Mechanisms Of Uptake And Tolerance Of Hyperaccumulator Plant Siegesbeckia Orientalis L.to Cadmium Stress

Applying hyperaccumulator to remove heavy metal from contaminated soil is a feasible remediation technique.Therefore,to screen some promising hyperaccumulator and explore their uptake and tolerance mechanism of heavy metals have become worldwide concerns last decades.Siegesbeckia orientalis L.,a cadmium(Cd)hyperaccumulator,owns high biomass and strong Cd accumulative capacity,and grows in the southwest China.As a result,it could be thought as a promising material to remediate the soil polluted by Cd.However,the mechanism of its uptake and tolerance to Cd is unclear.Hence,this research employed subcellular differential centrifugation,chemical morphological continuous extraction,root bag test,root system scanning,photosynthetic measurement experiment,high-throughput sequencing and bioinformatics to investigate the Cd accumulation in different organs,subcellular distribution and rhizosphere characteristics,physiological ecological response and molecular mechanism of S.orientalis under Cd stress.Moreover,the uptake and tolerance mechanism of Cd by the plant were systematically studied.The results were as follows:(1)The subcellular distribution and forms of Cd in S.orientalis were thought to contribute to the Cd accumulation and tolerance.The species could normally grow under high Cd stress,and accumulated a large amount of Cd in the shoots.Over 90% of Cd in roots,stems,and leaves of plants was stored in cell soluble fractions(48.42-76.54%)and cell wall fractions(19.94-46.34%),indicating that the two fractions were crucial parts for Cd compartmental distribution and detoxification.Similarly,the key reason for Cd hyperaccumulation of this species was that the proportions of Cd in the organelles were lower in roots than in shoots.The chemical forms of Cd in various organs showed that most of Cd(69.83-82.71%)existed in NaCl and HAc extraction states with low-toxicity,while the inorganic binding and water-soluble forms of Cd only accounted for 6.85-20.91%.It thus is a key mechanism for reducing Cd toxicity in the organs of the plant.(2)The effects of rhizosphere in S.orientalis on Cd activation and tolerance were reported.Readily oxidizable carbon played a part in activating Cd by the hyperaccumulator.The concentrations of different Cd fractions in rhizosphere soil were acid soluble fraction >reducible fraction > residual fraction > oxidizable fraction.And the Cd fractions transformedinto the acid soluble fraction with higher bioavailability as Cd treatment levels increased.The Cd activation in soil and tolerance of S.orientalis on Cd were related with the changes of rhizosphere properties.Some property indices between rhizosphere and non-rhizosphere soils differed.Specifically,the significant decrease of rhizosphere soil pH,and the obvious increasing concentrations of dissolved organic carbon and readily oxidizable carbon,were the major influence factors of Cd activation under Cd stress;The species could increase the tolerance of soil microbial groups on Cd and enhance the activation of Cd.Soil ammonification and nitrification were the key processes to maintain the numbers and activities of soil microorganisms,and could be used as assessment indexes for soil Cd pollution;the maintenance of rhizosphere soil enzyme activities corresponded to the high microbial biomass,and is conducive to the transformation of soil nitrogen and phosphorus,and promotes the growth of plants and root uptake of Cd.(3)The effective response of physio-ecological adaptation and defense mechanism of S.orientalis to Cd stress were elucidated.The total root length,surface area and volume increased significantly owing to its ecological adaptation mechanism under Cd stress.And the mechanism played a role of Cd uptake in the root.Oxidative damage of the plant strengthened and the contents of the soluble sugar of osmoregulatory substances in different plant organs enhanced observably with increasing Cd concentrations.Moreover,GR,GSH and APX of antioxidant systems also rose in different degrees.These substances effectively defensed oxidative damage from Cd stress.Similarly,the decline of photosynthetic rate in leaves resulted from the non-stomatal limitation under Cd stress.With the improvement of Cd concentrations,the ratio chlorophyll a/b and maximum photochemical quantum yield showed no obvious decrease,while the value of non-photochemical fluorescence quenching coefficient had a slight enhancement.These changes were the important photosynthetic physiological regulation mechanism of S.orientalis tolerance of Cd stress.(4)Based on RNA-sequence analysis,a schematic representation of main processes involved in differential Cd uptake and translocation in S.orientalis is proposed,and suggests that the multilevel coordination of root uptake,and exocytosis and coordination among cell components determines its high Cd accumulation and translocation of the plant.A total of5,953 genes were differentially expressed in control and Cd treatment at three stages,including 3,192 up-regulated and 2,761 down-regulated genes.The DEGs can be divided into four groups based on their expression patterns which showed different trends,and some DEGs were increasing with time.In this study,the gene information in continuous up-regulated trends was observed.The results showed heat shock protein 70,mental-tolerance protein,chlorophyll protein precursors and peroxidase were involved in Cd tolerance.The NRAMP participates in regulating the transmembrane transport of Cd in cells.The ABC family,P-type ATPase and ransporter OPT superfamily regulates the distribution of Cd in the vacuole.