A Study On The Role Of Sulfur On Responses Of Poplars To Cadmium And Lead

Modern industrial and agricultural processes may cause excessive cadmium(Cd)and lead(Pb)emissions that contaminate soil.Cd and Pb are nonessential elements that are highly toxic to plants and animals.The overdose of Cd and Pb in the soil can eventually threaten human health through the food chain,thus the remediation of Cd and/or Pb polluted soils is required.Phytoremediation is an eco-friendly,economical and no secondary pollution remediation technology which possess broad prospects.Populus species has been proposed for phyto-remediation because these woody species are characterized by fast-growth,large-biomass and strong adaptability.However,the accumulation of Cd/Pb in the poplars shoot tissues is lower than that of hyperaccumulators,and the remediation efficiency still needs to be improved.Sulfur(S)is an essential element for plant growth and development.The S absorption,assimilation and metabolism in plants are closely related to the heavy metals absorption,transportation and tolerance.Nevertheless,the role of S in response to the Cd/Pb stress is still not fully understood.Furthermore,the relationship between the S abosorption,assimilation and metabolism process and the tolerance of Cd/Pb still needs to be further confirmed,so as the mechanism of molecular regulation.This study aimed to further explore the physiological and molecular mechanisms of S in regulating Cd/Pb absorption,transportation,accumulation and tolerance in poplar.Firstly,the role of S in the Cd accumulation of poplar was investigated.Then,the physiological and molecular mechanism of S in Cd absorbance,transportation and accumulation in poplar was studied by varied levels of exogenous S.Finally,the mechanism of S in ameliorating toxic effects of Pb in poplar was explored.The main results were as follow:Two poplar genotypes with distinct Cd/Pb accumulation ability were treated with600?M Cd Cl₂ for 40 days.The results showed that the shoot of Populus deltoides accumulated more Cd than that of P.cathayana.The concentration of S in P.deltoides was also significantly higher when exposed to Cd.Moreover,the concentration of sulfur compounds which have high correlation with metal tolerance were also higher in P.deltoides than that P.cathayana,non-protein thiols and glutathione had the similar results.These results suggested that S assimilation and metabolism may play a vital role in Cd accumulation in poplar.S assimilation and metabolism may be the main reason of the different Cd accumulated ability between the two poplar species.To further explore the mechanism of S in response to Cd in poplars,P.deltoids were treated with different S levels(4.5,100 and1500?M S)in combination of 0 or 50?M Cd Cl_2.The results illustrated that the high S supply increased the Cd accumulation in the shoot,and S deficiency decreased the Cd contents.Moreover,the negative effects of Cd exposure on leaf biomass and phytosythesis were ameliorated by high S supply and exacerbated by S deficiency.Meanwhile,accumulation of reactive oxygen species(ROS)in Cd-treated poplars was alleviated by high S supply and exacerbated by S deficiency.The activity of?-ECS in roots,leaves and bark and the GST in shoot was increased due to high S supply.Consequently,the GSH contents in roots,leaves and bark were also increased significantly when high S was supply.The results proved that the increased GSH contents by S assimilation and metabolism may facilitate to GSH-Cd complexes formation,decreased ROS levels,and increased absorption,accumulation and tolerance to Cd in poplar.For further explore the molecular mechanism of S in regulating the Cd accumulation and tolerance in poplar,the transcriptome,micro RNA and degradome sequecing were adopted.The results showed that the transcriptional expression of YSL3(Yellow stripe like protein 3)and ZIP5(Zinc/iron regulated protein 5)were induced by high S supply,Cd absorption and translocation were enhanced.Meanwhile,the mi R828and mi R172 levels were decreased which may contribute to the upregulated of target gene MYB66 and AP2 and then enhance the regulation of downstream genes to increase the Cd tolerance in poplar under high S application.In the coexpression analysis,the ATPS(ATP sulfurylase)and NF-YA(Nuclea facteor Y subunit A)genes were discovered which participated in the S and Cd response simultaneously.The degradome analysis further confirmed that these two genes were regulated by mi R395 and mi R169,respectively.In addition,the results of coexpression network analysis suggested that ATPS was closely related with the S metabolism genes such as VTC(L-galactose-1-phosphate phosphatase)and DHAR2(Dehydroascorbate reductase 2),NF-YA was closely related with the metal transporter genes,such as CNGC1(Cyclic nucleotide gated channel 1),PDR(Pleiotropic drug resistance 12)and BCB(Blue-copper-binding protein).ATPS and NF-YA were simultaneously related with the signal transduction(Nucleoside diphosphate kinase,NDPK)and phosphate transport.These results suggested that S assimilation and Cd tolerance were highly correlation in regulating genes transcript,ATPS and NF-YA play a vitol role in this process.Pb is the associated mineral of Cd.To further explore the physiological and molecular mechanisms of S in response to Pb in poplars,P.deltoids were treated with different S levels(4.5,100 and1500?M S)in combination of 0 or 50?M Pb Cl₂.The results showed that the Pb contents in the shoot were increased with the increasing S levels.The Pb induced decline of photosynthesis,growth and ROS stress were alleviated by high S supply and exacerbated by S deficiency.High S supply led to the increased transcript levels of ATPS1,ATPS3 and GSHS1 which further resulted in the higher GSH contents.These results indicated that the absorption,transport,accumulation and tolerance of Pb in poplars were mainly through the enhancing GSH synthesis caused by S,which was similar with the mechanism of Cd.