Physiological And Molecular Mechanisms Of UVB On The Regulation Of Cadmium Enrichment And Its Tolerance In Poplar Trees

Human activities lead to a large number of heavy metals,which are deposited into the soil through multiple pathways,thus causing heavy metal pollution in the soil.Human activities also lead to the destruction of the ozone layer,increasing the amount of UVB radiation reaching the ground,so seriously affecting the life activities of plants and animals.Cadmium（Cd）contamination is one of the most problematic issues in soil heavy metal contamination.Scientists favor the application of phytoremediation techniques to treat Cd-contaminated soils.Compared with herbs,woody plants have large biomass and well-developed root systems,which are advantageous for the remediation of heavy metal contaminated soils.Poplar is fast-growing,resilient,and the first woody model plant to have its whole genome sequenced.Poplar has shown potential for remediation in the integrated management of Cd pollution.It was found that a moderate amount of UVB radiation can induce a resistance mechanism in plants.84K poplar（Populus alba×Populus glandulosa cv.?84K‘）has not only completed the whole genome sequencing but has the potential to remediate Cd-contaminated soil.When treated with moderate amounts of UVB,whether and how the resistance physiological mechanism of 84K poplar,can be activated,and how its enrichment,translocation,and tolerance to Cd will be affected,may provide new ideas for heavy metal remediation.In this study,84K poplar seedlings were used as test materials and indoor pot tests were taken to simulate the effects of UVB pretreatment on Cd transport,enrichment,and tolerance in poplar and the related mechanisms.Three treatment groups（UVB group,Cd group,UVB-Cd group）and one control group（CK group）were set up in the experiment.The main findings were as follows:（1）UVB and Cd treatments both inhibited plant height.The UVB dose in this experiment has not yet inhibited the accumulation of biomass,while the degree of Cd stress significantly inhibited the accumulation of fresh weight in each tissue.Compared with the Cd group,the net growth of plant height was significantly inhibited in the UVB-Cd group,but its biomass was unchanged.（2）UVB and Cd treatments both inhibited the photosynthetic capacity of plants.Net photosynthetic rate（Pn）,transpiration rate（Tr）,intercellular CO₂ concentration（Ci）,and stomatal conductance（Gs）indexes in the UVB-Cd group were the lowest among the four groups and differed significantly from those in the Cd group.The degree of photosynthetic inhibition was perhaps the strongest in the UVB-Cd group.Stomatal factors are the main reason for the limitation of leaf photosynthetic rate.（3）UVB and Cd treatments significantly inhibited photosynthetic pigment production.cd stress（Cd group,UVB-Cd group）may damage leaf photosynthetic pigments.Compared to the Cd group,the UVB-Cd group showed no difference in the content of chlorophyll a（Chla）,chlorophyll b（Chlb）,chlorophyll（Chl）,and carotenoid（Car）,except for a decrease in the content of proanthocyanidins（OPC）.（4）Compared with the Cd group,the UVB-Cd group plants had lower activities of ascorbate peroxidase（APX）,catalase（CAT）,and superoxide dismutase（SOD）,while a stronger degree of stress would in turn lead to a significant spike in their hydrogen peroxide（H₂O₂）content,which would induce a significant increase in intracellular malondialdehyde（MDA）and other toxic substances,thereby causing damage to the cell membrane and cytoplasmic leakage and causing an increase in relative conductivity values.（5）Localization of Cd:Cd was abundantly enriched in roots,distributed in epidermis,thick-horned tissue,cortex,bast,and formation layer in stems,and mostly concentrated in leaf veins and a small amount in leaf pulp in leaves.Compared with the Cd group,the Cd distribution density was higher in all organs of the UVB-Cd group.（6）The distribution of Cd concentration in each tissue of 84K poplar was:root>leaf>stem and underground part>aboveground part.Although,the enrichment factor（BF）and transport factor（TF）of poplar were less than 1,but the Cd concentration in its above-ground part was>100 mg?kg^-1.The Cd concentrations in leaves,stems,and roots of the UVB-Cd group,were significantly increased compared with the Cd group.（7）Compared with the Cd group,in the UVB-Cd group leaves,most genes were down-regulated except for F3H,MYB12,ABCC1,YSL,ZIP4,Ptr CAX-1,Ptr CAX-4 genes,which were significantly up-regulated;while in roots,most genes were up-regulated except for MYB12 and YSL genes,which were significantly down-regulated.Poplar is regulated by multiple genes synergistically to regulate the mechanism of UVB radiation on Cd uptake in poplar.In conclusion,84K poplar can be used as a candidate woody plant when remediating soil Cd contamination.Meanwhile,UVB radiation pretreatment significantly improved the uptake,translocation,and accumulation of Cd by 84K poplar.Exploring the physiological and molecular mechanisms of UVB on Cd enrichment and tolerance in poplar will provide new ideas and a theoretical basis for remediation of heavy metal pollution in the environment.