Studies On The Mechanisms Of Cadmium-induced Pectin Synthesis And Demethylesterification In The Cell Wall Of Rice Roots

Cadmium is a dangerous heavy metal,Cd lead to some life-threating disease including hypertension,cerebrovascular infarction,emphysema,serious damage in lungs,renal dysfunction,and cancer even from low-level chronic exposure.In general non-smoking population intake Cd is mainly absorbed from plant-derived food especially cereals.Rice is the main cereal source of Cd exposure.More than 10% of the rice exist cadmium content exceed in China.A lot of reports recently have demonstrated that Cd stress lead to pectin content and structure change,but the mechanism is still unclear.Another mode of Cd toxicity is caused oxidative stress.In our experiments,we found pectin content related to hydrogen peroxide.Therefore,we hypothesized that H2O2(hydrogen peroxide)regulating pectin synthesis and remodeling.In this study,we used hydroponics.We adopted different concentrations of Cd to investigate the changes of rice(Oryza Sativa)root elongation,endogenous H2O2 content and pectin content,PME(pectin methylesterase)activity,PMD(pectin methylesterification degree)in rice roots,H2O2 scavenger 4-hydroxy-Tempo and exogenous H2O2 were also used to explore the signaling roles of endogenous H2O2.In addition,the effects of Cd and H2O2 on pectin synthesis,pectin methylesterase activity and pectin methylesterification degree also were investigated.The results is as follows:1.Cd stress inhibited rice root elongation.The effects of different concentrations of Cd on rice growth showed that exogenous application of Cd significantly inhibited the growth of rice roots in a dose-dependent manner.High concentrations of Cd inhibited rice root elongation and protein accumulation significantly.2.In Cd-induced,H2O2 content increase intensively in a short time in rice roots,then pectin content increased significant.It also obviously increased PME activity and decreased PMD.And with increaseing of Cd concentration,pectin content,PME activity increased,PMD decreased.3.Single treatment with exogenous H2O2 similarly increased PME activity and decreased PMD.H2O2 scavenger 4-hydroxy-Tempo inhibited Cd-induced increases of pectin,PME activity and decrease of PMD by decreasing endogenous H2O2 content.4.In order to verify endogenous H2O2 acts as a pivotal downstream signaling molecule inCd-induced pectin increase and demethylesterification.We investigated in other heavy metals:aluminum,lead and copper induced the change of pectin and demethylesterification and H2O2 content.The experimental result indicated that in other heavy metals induced also increased pectin content,PME activity and decreased PMD,trend similar to Cd-induced.All these results indicate that Cd-induced endogenous H2O2 plays a novel signaling role in regulating pectin synthesis,PEM activity and pectin demethylesterificaiotn to resist Cd toxicity in rice.Previous report mostly focused on the role of pectin played in plant heavy metal detoxification,but it remains unclear why and how heavy metals induce pectin accumulation and reform in roots.In our research,we have creatively brought and testified endogenous H2O2 acts as a pivotal downstream signaling molecule in Cd-induced pectin increase and demethylesterification.Our results indicate that it is possible that H2O2 plays crucial signaling roles in response to Cd stress in rice roots.H2O2 also acts as a crucial signaling molecule downstream of other heavy metals in regulating pectin synthesis.