Alternative Respiratory Pathways Maintain PSⅡ Function By Regulating Antioxidant Defenses Under Environmental Stress

In mitochondrial respiration,there exists an alternative respiratory pathway（cyanide-resistant）,which is connected with the respiratory chain by an additional terminal oxidase—alternative oxidase（AOX）.In the present work,by using "Nongpu" No.12 bean seedling leaves（Phaseolus vulgaris L）and the DMTU was used to inhibit H₂O₂,the Tiron was used to inhibit O₂^-,the AOX inhibitor salicylhydroxamic acid（1 mM;SHAM）was used to inhibit AOX activity in vivo,the protective effects of alternative respiratory pathway on the photosystemⅡand its possible mechanisms under the environmental stress were studied.The main findings are follows:（1）Alternative respiratory pathways maintain PSⅡactivity under CuCl₂ stress by regulating antioxidant defense levels.The results showed that treatment with CuCl₂ caused a significant decrease in the photochemical efficiency of photosystemⅡand an increase of heat dissipation,accumulation of active oxygen molecules H₂O₂ and O₂^-,the activity of antioxidant enzymes CAT,POD,APX,SOD increased;Effect of CuCl₂ on the Photochemical Efficiency of PSⅡwere alleviated by inhibiting active oxygen generation.At the same time,the activation of alternative respiratory pathways were induced by CuCl₂ stress.But after inhibition of alternative respiratory pathways,CuCl₂ treatment caused further decrease of PSⅡphotochemical efficiency and further increase of heat dissipation,and further increase of reactive oxygen levels,and further decreased of antioxidant enzyme activity;the above results suggest that alternative respiratory pathways can maintain PSⅡactivity under CuCl₂ stress by regulating antioxidant defense levels.（2）Alternative respiratory pathways maintain PSⅡactivity under Cd（NO₃）₂ stress by regulating antioxidant defense levels.The results showed that treatment with Cd（NO₃）₂ caused a significant decrease in the photochemical efficiency of photosystemⅡand an increase of heat dissipation,accumulation of active oxygen molecules H₂O₂ and O₂^-,the activity of antioxidant enzymes CAT,POD,APX,SOD increased;Effect of Cd（NO₃）₂ on the Photochemical Efficiency of PSⅡwere alleviated by inhibiting active oxygen generation.At the same time,the activation of alternative respiratory pathways were induced by Cd（NO₃）₂ stress.But after inhibition of alternative respiratory pathways,Cd（NO₃）₂ treatment caused further decrease of PSⅡphotochemical efficiency and further increase of heat dissipation,and further increase of reactive oxygen levels,and further decreased of antioxidant enzyme activity;the above results suggest that alternative respiratory pathways can maintain PSⅡactivity under Cd（NO₃）₂ stress by regulating antioxidant defense levels.（3）Alternative respiratory pathways maintain PSⅡactivity under X.c.p stress by regulating antioxidant defense levels.The results showed that treatment with X.c.p caused a significant decrease in the photochemical efficiency of photosystemⅡand an increase of heat dissipation,accumulation of active oxygen molecules H₂O₂ and O₂^-,the activity of antioxidant enzymes CAT,POD,APX,SOD increased;Effect of X.c.p on the Photochemical Efficiency of PSⅡwere alleviated by inhibiting active oxygen generation.At the same time,the activation of alternative respiratory pathways were induced by X.c.p stress.But after inhibition of alternative respiratory pathways X.c.p.treatment caused further decrease of PSⅡphotochemical efficiency and further increase of heat dissipation,and further increase of reactive oxygen levels,and further decreased of antioxidant enzyme activity;the above results suggest that alternative respiratory pathways can maintain PSⅡactivity under X.c.p stress by regulating antioxidant defense levels.In summary,the alternative respiratory pathway can protect the photosystemⅡof the bean leaves from CuCl₂,Cd（NO₃）₂ or X.c.p stress by regulating the antioxidant defense level.