Functional Studies On The Inhibition Of Photosynthesis And Reactive Oxygen Species Metabolism In Tobacco By The Heavy Metals Cadmium And Zinc

The heavy metal cadmium is able to be taken up by plants and poison them.Zinc although it is a required trace element for plant growth,excessive zinc can also inhibit plant growth.To explore how the photosynthetic function and ROS metabolic system of plant leaves respond to heavy metals Cd and Zn stress,the present study investigated the effects of 1100μmol·L^-1 Cd and 200μmol·L^-1 Zn treatments on the chlorophyll and carotenoid contents of tobacco（Longjiang 911）leaves as well as photosynthetic function and ROS metabolism,and quantitatively analyzed the key proteins in the above physiological processes using the proteomic technique of TMT.The findings are as follows:（1）Cd stress inhibited the expression of key enzymes in the process of chlorophyll synthesis in tobacco leaves,resulting in the decrease of Chl content,while Zn stress did not significantly affect the change of chlorophyll content,and tobacco leaves could adapt to Zn stress by up regulating CAO expression and promoting the increase of Chl b content.Although the content of Car in tobacco leaves did not change significantly under Cd and Zn stress,the expression of Ze and VDE decreased significantly during Car metabolism under Cd stress,and the xanthophyll cycle and NPQ dependent energy dissipation mechanism were destroyed.Under Zn stress,tobacco leaves can adapt to Zn stress by increasing VDE expression and NPQ.（2）Donor side proteins and receptor side proteins of PSII and PSI reaction center core proteins were significantly down regulated in tobacco leaves under Cd Stress,but the expression of PPD6,OEE3-2,Psb L,Psb Q,Psb27-H1,psa L,psa OL decreased significantly under Zn stress.Under Cd stress,both donor and receptor sides of PSII in tobacco leaves were damaged,and PSII and PSI were severely photo inhibited,while Zn stress did not reduce the activity of PSII and PSI reaction centers in tobacco leaves.In addition,the expressions of Cytb6/f complex,PC,Fd,FNR and ATP synthase subunit,Rubisco subunit and RCA were significantly down regulated under Cd stress,but no significant changes of the above proteins were identified under Zn stress.（3）Although Cd stress could up regulate the expression of PGRL1A and PGRL1B in tobacco leaves and induce the increase of PGR5/PGRL1-CEF,NDH-CEF was significantly inhibited.Although the expressions of ndh H and PGRL1A in tobacco leaves were significantly up-regulated under Zn stress,no significant changes in NDH-CEF and PGR5/PGRL-CEF were observed.Under Cd stress,the expression of proteins related to FD dependent nitrogen metabolism and ROS clearance in tobacco leaves such as FTR,Fd-Ni R and Fd-GOGAT were significantly down regulated.Similarly,no significant changes of the above proteins were identified under Zn stress.（4）Cd stress led to a significant increase in O₂·^-production rate and H₂O₂Content in tobacco leaves,and a large amount of MDA accumulation,but Zn stress had little damage to ROS content and oxidation in tobacco leaves.Cd stress induced the increase of POD activity in tobacco leaves,but inhibited the activities of SOD and CAT.Zn stress significantly increased the activities of SOD and POD in tobacco leaves.（5）Cd stress inhibited the activity of APX and its protein expression in As A-GSH cycle,but induced the significant up-regulation of AO expression.However,tobacco leaves can promote the regeneration of As A by increasing the activity and protein expression of DHAR and MDHAR.The up-regulation and activity of GPX,GR and GST are also important ways for tobacco to adapt to Cd stress.Although Zn stress inhibited the activities of APX and GPX in tobacco leaves,it had little effect on the activities of other enzymes and protein expression during As A-GSH cycle.（6）Trx-Prx pathway related proteins were very sensitive to Cd stress.The expression of Trx electron donor FTR and different types of Trx and Prx decreased significantly under Cd stress,but only the expression of Trx-HFC164,Prx Q and 2-Cys Prx BAS1 in Trx-Prx pathway decreased significantly under Zn stress.