Physiological And Molecular Responses To Cadmium And Zinc Combined Stress In Cosmos Bipinnatus Seedlings

Cadmium（Cd）and zinc（Zn）often coexist in soil,which have serious adverse effects on plants and ecological security.Cosmos bipinnatus is a Cd accumulation plant with high ornamental value and strong resistance.This study determined Cd/Zn absorption and transportion,Cd/Zn subcellular distributions and chemical forms,antioxidant enzyme activities and analysised transcriptome in roots to explore physiological and molecular responses to Cd,Zn and Cd+Zn stresses in C.bipinnatus.The main results of the research are as follows:1)The characteristics of phenotype and Cd-Zn accumulation showed that:Cd,Zn and Cd+Zn treatments significantly affected plant growth;the shoots biomass under Cd+Zn treatment were significantly higher than that under Cd treatment;the roots and the aerial parts accumulated 472.15±23.63 mg kg^-1 and 796.12±6.52 mg kg^-1 Cd,respectively.Zn accumulated 2994±218.36 mg kg^-1 in roots and 4221.13±87.99 mg kg^-1 in aerial parts under Zn treatment.The results showed that C.bipinnatus can accumulate large amounts of Cd and Zn,which were mainly stored in the shoots.2)By analyzing the absorption and transportion characteristics of Cd and Zn,we found that Zn significantly inhibited the absorption and transportion of Cd.Cd merely inhibited Zn absorption.3)By analyzing the characteristics of Cd and Zn subcellular distributions and chemical forms,we found that Zn significantly increased the concentration and percentage of Cd in root cell wall,and reduced those in root soluble fraction;Cd reduced the Zn concentration in the subcellular components in roots and shoots.The concentration and percentage of Cd/Zn in the water-soluble form and ethanol-extracted form in roots significantly declined,and the percentage of Cd in the Na Cl-extractable form significantly increased.These results indicated that Zn may enhance the tolerance of Cd in C.bipinnatus by increasing the capacity of Cd in the root cell wall and reducing the more toxic soluble Cd forms,and thus prevent Cd from being transported to the ground.4)The results of antioxidant enzymes activity indicated that the activities of SOD,POD,and CAT in roots and shoots significantly increased under Cd,Zn and Cd+Zn treatments;compared with Cd and Zn treatments,Cd+Zn treatments enhanced the activities of SOD,POD and CAT in roots.The results suggested that the antioxidant system of root system had a positive synergistic effect on the detoxification of Cd and Zn in C.bipinnatus.5)By analyzing the root transcriptome data,it was found that the expression of 996,2765,and 3023 unigenes was significantly affected by Cd,Zn and Cd+Zn treatments,respectively.The expression of 1465 and 952 unigenes was significantly affected by Cd+Zn treatment compared with Cd and Zn treatment,respectively.The transcriptome revealed that different expression patterns of some mental transporters among Cd,Zn and Cd+Zn treatments,which may be an important reason for the different characteristics of Cd and Zn absorption,transportion and subcellular distribution.In addition,some genes involved glucose metabolism,phenylalanine metabolism,hormone biosynthesis,and signal transduction pathways were also significantly induced by Cd,Zn and Cd+Zn treatments,suggesting that these biological processes played an important role in Cd/Zn absorbtion,transportion and detoxification in C.bipinnatus.