Mechanism Of The Effect Of Salicylic Acid On Cadmium Accumulation In Rice And Its Signal Molecule

Cadmium is one of the main heavy metal pollutants in rice grain in China.Reducing Cd accumulation in rice is crucial for food safety.Salicylic acid,as an important endogenous phytohormone,plays an important role in plant response to biotic and abiotic stresses.Recent studies indicated that salicylic acid can effectively reduce cadmium accumulation in crops and alleviate cadmium toxicity in crop plants.however,the physiological and molecular mechanisms of SA reducing Cd accumulation in rice plants and its possible signal molecule are still not clear.In this study,hydroponic experiment was used to investigate the effect of SA on Cd accumulation in rice seedlings.Then,the effect of SA on Cd subcellular distribution,cell wall composition and Cd adsorption,Cd chelation and vacuolar compartmentalization in Cd-stressed rice plants were examined.Furthermore,the interaction between SA and NO signaling in regulating cell wall composition under Cd stress was investigated.The objective of this study was to investigate the physiological and molecular mechanisms of SA regulating Cd accumulation and its possible signal molecule.The main results are as follows:（1）An optimized method using LC-MS/MS,coupled with deuterium-labeled internal standards was established for simultaneous quantification of SA and ABA in rice seedlings exposed to Cd stress.The method showed good linearity（r>0.999）for SA and ABA in the range of 1.0-500.0 and 0.3-100.0ng·m L^-1,respectively.The detection limits ranged from 0.5-1.75 ng·g^-1,and the recoveries ranged from96.3%to 111.3%.The validated method was successfully applied to investigate the temporal responses of SA and ABA in Cd stressed rice roots and shoots.The endogenous levels of SA and ABA significantly increased in rice roots after Cd exposure,and SA was more responsive to Cd stress than ABA.Therefore,SA was selected for the following study.（2）Hydroponic experiment was used to investigate the effect of SA on Cd accumulation in rice seedlings and its mechanism.In the present study,SA application remarkably decreased Cd concentrations in root and shoot of rice seedling and grain compared with the Cd treatment alone.SA application increased the distribution ratio of Cd in cell wall fraction and decreased Cd distribution in other fractions in rice roots,indicating that SA application increases Cd deposition in the root cell wall,thus reducing Cd toxicity to plant cells.（3）Compared to the Cd treatment alone,Cd+SA treatment significantly increased absorbances of the negative charged groups and Cd contents in pectin,hemicelluloseⅠ（HCⅠ）,hemicelluloseⅡ（HCⅡ）and cellulose,which indicated that SA induces the synthesis of cell wall polysaccharides and contributes to increase Cd deposition in the root cell wall.Pectin plays a predominant role in Cd deposition in root cell wall.Furthermore,Cd+SA treatment significantly increased pectin content,PME activity and pectin demethylesterification in the root cell wall compared with Cd treatment alone,which indicated that SA can induce pectin synthesis and demethylesterification.Moreover,SA application promotes lignin biosynthesis to thicken the cell wall in rice roots,thereby enhancing defense against Cd toxicity.However,SA application has no obvious effect on Cd distribution in soluble fractions including vacuole,GSH and PCs contents,and OsPCS2 and OsHMA3 expressions.Thus,it is conclude that SA reduces Cd accumulation in rice by promoting pectin synthesis and demethylesterification,together with lignin biosynthesis.（4）The interaction between SA and NO signaling in regulating cell wall composition under Cd stress was investigated by application of SA,NO donor SNP and NO scavenger c PTIO.SA can effectively induce endogenous NO generation in rice roots.SA pretreatment significantly decreased Cd contents in rice roots and shoots compared with the control,and the combination of SA and SNP pretreatment was more effective in reducing Cd accumulation than SA alone,while application of c PTIO,a scavenger of NO,partially recovered the SA-induced decrease in Cd accumulation in roots and shoots.Moreover,SA pretreatment significantly increased pectin synthesis,pectin demethylesterification and lignin biosynthesis,and the regulatory effect of SA was increased with the addition of SNP but was inhibited by NO scavenger c PTIO.These results indicate that NO may be involved in SA-induced cell wall components synthesis under Cd stress as a downstream signaling molecule.