The genes such as tomQ`b, LeEXP1and TAPG2which relate to Cdsequestration in leaf cell walls of tomato seedlings inoculated with strain D54wereup-regulated under Cd stress. To alleviate the toxity of reactive oxygen intermediates(ROI) to leaf cells under Cd stress, inoculated with strain D54induced the gene TAL(transaldolase) up-regulated to maintain the reductive GSH concentration in leaf cells.Moreover, the gene agpL3related to the synthesis of secondary metabolites wasdown-regulated in leaf cells of tomato seedlings inoculated with strain D54. |
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