Physiological And Molecular Response Of Horticultural Plant Tagetes Erecta L.to Cadmium Stress

With the accelerating process of urbanization and industrialization,environmental problems have become increasingly serious,and soil heavy metal pollution has seriously affected food safety,then endanger people’s health.Therefore,soil remediation of heavy metal contaminated is urgent.Phytoremediation is an environmentally friendly and energy efficient technology,and has great potential to remove heavy metal of soil.In this study,cadmium（Cd）,which is the most serious non-point source pollution in China,was selected as the research object.Through screening horticultural plants suitable for remediation of soil Cd pollution,the effects of Cd on plants and rhizosphere soil ecological environment were explored,and the tolerance capacity and response mechanism of plants were systematically evaluated.The conclusions are as follows:1.Screening of plants for remediation of Cd in soil.1)The tolerance characteristics of different plants under Cd stress could be divided into three categories:strongly inhibited,such as average plant height increment of Ceratostigma plumbaginoides Bunge decreased significantly by 23.24%～152.16%.Inhibited effect.For example,Celosia cristata L.and Dendranthema morifolium Tzvel.Promoted and inhibited coexist effect such as Capsicum annuum L.and Tagetes erecta L..The increment of ground diameter was consistent with the increment of plant height.2)Under different concentrations of Cd,biomass accumulation of Rosa chinensis Jacq,Ceratostigma plumbaginoides Bunge and Dianthus caryophyllus L.were significantly inhibited,and the inhibition gradually increased with the increase of Cd concentration.However,the biomass accumulation of Tagetes erecta L.and Dendranthema morifolium Tzvel showed no significant downward trend compared with CK,even the biomass accumulation of Septemberum under 225 mg·kg^-1 significantly increased by38.81%compared with CK.The results showed that Tagetes erecta L.and Dendranthema morifolium Tzvel had certain tolerance to Cd stress,and high concentration of Cd stress had little or even promoted effect on the accumulation of substances.3)The root-shoot ratio of each plants differed in response to Cd stress,but there was no significant difference in the root-shoot ratio of Dendranthema morifolium Tzvel,Ceratostigma plumbaginoides Bunge and Tagetes erecta L.under different concentrations of Cd compared with CK,which indicated that Cd stress had similar effects on aboveground and underground of these plants.In conclusion,under Cd stress,Dendranthema morifolium Tzvel and Tagetes erecta L.maintained a relatively stable root-shoot ratio,expanded the contact area between roots and soil,promoted nutrient absorption and ensured the normal growth and metabolism of plants by increasing root growth or reducing the growth of aboveground plants.Because the flowers of Tagetes erecta L.could extract essential oil,which had certain economic value,also its self-seeding and propagation characteristics,low cost,high value,which was conducive to promotion.This study believes that Tagetes erecta L.was an ideal horticultural plant for remediation of soil Cd pollution.Therefore,Tagetes erecta L.was selected as the target plant for the mechanism study of Cd tolerance and enrichment in this study.2.Growth,Cd accumulation and distribution characteristics of marigold under Cd stress.1)The content of Cd in all organs increased significantly with the increasing Cd concentration.The content of Cd in stem was maximum and root was minimum in seedling stage,while the content of Cd in root was maximum and inflorescence was minimum,only 2.46%of root,5.49%of the stem and 2.62%of leaf.2)Nitrogen absorption was increased under Cd treatments,and phosphorus content was also increased under concentration≤150 mg·kg^-1.Nevertheless,the potassium content in the flower was decreased under 300 mg·kg^-1.The maximum inflorescence diameter of marigold was significantly smaller under the highest concentration of Cd（L300）,however there was no significant difference between the other concentrations and CK.The maximum flowering amount of plants treated by 300 mg·kg-1 decreased significantly,which was less than 1/5 of that by control.There were significant differences in the distribution of the N、P and K among different organs.Such as,K was actively transferred from vegetative organs（especially leaves）to flower parts,indicated that the transport of these elements to flower parts was not affected by Cd treatment.3)The contents of H₂O₂,·O₂^-and malondialdehyde（MDA）was increased during the seedling phase,especially when Cd concentration≤30 mg·kg^-1.Furthermore,with growth of plants,these indexes significantly increased from 30mg·kg^-1 to 150 mg·kg^-1 by the required Cd concentration.4)The germination rate of seeds under 7.5 mg·kg^-1,15 mg·kg^-1 and 30 mg·kg^-1 treatments was significantly increased by 67.83%,104.46%and 73.92%compared with CK treatment.However,when Cd concentration was higher than 30 mg·kg^-1,the germination rate of seeds showed a downward trend.When Cd concentration reached 75 mg·kg^-1,the number of pollen variation such as square or irregular deformity increased significantly.The results indicated that the seed vigor was affected by Cd stress.3.Transcriptome analysis showed that:1)Under 30 mg·kg^-1 Cd stress,494 genes were significantly up-regulated and 83 genes were significantly down-regulated in T.patula leaves.GO function was mainly enriched in secondary metabolic process e.g.phenylpropane biosynthesis.Under 150 mg·kg^-1 stress,622 genes were significantly up-regulated and 506 genes were significantly down-regulated,respectively.These were mainly enriched in light harvesting in photosystem,photosystem I,photosystem II and photosynthetic enzyme activities.2)The production and distribution of secondary metabolic process,which included phenylpropanoid biosynthesis,flavone,phytohormone,phenylalanine,tyrosine and tryptophan biosynthesis,were of great significance in improving the ability of marigold to resist Cd stress.3)High concentrations of Cd induced genes downregulation of photosynthesis-antenna proteins,porphyrin and chlorophyll metabolism,carotenoid biosynthesis,circadian rhythm-plant,which reduced light energy absorption and photosynthetic efficiency.These suggested that the synergistic action of transcription,secondary metabolite biosynthesis,photosynthesis,carbohydrate metabolism,and plant hormone signal transduction,enhanced Cd resistance in marigold,which broadened our understanding of the mechanism of resistance to Cd toxicity in marigold.4)Under 30 mg·kg^-1treatment,PR1 gene of H₂O₂ signaling pathway and Ethylene signaling pathway Chi B gene were up-regulated,which resulted in enhanced defense response.Under 150mg·kg^-1 treatment,the down-regulation of MKK9 gene in ethylene pathway and PYR/PYL gene in abscisic acid signaling pathway might weaken the adaptability of marigold to Cd stress.4.Soil microorganism and soil enzyme results showed that:1)The alpha diversity of the bacterial community under 150 mg·kg^-1 Cd concentration was significantly reduced.Actinobacteria,alphaproteobacteria and gammaproteobacteria were dominant classes,and their abundances were significantly higher than the control.Streptomyces in the actinobacteria had the strongest resistance,and acidobacteriota and chloroflexi had weaker resistance to high-concentrations Cd.2)Among the top 30 families in bacterial community abundance,families with close relationships did not show uniform resistance or sensitivity to high-concentrations Cd necessarily.3)High-concentration Cd could promote acid phosphatase activity and inhibit catalase activity,which might be related to microbes that secrete or act on the enzyme.4)The biomass of T.patula was significantly negatively correlated with Cd concentration,but positively correlated with the number of bacteria and catalase activity.However,that had no significant correlation with acid phosphatase activity.The results indicated that high-concentration Cd changed the number and structure of microbial community in the rhizosphere soil of T.patula,and significantly inhibited soil catalase activity,and ultimately decreased the biomass of T.patula.5.The Cd tolerance mechanism of T.patula mainly involved avoidance mechanism and tolerance mechanis.Avoidance mechanism might involve stem-root synergism avoided Cd transfered to leaves at initial stage,and using root exudates increased the number of soil bacteria and catalase activity in soil,restricted of Cd transfered to aboveground part at later stage,reducted Cd content in inflorescence.Tolerance mechanism might involve guarantee nutrient absorption and rational distribution,and removal of excess reactive oxygen species to avoid membrane peroxidation injury.The tolerance could be improved with its growth.In addition,the synergistic action of transcription,secondary metabolite biosynthesis,photosynthesis,carbohydrate metabolism,and plant hormone signal transduction,enhanced Cd resistance in marigold.