Study On The Responses Of Ricinus Communis L.to Lead And Zinc Stress And Their Mechanisms

Heavy metal pollution is one of the most widespread contaminants in soils worldwide,and phytoremediation is a relatively cheap and environmentally friendly technique and has drawn much attention.Ricinus communis L.(Castor)was considered as a potential industrial oil plant for remediating heavy metal contaminated soil because of its rapid growth,strong deep root system and highly tolerance and accumulation to a variety of heavy metals.Pb and its associated heavy metal Zn could cause toxic effects in plants,including disruption water balance,mineral nutrition,photosynthesis and enzyme activity.Pb and Zn also may enter in human body and causes serious diseases.In this research,taking castor as research object,we observe the root and leaf morphology and its anatomical features of castor under Pb and Zn single and compound stress and analyze the characteristics of metals uptake,transportation and accumulation potential to Pb and Zn.We also probe into the mechanisms of Pb and Zn tolerance in castor by studying responses of photosynthetic physiology,antioxidant enzymes and osmotic regulation substances.We employed the RNA-seq platforms to expore the transcriptome of castor and to investigated its transcriptional changes in response to Pb and Zn stress,meanwhile,to identify the candidate genes and pathways involved in Pb and Zn stress response in roots of castor thereby elucidating the molecular mechanism of Pb and Zn stress tolerance.The results were as follows:1.Pb and Zn single and compound stress significantly decreased the root activity and growth and reduce the cell division and cell enlogation.At the same stress concentration,Zn had stronger inhibitory on root growth than that of Pb.The total root length,main root length,root volume and root surface area of castor seedling in Zn600 group were 2.72cm,1.42cm,0.073cm~3 and 1.604cm~2,significantly decreased 92.67%,67.95%,63.32%and 66.25%respectively than Pb600 group.The compound stress of Pb and Zn shows addictive effect to restrains root growth.The total root length and main root length of seedling reched the minimum(1.5 and 0.9cm)at Zn600+Pb600 group,and significantly decreased 98.97%and 95.6%than that of control group.Results indicated that the inhibitory effect on root morphology indexes ranked as:Pb<Zn<Pb+Zn.2.The heavy metal uptake and distribution of castor were different,as a result of different elements and organs.The Pb and Zn concentration in different organs increased with the Pb and Zn in the group,and the Pb were mainly accumulated in roots,but the Zn in aboveground part was higher than that in roots.The TF of Zinc in castor plant reached maximum(2.93)in the single stress of Zn200,wheras,the TF of Pb is only 0.35 in Pb200 group.The plant growth and biomass increased under the group with single Pb at low concentration,the height and ground diameter comes up to maximum(5.81mm and 29.48cm)and significantly increased 3.6%and 3.8%than that of the control.But the growth and biomass could reduce in the group of Pb600,single Zn stress and the compound Pb and Zn stress.The correlation analysis indicated that the accumulation of P and K had high significant positive correlation with the Pb content in the aboveground,but negatively correlated with Pb and Zn content in root and the Zn content in aboveground part.The accumulation of N had a negative correlation with Pb content in plant,but positive correlation with Zn content.3.The single Zn stress and compound Pb and Zn stress resulted in changes of root anatomical structures in castor plant.The volume of root cap and root cap cell would became progressively smaller and the arrangement of root cap cell was in irregular in the group of Zn600 and Zn600+Pb600.The root tip meristematic cells became smaller,deeply stained and presented in irregular arrangement.The degree of suberification of cell walls of epidermis cell and exodermis cell increased,the parenchymal cell in cortex became smaller and the cytoplasm of it were colored deeply,and there were crystal in the cell.Its color and size became deeper and bigger in the group of Zn600+Pb600.The single Zn stress and compound Pb and Zn stress significantly reduced the total thickness of the leaves and the thickness of palisade tissue,and the palisade tissue cells became smaller and presented in irregular arrangement.Meanwhile,the number of parenchymatous cells and catheter also decreased,but the thickness of sponge layer became thicker.The correlation analysis showed that the Zn concentration in the group had significantly negative correlation with upper epidermis,thickness of palisade tissue and the ratio of the palisade tissue to spongy tissue.4.A significant inhibition of chlorophyll content was induced by high Pb and Zn concentration.The chlorophyll(a),chlorophyll(b)and chlorophyll content in the group of Pb600 decreased 9.12%、39.73%and 19.53%than control group,and the above indexes of group Zn600+Pb600 also significantly decreased 73.49%、87.11%and 78.12%than that of control.The Pn decreased with the increasing stress concentration,but the Ci increased.The decreasing of Pn of Castor under Pb and Zn stress was caused by nonstomatal limitation.The F0、Fv/Fm、ΦPS Ⅱ、qP and ETR of plant obviously decreased with the increasing concentration of groups.It showed PS II was suffered from being close and injured.The Pb and Zn single and compound stress made the membrane leakage enlarged and membrane lipid peroxidation enhanced.The POD and CAT activity in plant increased under the low Pb and Zn concentration,but the antioxidant enzyme activity could reduce when Pb and Zn reached high concentration.The proline content and and soluble phenol were significantly increased by Pb and Zn single and compound stress.The osmotic potential decreased and cell uptake strengthened with increasing proline content.As a result,castor can enhance their ability to endure Pb and Zn stress.5.The transcriptome of Castor roots at CK,Pb600 and Zn600 group were analyzed.Approximately 55,000,000 high quality clean reads were generated.The differentially expressed unigenes(DEGs)were screened out using differentially expressed sequencing(DESeq)method.Compared to the control group,there were 1,213 DEGs in group of Pb600 in which 864 expressed unigenes down-regulated and 368 expressed unigenes up-regulated.Of the 5,724 DEGs in the group of Zn600,3,806 and 1,918 genes were detected to be down-regulated and up-regulated.Among the 864 down-regulated gene under Pb group,745 genes were also down-regulated in Zn group,and the 368 up-regulated gene under Pb group,215 genes were up-regulated and 19 genes were down-regulated in Zn group.The rest were express in their own group only.Go enrichment analysis of these DEGs indicated that cell component,catalytic activity,metals transport function and many metabolic process were regulated under heavy metal stressKEGG enrichment analysis indicated that 20 pathways associated with heavy metal tolerance,for example,phenylpropanoid biosynthesis,ABC transporters and Glutathione metabolism,were highlighted.34 and 100 genes were annotatied in pathway of phenylpropanoid biosynthesis,5 and 14 genes were annotatied in pathway of ABC transporters and 8 and 30 genes were annotatied in pathway of Glutathione metabolism under Pb and Zn group.These pathways indicated that Pb and Zn influenced the related physiological processes involved in morphology,heavy metal uptake and transport and heavy metal tolerance.