Physiologicaland Molecular Mechanism Of Cadmium Tolerance In Different Materials Of Vigna Radiata L.

Cadmium（Cd）is one of the main inorganic contaminants in soil.Excessive amounts of Cd enter the food chain and cause harm to human health.Mung bean（Vigna radiata L.）is one of the main edible legume crops in China.It is a nutrient-rich special grains and functional health products;and it is also a suitable green manure crop and a good precedent for cereal crops.In recent years,there have been studies on the physiological and biochemical aspects of mung bean in response to heavy metal stress,but the mechanism of mung bean’s absorption and transport of Cd is still unclear.In this study,mung bean mutants with Cd tolerance（17#and 20#）and their wild types（Sulu）were used as research materials to explore the mechanisms of mung bean responses to Cd stress,so as to clarify the reasons for the different Cd tolerance among different mung bean materials.The following results are obtained:1.Under different concentrations of cadmium stress,the inhibition of growth of wild-type Sulu was significantly greater than that of the two mutants 17#and 20#.The concentration of Cd in roots,stems and leaves of the two mutants was significantly lower Sulu,and 20#has the lowest Cd content.Under 2μM Cd for 15 days,the Cd accumulation in roots of 20#was significantly lower than that in Sulu roots,but the Cd accumulation in shoots was significantly higher than that in Sulu shoots.Dithizone staining showed that Sulu had deeper Cd staining than 20#.The results of NMT showed that the influx of Cd was the highest at the location 300μm from the root tip,and the 20#root has a lower Cd influx.The Cd in root cells mainly accumulated in the cell wall,and the Cd in stem cells mainly accumulated in soluble components and cell walls.The Cd content in 20#cells were lower than Sulu.These results indicated that the higher Cd tolerance of 20#compared with wild-type Sulu was mainly attributed to the lower Cd uptake in the root and the higher up-ground transport coefficient.2.Transcriptome analysis showed that a large number of genes were differentially expressed in Sulu and 20#roots under cadmium stress,and the metabolic pathways involved in these genes were mainly enriched in phenylpropane biosynthesis,amino acid biosynthesis,carbon metabolism,GSH and sulfur metabolism related processes,oxidative stress,metal ion binding and transmembrane transport.The metal transporter genes with large differences in expression between 20#and Sulu included:the root metal ion uptake related transporter genes VrZIP8 and VrNramp5 and vacuolar isolation related gene VrABCC15-1.Under Cd stress,the up-regulation level of VrZIP8 and VrABCC15-1 in 20#roots were significantly higher than that in Sulu,and the 20#roots has a higher down-regulation level of VrNramp5.In conclusion,VrZIP8,VrNramp5 and VrABCC15-1transporters may play an important role in regulating Cd absorption in Vigna radiata L.3.Cd stress increased Fe accumulation in roots of Vigna radiata L.,but decreased Mn accumulation in roots.The absorption of Fe by 20#was higher than Sulu,and the accumulation of Mn in the root of Sulu was higher than 20#during the whole treatment time.Cd uptake kinetics showed that Sulu roots had a higher affinity for Cd.The addition of 6μM MnCl₂increased the Km value of Sulu and 20#,suggesting that there might be a competition in the absorption of Mn and Cd in Vigna radiata L.Analysis of gene expression patterns showed that VrZIP8 was mainly expressed in stem and leaves,the expression level of VrNamp5 was higher in roots,and the expression level of VrABCC15-1was the highest in leaves.The up-regulation fold of VrZIP8 and VrABCC15-1 in the root of mutant 20#induced by Cd was significantly higher than that in Sulu,and the down-regulation fold of VrNramp5 in the root of mutant 20#was significantly higher than wild-type Sulu.The results of yeast Cd tolerance showed that VrNramp5 decreased the Cd tolerance with increase absorption of Mn and Cd in yeast.VrZIP8 increased the absorption of Fe and Zn,but did not change the Cd tolerance and Cd content of yeast.VrABCC15-1 did not change the tolerance and absorption of Cd in yeast.These results indicate that VrNramp5 plays an important role in regulating the absorption and accumulation of Mn and Cd in Vigna radiata L.,thereby affects the tolerance to Cd stress.Under Cd stress,the changes of VrNramp5 expression level in the roots of wild type Sulu and mutant 20#might be one of the important reasons for the differences in Cd accumulation and tolerance.