ACC Deaminase-Producing Bacteria Enhance The Tolerance To Cadmium In Maize

With the rapid development of economy,heavy metal pollution in soil has gradually become a serious environmental problem.China is a large agricultural country,the cultivated land area accounts for about 7% of the total cultivated land area in the world,and the problem of heavy metal pollution in farmland has seriously affected our food security.As a new method to control heavy metal pollution in soil,microbe-plant combined remediation technology with the advantages of safety and high efficiency,has received much attention.In this study,the four strains A1(Achromobacter),YM3(Burkholderia),4C7(Serratia)and YS4(Ochrobactrum)were selected as the tested strains,and Longping 206 was used as the tested maize varieties.Hydroponic experiments were carried out at three cadmium concentrations(0 mg/L、8mg/L、12mg/L)to evaluate the effect of inoculation of bacteria on the growth and Cd uptake,and the mechanism involved in improvement of maize tolerance to cadmium.The main findings are as follows:The results of the growth-promoting characteristics showed that strain A1 and strain YM3 have the ability to produce ACC deaminase and siderophore at 0 mg/L、50 mg/L and100 mg/L cadmium concentrations.Strain 4C7、YS4 and YM3 have the ability to produce IAA and dissolved phosphate.Moreover,cadmium significantly promoted the ability of producing IAA and ACC deaminase.The results of the growth curve of the strains at three cadmium concentrations showed that all four strains could grow at 0 mg/L、50 mg/L and100 mg/L cadmium concentration,among which the strains YM3、A1、4C7 had strong growth ability at cadmium concentration.The effects of different concentrations of cadmium and inoculated bacteria on maize biomass were studied by hydroponic experiments.The results showed that cadmium could significantly reduce maize biomass.During cadmium stress,the stem and leaf dry weights of maize were significantly increased by inoculation of 4 strains(A1、YM3、4C7、YS4)compared with the non-inoculation control.Under 8 mg/L cadmium stress,inoculation with strains A1、4C7、YM3、YS4 increased the stem dry weight of maize by 100%,33.3%,63.3% and 25.6%,and the leaves dry weight by 103.5%,41.5%,48.5% and 35.0%;Under12 mg/L cadmium stress,the stem dry weight were increased by 68.66 %、59.70%、83.58%and 39.25%,,and the leaves dry weight were increased by 45.40%、45.40%、67.48% and24.54% by inoculation with strains A1、4C7、YM3、YS4.Under 8 mg/L cadmium stress,strains A1 and 4C7 increased the roots biomass by 45.45% and 36.36%.Under 12 mg/L cadmium stress,the biomass of maize roots increased by 40% and 40% by inoculation with strains A1 and YM3.The effects of cadmium and bacteria on MDA,proline,chlorophyll and antioxidant enzymes in maize leaves were studied.Cadmium treatment significantly increased MDA and proline content and CAT、SOD and POD enzyme activities in maize leaves.The effect of inoculation treatment on MDA and proline content in maize leaves and CAT,SOD and POD enzyme activities was affected by cadmium concentration,showing inconsistency.Under the stress of 8 mg/L cadmium,inoculation with strains A1 and 4C7 significantly reduced the MDA content of maize leaves,inoculation with strains YM3 and YS4 significantly increased the MDA content of maize leaves,and inoculation with strain 4C7 significantly reduced the proline content of maize leaves.Under the stress of cadmium concentration of 12 mg/L,inoculation with 4 strains significantly reduced the MDA and proline content of maize leaves,and inoculation with strain 4C7 had a better effect on reducing the proline and MDA content of maize leaves.Regarding the antioxidant enzyme system,the inoculation with the four strains significantly increased the POD activity under cadmium stress.Except for YS4,the SOD activity decreased and reached a significant difference at 12 mg/L Cd.Except for strain YS4,which significantly reduced GSH content in leaves,other bacteria had no significant effect on GSH content.Inoculation treatment increased the root activity to a certain extent.All four strains could increase the chlorophyll content of maize leaves,and the difference was significant at 12 mg/L Cd.The results of cadmium content showed that inoculation treatment at cadmium stress significantly increased the cadmium content in roots and decreased the cadmium content in leaves.Inoculation with strains increased the cadmium content in roots by 8.19%-37.95%(8 mg/L)and 7.48%-50%(12 mg/L),and decreased the leaf cadmium content by 12.98%-33.68%(8 mg/L)and 4.43%-32.34%(12 mg/L),respectively.Strain 4C7 had a significant effect on reducing the accumulation of cadmium in the maize stalk at cadmium stress.Strains A1 and YM3 had significant effects on reducing cadmium accumulation in maize stem and leaf.Based on the enrichment coefficient,it can be found that inoculation with strains can increase the accumulation of cadmium in the roots of maize,and reduce the enrichment coefficient of leaves,and reduce the transfer of cadmium to the above-ground part.Among them,the three strains A1,4C7 and YM3 have significant effects.The four strains could significantly reduce the root-stem Cd transport coefficient and root-leaf Cd transport coefficient under the stress of two cadmium concentrations,and the strains 4C7,YM3 and A1 could significantly hinder the transfer of Cd to the above-ground parts of maize.