Cadmium Tolerance In Different Hemp Varieties And Analysis Of Proanthocyanins In Relieving Cadmium Stress

Hemp,as an economic crop,is an important raw material for China’s hemp spining industry,paper industry and drug production.As an annual plant,it has the characteristics of short growth cycle,large biomass,strong resistance to heavy metals,and difficulty in bringing heavy metals into the food chain.Therefore,it is considered as an ideal phytoremediation material for repairing Cadmuim pollution.In this study,two Cd-resistant extreme materials were screened by Cd stress tests on six main industrial hemp varieties in China.Then,the physiological response and transcriptome analysis of the two materials were analyzed and compared.Finally,the regulation of Grape seed Proanthocyanidins（GSP）on Cd toxicity in hemp was detected.These results can provide evidence for excavating Cd-resistant genes in hemp,understanding the regulation of GSP on Cd toxicity,and using hemp plants to repair Cd-contaminated soil.The results details are as follows:1.The hydroponics method was used to investigate the seed germination rate and growth of six varieties of hemp under the treatment of 100μM CdCl₂.The results showed,under Cd treatment,the germination rate of Ym was 55.2%.Compared with the control,the plant height decreased by 10.5%,the fresh weight decreased by 5%,however.Nx was greatly affected by Cd,under Cd treatment,the Nx germination rate was 52.7%,while the plant height and fresh weight decreased by 66.1%and 42.7%respectively.Then,the six varieties were sown in soil with higher Cd content,and compared the different tissues and periods of the six varieties responsed to Cd-containded soil.The results indicated that Cd content in soil that hemp plants took away was the highest in Ym（450.88mg）,but was the least in Nx（44.7mg）,as well as,in different periods,the capacity of enrich and transport Cd among different varieties had significant differences.Among them,Ym has the strongest Cd resistance while Nx has the worst Cd resistance.2.Ym and Nx were treated with 0 and 100μM CdCl₂ respectively,and their physiological responses were detected.The results showed that the growth rate（17%）,fresh weight（5.96g）,height（9.58cm）and antioxidant enzymes activities of Ym were significantly higher than Nx（growth rate:4%,fresh weight:4.14g,height:7.16cm）under Cd stress,in addition,the degree of cell membrane damage of Ym was significantly lower than Nx exposed to Cd,which indicated that Ym was more resistant to Cd than Nx.3.Furthermore,transcriptome sequencing was performed on two varieties with 0 and 100μM CdCl₂ for 12h.Compared with the Gene Ontology（GO）enrichment results of the differentially expressed genes（DEGs）,in Y_CK vs Y_Cd group,there were 1405 genes up-regulated,881 genes down-regulated,while in N_CK vs N_Cd group,there were 79 genes up-regulated and 32 genes down-regulated.According to the Encyclopedia of Genes and Genomics（KEGG）metabolic pathway analysis,the DEGs in Nx and Ym were significantly enriched in glutathione metabolism,phenylpropanoid biosynthetic metabolic pathway under Cd stress.In addition,DEGs in Ym were also significantly enriched in sulfur metabolism and phytohormone signal transduction pathways.Heatmap results showed that Cd induced an increase in the expression of some related genes such as transporters,antioxidant-related enzymes,plant hormones,cell wall components,and transcription factors in Ym and Nx.Among them,the genes involved in Ym expression were significantly more than Nx,indicating that Ym is more tolerant to Cd toxicity than Nx.The expression levels of 10 DEGs were verified by RT-PCR,and more than 90%of the genes expression trends were consistent with the transcriptome sequencing results.4.The physiological and biochemical indexes of Ym seedlings induced by 100μM CdCl₂and the regulation of exogenous application of GSP were detected.The results showed that the exogenous application of GSP could promote the growth of root length of seedlings,reduce the content of malondialdehyde（MDA）,enhance the activities of antioxidant enzymes（SOD,POD,GSH）,and inhibit the production of ROS under Cd stress,which all can weak the effect of Cd toxicity of hemp.