The Mechanism Of Arbuscular Mycorrhizal Fungi Reducing Cadmium Uptake And Accumulation In Upland Rice

The emission of a large amount of pollutants from industrial and agricultural production aggravates the cadmium?Cd?contamination for farmland soil in China,which threatens the safety of crops.Rice is the largest food crop in China,which has the traits of absorbing and accumulating Cd.The enrichment of Cd seriously affects the yield and quality of rice,and then enters the human body through the food chain,endangering human health.Arbuscular mycorrhizal fungi?AMF?is a key microorganism that can form a reciprocal symbiosis with more than 90%of terrestrial plants.It not only plays an important role in the absorption and defense of heavy metals in host plants,but also effectively improve the quality and safety of agricultural products.Due to the shortage of fresh water resources in China,the development of upland rice production is of great significance for water saving and drought resistance.Therefore,this study took upland rice?Oryza sativa L.?as the research object,and explored the effect and mechanism of inoculating arbuscular mycorrhizal fungi on cadmium uptake and accumulation in upland rice through soil pot experiments under different cadmium treatments.Our research provides theoretical basis and practical technology for improving rice yield and quality in cadmium-contaminated areas.?1?A pot experiment was set to study the effect of the Rhizophagus intraradices?RI?and the Fulceliformis mosseae?FM?in Cd contaminated soil for the plant biomass,Cd accumulation,photosynthetic rate,photosynthetic pigment content,reactive oxygen species,antioxidant system,ultrastructure of mesophyll cells and root anatomical structure,in upland rice by using hanyou 3as the test crop.Three concentrations were set in the experiment:no Cd was added,low Cd(2mg kg^-1)and high Cd(10 mg kg^-1).Results show that both FM and RI can form a good symbiotic relationship with upland rice and immunity to Cd pollution.The growth of upland rice was inhibited significantly under Cd stress.The growth,photosynthesis rate and photosynthetic pigment content of upland rice decreased significantly under Cd stress,which seriously affected the photosynthesis of plants.In addition,the active oxygen balance of upland rice were broken and the content of O₂.^-,H₂O₂ and MDA were increased significantly under high Cd stress.The deepening of lipid peroxidation activated the oxidative stress response of upland rice,which increased the content of antioxidant GSH,AsA and antioxidant enzyme GPX,APX.At the same time,Cd damaged the ultrastructure of mesophyll cells and the anatomical structure of root system.The chloroplast of mesophyll cells was severely deformed,starch grain expand and increase,the root epidermis exfoliated and the stele ruptured.Inoculating AMF can increase the shoots and root biomass of plants significantly,promoted the growth of upland rice.At the same time,the inoculation of FM and RI under the pollution of low cadmium(2 mg kg^-1)and high cadmium(10 mg kg^-1)reduced the Cd content significantly in the shoots and roots of upland rice,which proved that AMF can reduce the absorption and accumulation of Cd in upland rice.In addition,AMF symbiosis alleviated the inhibitory effect of Cd on photosynthesis and regulated the protective mechanism of upland rice adaptation.Furthermore,the toxicity of lipid peroxidation to upland rice under Cd stress was reduced by increasing the activities of oxidases and non-enzyme oxides in plants.At the same time,AMF treatment alleviated the toxicity of Cd to leaf mesophyll cells,and the root system of upland rice showed a more complete tissue structure,which further confirmed that AMF could improve the tolerance of upland rice to Cd toxicity.In addition,RI are more effective under high Cd pollution.?2?A hydroponic experiments experiment was set to investigate the effect of the Rhizophagus intraradices?RI?for the plant height,root length,Cd accumulation,contents of different polysaccharide components in the root cell wall and the Cd accumulation of these components.Meanwhile,the mechanism were explored on pectin methylesterase?PME?activity and functional group characteristic absorption peak of root cell wall as well as the phenylalanine ammonialyase?PAL?activity,lignin content,and the anatomical structure and ultrastructure of the root system in upland rice by using hanyou 3 as the test crop in Cd contaminated stress.Two concentrations were set in the experiment:no Cd was added and 50?mol L^-11 Cd.Results show that RI could form a good symbiotic relationship with upland rice and was not affected by Cd pollution.The plant height and root length of upland rice was inhibited significantly,the accumulation of Cd in shoot and root was increased,and Cd²⁺mainly accumulated in root under Cd²⁺stress.The content of polysaccharide in root wall of upland rice was changed under Cd stress treatment,the total sugar and uronic acid contents of pectin and cellulose 1?HC1?were increased.And the characteristic peaks of polysaccharide chains C-C and C-O in cell wall,-COOH in cellulose,-NH₂ in protein and-OH in pectin were shifted to a certain extent under Cd stress.At the same time,the root structure was seriously injured,the stele collapsed,the diameter and area of the stele decreased,and the membrane system of the root cell was destroyed obviously.In addition,serious plasmalemma separation occurred,mitochondria became smaller or even disintegrated,cytoplasm distribution was disordered,root cell wall was distorted and deformed.Under the condition of Cd pollution,inoculation of AMF can increase the plant height and root length of upland rice significantly,decreased the content of Cd in shoots,and promoted the growth of upland rice.Furthermore,the total sugar and uronic acid contents of pectin and HC1 in root cell wall were increased,which significantly improved the binding of Cd to root cell wall.AMF symbiosis also increased the concentration and proportion of Cd in pectin and HC1,which suggesting that AMF could redistribute Cd in the reconstituted cell wall of upland rice.At the same time,AMF had a certain effect on the adsorption and fixation of functional groups of Cd²⁺in the root wall of upland rice.The stretching vibration peak of-C=O was enhanced,which indicated that more carboxyl groups were esterified in the pectin after inoculation.Polysaccharide chain C-C and C-O shifted to a certain extent,which indicated that Cd stress and AMF inoculation could modify cell wall cellulose and other polysaccharides,which affected the structure of cell wall polysaccharides.Moreover,-NH₂ in protein,-COOH in cellulose and-OH in pectin are the main sites of AMF action.In addition,AMF treatment increased the thickness of root outer cortex and the area of stele and xylem,the stele was intact,plasmolysis of cell was relieved,and the root cell wall was further thickened to resist the entry of Cd.