Molecular Mechanism Of Phosphates Reducing Cadmium Toxicity In Rice

With the development of urbanization and industrialization in our country,the corresponding environmental problems are appearing constantly.In recent years,the problem of heavy metal pollution in agricultural soils has gradually become the focus of our country’s environment and has been widely concerned.Many studies have shown that the accumulation of cadmium in rice is affected by a large number of genes,such as NRAMP family and HMA family.However,a large number of genes involved in plant cadmium accumulation have not been reported.Phosphate is one of the most widely used fertilizers in the world,and the study on phosphate can provide a theoretical basis for reducing the accumulation of cadmium in rice through fertilization management.In this study,a high cadmium accumulation variety(Cd-T)and two corresponding low cadmium accumulation variety(Cd-N1 and Cd-N2)were found by screening 14 rice varieties planted in the Three Gorges Reservoir Area.Firstly,by comparing Cd-T and Cd-N1 in soil culture,the accumulation and transportability of metal ions between high and low cadmium cumulative varieties were determined,and then the key genes were identified by transcriptome analysis.Then,the interaction between Cd-T and Cd-N2 was studied.The physiological differences between the two cultivars under different treatments of P and Cd were measured.The basic results are as follows:(1)Screening of High and Low Cadmium Cumulative Rice VarietyCd-T is a rice cultivar with high cadmium accumulation,the cadmium content in the ground reaches 118.82mg/kg and 196.16mg/kg under 1 mg/L and 10 mg/L cadmium treatments,respectively,which are significantly higher than Cd-N1 and Cd-N2.(2)Screening and Verification of Cadmium Related GenesThe ability of Cd-T root to absorb cadmium and transport cadmium from root to shoot was significantly higher than that of Cd-N1 under the condition of low concentration cadmium.In low concentrations of cadmium contaminated soils(0.28 mg/kg),the cadmium content in roots stems and leaves reached 7.23,0.96,and 1.97 mg/kg,respectively,3.53,6.40,and 4.11 times that of Cd-N1.However,there was no significant difference between 1.28 mg/kg and 50.28 mg/kg cadmium treatments.At the same time,Cd-T and Cd-N have some differences in the enrichment ability of various trace elements.The enrichment and transport coefficients of Cd-T root were 25.81 and 0.14,respectively,higher than Cd-N1 7.31 and 0.07,indicating that Cd-T accumulation ability of cadmium was stronger than Cd-N1 because Cd-T root absorption and transportability of cadmium were significantly stronger than Cd-N1.The photosynthetic capacity of Cd-T was stronger than that of Cd-N under the condition of low concentration cadmium.However,there was no significant difference in chlorophyll content between the two rice varieties.The transcriptome analysis showed that the expression of OsNRAMP2,OsZIP2,and OsIRT1 in the root and stem of Cd-T was significantly higher than that in Cd-N under the condition of 0.28mg/kg cadmium contaminated soil culture.Overexpression of OsZIP2 gene in wild rice could increase Cd content in rice grains,and the content of Cd in overexpression plant seeds was 38.73μg/kg,higher than that in 29.15μg/kg in the control group,and the content of Cd in over-expression plant under 10 mg/L Cd treatment was 437.13mg/kg in hydroponic culture experiment,higher than that in 178.26mg/kg in the control group,indicating that OsZIP2 had the function of promoting Cd transport to rice.(3)Effects of Phosphate on Response to Cadmium toxicity in RiceThe biomass,plant height,and root length of Cd-T and Cd-N2 decreased to a certain extent under 0 mg/L and 10 mg/L phosphate treatments,indicating that both low and high phosphorus stress rice.Both medium and high phosphorus treatments(3 mg/kg and 10 mg/kg)reduced cadmium content in two rice varieties,with Cd-T’s upper-ground cadmium content of 82.61 mg/kg and 61.57 mg/kg under 3 mg/L phosphate treatment significantly lower than that under 1 mg/L phosphate treatment,121.47 mg/kg,respectively.This indicated that higher phosphate concentration could effectively enhance the resistance of rice roots to cadmium poisoning.Cadmium content in different tissues of Cd-N2 rice varieties decreased significantly at low phosphorus concentration,but Cadmium content in different tissues of Cd-T was not affected.The phosphate of 3mM could promote the activities of antioxidant enzymes of Cd-T and Cd-N2 rice,but 10 mM and 0 mM phosphorus treatment could decrease the activities of the antioxidant enzymes of rice.The chlorophyll content of Cd-T and Cd-N2 seedlings increased with the increase of phosphate application,indicating that the cadmium toxicity of rice seedlings was alleviated with phosphate application.Transcriptome analysis showed that the expression of OsNRAMP5,OsHMA2,OsIRT1,OsZIP2 in roots of Cd-N2 plants decreased significantly under long-term cadmium treatment at 3 mg/kg phosphorus treatment.These results suggested that the 4 Cd-transport-associated genes might be involved in the response of rice to cadmium under phosphorus nutrition.Phosphate application could significantly inhibit the expression of OsHMA2,OsNRAMP5,and OsIRT1,and reduce the accumulation of Cd in rice roots.However,the decrease of Cd content in rice tissue resulted in an increase of Cd-T and Cd-N2 biomass and chlorophyll content after increasing phosphate application under Cd pollution.