| Soil contamination has become a global environmental problem in recent decades.Polycyclic aromatic hydrocarbons?PAHs?are a class of common persistent organic pollutants in the environment with characteristics of teratogenic,carcinogenic and mutagenic,and can be absorbed and accumulated in plants and translocated into shoots,which are most part will be eaten,resulting in serious threats to agricultural products and human being's health.Clearly,it is essential to reduce PAHs contamination in the plants and their harm to human being's health.Pyrene,a typical representative of PAHs,has been an indicator widely used to test PAHs of the invironment and model compound to study biodegradation of high-molecular-weigh PAHs.The objectives of this study were to isolate and characterize an efficient pyrene-degrading endophytic bacterium PW7 from the healthy plant Plant ago asiatica Willd grown in PAHs contaminated sites.Then growth characteristics and pyrene-degrading of the stain PW7 were elucidated with different environmental conditions.The conditions for pyrene-degrading and growth of the strain were optimized.Finally,the strain PW7 was colonized into wheat by the three different methods of colonization.The influences of colonization on plant growth,pyrene uptake,accumulation and metabolization were investigated.The results of this study will be benefit for reducing the plant PAHs pollution levels and protecting agricultural products against PAHs contamination.1.Isolation and characteristics of an efficient pyrene-degrading endophytic bacterium strain from plants and optimize the conditions.?1?A high-efficiency pyrene-degrading endophytic bacterial strain PW7 was isolated from the healthy plant Plantago asiatica Willd grown in PAHs-contaminated soils by enrichment culture.It was identified as Serratia sp.based on the morphology,physiological characteristics and 16S rDNA sequence analysis.Strain PW7 had secondary growth characteristics in LB liquid medium.The study about influences of environmental conditions on the growth characteristics of the strain PW7 revealed that,the best condition of temperature was 30?,the suitable pH was 7.0,the best condition for the strain PW7 was the salt concentration of 5 g·L-1.The strain PW7 was of aerobic bacteria and could grow well when the inventory was 50 mL/100 mL flask.The strain PW7 had good salt tolerance.?2?Strain PW7 could remove 47.52%of pyrene from media within 10 days.Research of the strain PW7 pyrene degradation showed that the optimal biodegradation conditions for pyrene by strain PW7 were at pH 7.0 and temperature 30?.In our test,the substrate concentration was negatively correlated with the growth of the strain and the degradation of pyrene,the inoculation amount was positively correlated with the growth of the strain and the degradation of pyrene.The pyrene degradation rate was 70.81%in media added with 20 mg·L-1 pyrene.Different additional carbon and nitrogen sources had different influence on pyrene-degrading.Within 10 days,strain PW7 could degrade more than 60%pyrene with the additional glucose,maltose,sucrose,and peptones,respectively.2.Inoculation of endophytic bacteria strain PW7 in wheatThis study evaluated the performance of different plant colonization methods?soak seed,SS;soak root,SR;paint leaf,PL;control,CK?for reducing the pollution of pyrene in the plant.?1?Endophytic bacteria strain PW7 could be successfully colonized in the wheat,it could also be transferred from the roots to the shoots.Endophytic bacteria strain PW7 could actively colonize inoculated plants internally,and inoculating wheat plants with strain PW7 can promote the growth of the wheat.Compared with CK,the fresh of the wheat with SR,SS,PL were increased by 25.00%,20.76%,15.45%.The promotion of wheat with SR is strongest.?2?The wheat can be absorbed pyrene of the culture medium,and conducted by the roots to shoots.Inoculating wheat plants with strain PW7 could reduce the pyrene concentration in medium,the order of the concentrations of pyrene in medium from the low to high was SR<SS<PL<CK.Compared with CK,the reduction rate of pyrene concentrations with SR,SS,PL were 51.36%,23.64%,12.11%.With the extension of exposure time,the concentrations of pyrene in medium and the rate of pyrene removal decreased gradually.Inoculating wheat plants with strain PW7 via SR,SS,PL could increase the rate of pyrene removal,the order of the rate of pyrene removal from high to low was SR>SS>PL>CK,the rate of pyrene removal with SR was up to 78.28%,PL was the worst.?3?Inoculating wheat plants with strain PW7 by SS,SR or PL revealed that strain PW7 can induce the concentrations and accumulation of pyrene.In comparison with CK,inoculating wheat plants with strain PW7 reduced the concentrations and accumulation of pyrene in wheat,and the order was SR<SS<PL<CK.It was indicated that strains PW7 inoculated into wheat can promote the absorption and metabolism of the pyrene.The concentrations and accumulation of pyrene of the wheat with SR were the lowest,were 29.49 mg·kg-1,3.66 ?g·pot-1.Compared with CK,the reduction rate were the highest,were respectively 61.99%,53.96%.With the extension of exposure time and reducing pyrene concentrations,the concentrations and accumulation of pyrene in wheat decreased gradually.Inoculating wheat plants with strain PW7 by SR,SS or PL,the concentrations and accumulation of pyrene were different,the order was SR<SS<PL<CK.Compared with the control reduction rate was SR>SS>PL.Inoculating wheat plants with strain PW7 can induce the concentrations and accumulation of pyrene in wheat.?4?Inoculating wheat plants with strain PW7 by SR,SS or PL revealed that strain PW7 can induce the concentration factors and transmission factors.With increasing pyrene concentrations and the extension of exposure time,the concentration factors and transmission factors decreased gradually.Colonized in different methods,the concentration factors and transmission factors of the wheat was SR<SS<PL<CK,but the pyrene concentration factors of roots changed little.Most pyrene was degraded in the roots,and the rate of pyrene removal in medium and the wheat was SR>SS>PL.Our results indicated that inoculation of strain PW7 in wheat could reduce the pyrene contamination in wheat.Among three inoculation ways in this test,soaking roots in inocula was the best way for increasing pyrene removal from plants. |
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