Physiological Response And Mechanism Of Mineral Metabolism Of Ipomoea Batatas L. To Cadmium Pollution

Mineral absorption and metabolism is the basis of plant absorption,fixation,transformation and enrichment.At present,there is a significant contradiction between heavy metal hyperaccumulation and efficient absorption of mineral elements in selected enrichment plants.In order to explore the effects of mineral elements on cadmium(Cd)accumulation in plants,we analyzed the difference of mineral nutrition metabolism of purple sweet potato(Ipomoea batatas L.)at different growth periods under Cd stress by pot experiment.And we further analyzed the interaction between Cd and mineral elements in purple sweet potato by hydroponic experiments.In order to analyze the response mechanism of purple sweet potato roots to Cd stress,we used transcriptome and metabolome sequencing to screen differentially expressed genes and metabolites related to Cd transport.The main results are as follows:Firstly,the Cd content in different parts of purple sweet potato is different at different growth periods.The order of Cd content is root > stem > tuberous root > leaf.The root is the main part of Cd accumulation.After 120 days of growth,when the concentration of Cd was 2mg/kg and 4 mg/kg,the bioconcentration factors(BCF)of purple sweet potato roots were greater than 1,which were 1.736 and 1.119,respectively.The effects of Cd stress on the content of mineral elements in different parts of purple sweet potato during the whole growth period are both antagonistic and synergistic,which are related to the types of mineral elements,plant parts,growth periods and Cd concentration.After 30 days of growth,the correlation between Cd and Fe was the strongest in roots and leaves(r = 0.783** and 0.880**),while the correlation between Cd and Mg was the strongest in stems(r = 0.955**).After 30 d,60 d and90 d of growth,the elements with the strongest correlation with Cd in purple potato roots were Fe(Correlation coefficient,r = 0.783**),Zn(r =-0.839**)and Mg(r = 0.944**),respectively.After 120 d of growth,Cd in purple sweet potato roots had no significant correlation with the eight mineral elements.The results of principal component analysis(PCA)showed that there were differences in ions between roots,tubers,stems and leaves,and the cumulative contribution rate of the first two principal components was 68.9%.Secondly,under the condition of hydroponic culture,compared with the control,when the concentration of Cd was 5～25 μmol/L,the Fe content in the underground part of purple sweet potato seedlings increased by 26.54%～115.24%,while the K content in the aboveground part decreased by 6.75%～16.73%,and the Mn content decreased by 49.15%～65.33%.In shoots,there was a significant negative correlation between Cd and K(r =-0.613**),and a significant positive correlation between Cd and Fe(r = 0.543*).In the underground part,Cd was positively correlated with Fe,Mn and Zn(r = 0.777**,0.746**,0.588**,respectively).Among them,Cd and Fe had synergistic effect in purple sweet potato.Under the single treatment and combined treatment of 400 μmol/L Fe and 180 μmol/L Mn,the Cd content in the shoot of purple sweet potato seedlings treated increased by 96.01%,151.09% and 196.38%respectively,while the Cd content in the underground part decreased by 27.94%,20.92% and26.49% respectively.The treatments of Fe and Mn can promote the transfer of Cd in purple sweet potato seedlings.Lastly,transcriptome sequencing showed that Cd stress induced the expression of Cd transport related genes.We screened 17 DEGs(differentially expressed genes)related to Cd transport,12 of which were up-regulated and 5 down-regulated,including ZIP gene family(ZIP1,ZIP8,ZIP9 and ZIP11),PCR gene family(PCR2,PCR8 and PCR9),HMA5,CHX18,Nramp gene family(Nramp1,Nramp5 and Nramp6),and MTP gene family Family(MTP4,MTP10 and MTP11)and VIT gene family(VITH4A and VITH4B).Among them,the key genes may be related to Fe transport.The results of metabonomic sequencing showed that the content of glutathione increased 127.689 times significantly,which may be the key substance of Cd chelation.Combined analysis showed that under Cd stress,genes related to cysteine and methionine metabolism were mainly up-regulated,and the contents of related metabolites were significantly increased.In conclusion,Cd stress promoted the absorption and transport of Fe,and enhanced the process of sulfur assimilation and metabolism in roots of purple sweet potato seedlings.