The Functions Of The Cyanide-resistant Respiration Pathway During The Chilling Stress Of Rice Seedlings And The Greening Of Etiolated Rice Seedlings

We explored the contribution of the cyanide-resistant respiration to increasing chilling tolerance and decreasing chloroplast damage in rice seedlings,the function of cyanide-resistant respiration during the greening of etiolated rice seedlings and effects of exogenous H₂O₂ on chilling-resistance of rice seedlings.The results of experiments revealed:1.After rice seedlings were exposed to low temperature for 12h,relative water content (RWC) was decreased significantly and both relative membrane penetration and reactive oxygen species(ROS) were increased.Total respiration(V_t),capacity of the alternative pathway(Va_alt) and the value of V_alt/V_t were all increased in the chilled leaves,compared with the control leaves (under normal temperature).Under chilling condition,the leaves pretreated with salicylhydroxamic acid(SHAM) had lower RWC and higher relative membrane penetration, levels of the generating rate of O₂^- and H₂O₂ content and than the control leaves(untreated with chemical) had.Electron microscopy results showed that under the normal condition,SHAM had little effect on the structure of the chloroplast and H₂O₂ accumulation.Under the condition of chilling,SHAM made the chloroplasts suffer more serious damage.The amount of H₂O₂ also increased.The accumulation of H₂O₂ might be related to the disintegration and unstacking of grana.These results showed that the alternative pathway was related to enhance rice seedlings chilling tolerance by restraining the ROS level.2.Under constant light for 12h,decrease in chlorophyll content and increases in the generating rate of O₂^- and H₂O₂ content in fully etiolated rice seedlings leaves which pretreated with SHAM were found.This showed that alternative pathway played an important role on chloroplast development and revealed connection between alternative pathway and ROS. 4,5-dihydroxy-1,3-benzene disulfonic acid(Trion) which is the scavenger of O₂^- could suppress the chloroplast development obviously,but N,N'-Dimethylthiourea(DMTU) which is the scavenger of H₂O₂ could increase the content of chlorophyll.Exogenous O₂^- contributed to the chloroplast development but higher concentration played an inhibited role.Chlorophyll content decreased constantly with increasing the concentration of H₂O₂.This might be the mechanism of the dual effect of O₂^- and H₂O₂ played a toxic effect on the greening of etiolated rice seedlings. Alternative pathway decreased the generating rate of O₂^- and H₂O₂ content and increased the chlorophyll content.And what's more,alternative pathway played a role on suppressing the toxicity of H₂O₂.3.After 12h of chilling stress at 6℃,pretreatment with H₂O₂ 0-20mM as foliar spray under normal growth conditions was measured.0-8mM H₂O₂ could significantly enhance RWC and decrease relative membrane penetration.The obvious effect of H₂O₂ occurred at 4mM.After chilling stress,compared with water-treated seedlings(control),4mM H₂O₂ inhibited the malonaldehyde(MDA) content and increased V_t,V_alt,the soluble sugar content,soluble protein content and proline content.However,pretreatment of rice plants with 4mM H₂O₂ showed no obvious effect on endogenous H₂O₂ content and the generating rate of O₂^-.These results suggest that the exogenous H₂O₂ could enhance rice seedlings chilling tolerance by enhancing respiration rate,the metabolism of carbohydrate and nitrogen and decreasing lipid peroxidation.And exogenous H₂O₂ may be isolated from the endogenous ROS pool in a certain way.