Mechanism Of Exogenous Glycerol Alleviating Cadmium Stress In Maize Seedlings

Maize is an important food,economic and feed crop,which has been planted in a large area in China and even in the world.Soil cadmium（Cd）pollution is the most common pollutant among soil heavy metal pollutants,which seriously threatens the growth and development of maize seedlings and human health.In this experiment,the effects of exogenous glycerol on the growth,photosynthetic characteristics,reactive oxygen species metabolism,antioxidant capacity,mineral element absorption,and RNA transcription level of maize seedlings under Cd stress were studied by potting,and set two Cd stress concentrations of 400μM and 800μM,to clarify the response and mitigation mechanism of glycerol on maize seedlings under Cd stress,and provide theoretical basis and technical support for maize stress resistance cultivation.The main research results are as follows:（1）Exogenous glycerol promoted the growth and development of maize seedlings under Cd stress.Under Cd stress of 400μM and 800μM,maize seedlings were short,the dry and fresh weight of shoot and root decreased significantly compared with the control,and the root growth was inhibited.Exogenous glycerol promoted the increase of aboveground dry and fresh weight,root dry and fresh weight and maize plant height of maize seedlings under Cd stress,improved root characteristic parameters and alleviated the inhibitory effect of Cd stress on maize root development.（2）Exogenous glycerol increased the photosynthetic enzyme activity and photosynthetic efficiency of maize leaves under Cd stress,and increased the content of carbohydrate.Under Cd stress,exogenous glycerol increased the contents of chlorophyll a,chlorophyll b and chlorophyll a+b in maize leaves under Cd stress,enhanced Ru BP carboxylase and PEP carboxylase activities,and significantly increased the net photosynthetic rate（Pn）,and the intercellular carbon dioxide concentration（Ci）,transpiration rate（Tr）and stomatal conductance（Gs）were also increased to a certain extent of leaves.The initial fluorescence（Fo）and the quantum ratio of heat dissipation（Fo/Fm）of chlorophyll fluorescence parameters reduced,and the maximum photochemical efficiency of PSⅡ（Fv/Fm）and photosynthetic performance index（PIabs）increased,which was beneficial to reduce the damage of Cd to the photosynthetic mechanism of maize leaves;Due to the improvement of photosynthetic carbon assimilation ability of maize,the content of sucrose and total carbohydrate in maize leaves increased significantly.（3）Exogenous glycerol enhanced the scavenging capacity of active oxygen species and the balance of antioxidant system of maize seedlings under Cd stress.Exogenous glycerol reduced the content of H₂O₂ and MDA in maize leaves and roots under Cd stress,decreased the electrolyte leakage rate（EL）of leaves and roots,reduced the damage of reactive oxygen species to cell membrane,and maintained the normal structure and function of cell membrane under Cd stress.The application of exogenous glycerol increased the activities of SOD,POD,CAT and APX in maize leaves and roots under Cd stress,and the increase of antioxidant enzyme activity in maize roots was smaller under Cd stress;At the same time,exogenous glycerol also promoted the content of non-enzymatic substances ascorbic acid（ASA）and reduced glutathione（GSH）in the antioxidant system under Cd stress.The increase of antioxidant enzyme activity and antioxidant substance content is conducive to the clearance of reactive oxygen species in cells and improves the resistance of maize seedlings to Cd stress.（4）Exogenous glycerol regulated the absorption,distribution and transport of Cd in maize seedlings under Cd stress,and promoted the absorption of essential mineral elements.Under Cd stress,foliar spraying of glycerol decreased the content of Cd in the shoot of maize seedlings,but increased the content of Cd in the root and decreased the transformation factor（TF）.In different subcellular components of maize root,the content of Cd in cell wall and soluble components increased,and the content of Cd in organelles decreased.In subcellular components of maize leaves,the content of Cd in organelles and soluble components decreased significantly,and the content of Cd in cell wall increased slightly.Cd stress inhibited the absorption of essential mineral elements by maize plants,while exogenous glycerol promoted the increase of Mg,Ca,Mn,Fe and Zn contents in maize shoots and roots to a certain extent.（5）The transcriptome sequencing of all treated maize leaves and the analysis of differentially expressed genes showed that there were 123 up-regulated and 115 down-regulated genes in CK and Glycerol treatment groups;In CK and Cd400 treatment groups,865 differential genes were up-regulated and 696 genes were down-regulated;There were 408 up-regulated genes and 354 down-regulated genes in CK and Cd800 treatment groups;There were 121 up-regulated genes and 49down-regulated genes in the Cd400 and Cd400+Glycerol treatment groups;There were 48 up-regulated genes and 31 down-regulated genes in the Cd800 and Cd800+Glycerol treatment groups.（6）The enrichment analysis of differentially expressed genes showed that in CK and Glycerol treatment group,the differentially expressed genes were mainly enriched in phenylpropane biosynthesis,nitrogen metabolism,starch and sucrose metabolism,carbon sequestration of photosynthetic organisms and other pathways.The differential genes in CK and Cd400 treatment group,CK and Cd800 treatment group were mainly enriched in photosynthesis,photosystem,glutathione metabolism,redox process and other pathways.Cd400 and Cd400+Glycerol treatment group,Cd800 and Cd800+Glycerol treatment group were jointly enriched in phenylpropane biosynthesis,plant-pathogen interaction and photosynthesis-related pathways.It is speculated that the molecular mechanism of exogenous glycerol alleviating Cd stress in maize seedlings may be related to phenylpropane biosynthesis,plant-pathogen interaction and photosynthesis.