Physiological And Molecular Mechanisms Of Responses To Salt Stress And Solar Ultraviolet-B Radiation In Poplar

Soil salinity and the increased of Ultraviolet-B radiation(UV-B,280-315nm)causes increasingly agricultural and environmental problems on the worldwide scale,especially in the arid and semi-arid areas.Poplar plants is one of the most widely distributed species of woody which have a highly adaptable in the world,Populus alba and Populus russkii are widely distributed in China's Northwest,and these two species of Populus has a distinct difference to the salinity stress.The present study is investigate the physiological &biochemical responses as well as the molecular mechanism in Populus alba and Populus russkii under salinity stress,UV-B radiation and UV-B&Salt combined treatments.In the meanwhile we have researched the transport characteristics of Na~+/K~+ in two species of Populus under highly salinity treatment using hydroponic conditions,as well as series of the adaptation mechanism of physiological and molecular through the method of high throughput gene expression screening.The main results show as follows:(1)The photosynthetic system and reactive oxygen species(ROS)metabolism of two species of Populus were influenced significantly under salinity stress,UV-B radiation and UV-B&Salt composite stresses,for instance the net photosynthesis(Pn),the maximal photochemical efficiency(Fv/Fm)and the content of photosynthetic pigments were decreased respectively,the thermal dissipation quantum rate(Fo/Fm)was increased;the concentrations of hydrogen peroxide(H2O2)and malondialdehyde(MDA)were enhanced,as well as the series of antioxidant enzymes,such as POD,CAT,APX and GR.In addition,the contents of osmotic protective substances such as proline,glycine betaine and soluble protein were increased also.All in all,two Populus has a similarity property of physiological response to salt stress and UV-B radiation,and the defense responses was the strongest under the UV-B&Salt composite stresses in Populus.It's indicated that moderate salinity has irritated the defense system timely and alleviated the UV-B damage in Populus.(2)Many of key genes which were involved in the UV-B signaling pathway were expressed under UV-B radiation and UV-B&Salt combined treatments,such as HY5,COP1,STO/BBX24 and STH2,which has facilitated the UV-B photomorphogenesis.Simultaneously,series of key genes in the phenypropanoid metabolic were up-regulated such as CHS1,CHS4,FLS1 and FLS2.In addition,salinity can also stimulate the expression of these genes in P.alba.The Populus could defense the damage caused by UV-B radiation effectively that was mainly depends on the expression of these genes which could sense or respond to UV-B radiation timely,and they could initiate or promote the biosynthesis of the phenypropanoid metabolite synchronously.(3)In our study we have detect the degrees of DNA lesions in the genome of P.alba leaves caused by different stresses through the method of q PCR.It's indicated that the damage caused by UV-B radiation was the most seriously,however,the degree of DNA lesions under UV-B&Salt combined were lower than the sole UV-B treatment significantly;In the meanwhile,we found that the expression of DNA-repair genes were up-regulated significantly under UV-B&Salt combined stress,such as DNA photolyases PHR,MMR system gene MSH2-MSH6,and damaged-DNA binding protein complex DDB1 a and DDB2,and the extent of up-regulate of these genes under UV-B&Salt combined treatment was more than the sole UV-B radiation,it's manifested that moderate salinity could alleviate UV-B damages in Populus significantly.(4)We have investigated the property of Na~+ transport with LAST in Populus by pharmacological method,which was processing by add channel inhibitors and the inhibitors of transporter which was possibility involved in Na~+ uptake and transport under high salinity treatment with hydroponic conditions.The results show that the low affinity absorption of Na~+ in two species of Populus has a distinctly differences.It's indicated that the Na~+ low affinity uptake in P.alba were mainly mediated by K~+ channels and non-selective cation channels(NSCCs);which K~+ channel,the high affinity transporter of K and non-selective cation channel(NSCCs)in P.russkii.(5)In our present study,we have discovered that the Na~+ absorption or distribution,antioxidant system and osmoprotectant synthesis,as well as the expression of Na~+/K~+ transporter in two species of Populus have a distinct difference under short-term(24h)or long-term(12-15d)salinity stress treatment through hydroponic conditions.P.alba could irritate the defense mechanism under short-term salinity stress effectively and timely through up-regulated of a series of salt resistant genes,such as Na~+/H+ antiporter in the vacuole membrane(NHX),plasma membrane Na~+/H+ antiporter SOS1 and Na~+/K~+ co-transporter HKT1;1,while the expression of these genes no up-regulated trend in P.russkii.(6)It's showed that there are a large number of differentially expressed genes in the roots of two species of Populus under short-term and long-term salinity stress by analysis of the gene expression profile through RNA-Seq sequencing.The GO enrichment and classification analysis of the differential expressed genes show that these genes mainly in the physiological metabolic pathways,which have different catalytic activity and binding property,thus it has lead to a distinctly different tolerance of salt in two species of Populus under salinity stress.