The Effect Of Exogenous Lanthanum On Endophytic Bacteria In Plants

In recent years,the large-scale development and utilization of rare earth elements(REEs)by human have caused negative impacts on ecosystems.As ecosystem producers,plants are most severely affected by REEs.Researches have shown that REEs not only inhibit plant growth and development,but also enrich them in plants,posing a threat to humans at the top of the food chain.Therefore,it is urgent to clarify the mechanism by which REEs affect plants.Plant endophytic bacteria are a kind of microorganism that parasitizes in plants and has a mutualism relationship with the host plant.Plant endophytic bacteria have the functions of dissolving phosphorus and fixing nitrogen,secreting plant hormones and iron carriers and regulating plant resistance,thus enhancing plant resistance to external stress and promoting plant growth.Previous studies have shown that REEs can affect plant growth and development,but there have been no reports on effects of REEs on endophytic bacteria in plants and the role of endophytic bacteria in the regulation of plant growth and development by REEs.Therefore,in this thesis,lanthanum(La)with high accumulation in the environment and Arabidopsis(a model plant)were selected as representatives of REEs and plants,and skotomorphogenesis was taken as the main developmental stage of plants to explore the role of endophytic bacteria in the regulation of plant growth and development by REEs.The main results are as follows:(1)Obtain the effect of La(Ⅲ)on the endophytic bacteria and growth of Arabidopsis thaliana.Using 16 s r DNA amplification and sequencing analysis,it was found that after La(Ⅲ)exposure,the diversity of endophytic bacteria in Arabidopsis decreased,and the decrease amplitude increased with increasing the concentration of La(Ⅲ).In addition,the dominant endophytic genera in La(Ⅲ)-exposed Arabidopsis are Paenibacillus,Bacillus and Enterobacter.Among them,the abundance of Paenibacillus increased,while the relative abundance of Bacillus and Enterobacter decreased with increasing the concentration of La(Ⅲ).Through KEGG analysis,it was found that endophytic bacteria mediated transmembrane transport,signal transduction,carbohydrate metabolism,amino acid metabolism,and cell motility abundance values decreased with increasing the concentration of La(Ⅲ).Inductively coupled plasma mass spectrometry was used to analyze the elemental content of Arabidopsis exposed to different concentrations of La(Ⅲ).It was found that Arabidopsis can enrich La,and the enrichment amount increases with with increasing the concentration of La(Ⅲ),which means that La(Ⅲ)can enter the body of Arabidopsis,thereby threatening the survival of some endophytic bacteria.Meanwhile,La(Ⅲ)caused changes in skotomorphogenesis in Arabidopsis,which are externalized as shortening of hypocotyls,obstruction of protochlorophyllin ester accumulation,and promotion of hook formation.Correlation analysis showed that after exposure to La(Ⅲ),endophytic bacteria in Arabidopsis regulate skotomorphogenesis by increasing or decreasing nutrient element contents.(2)Explored the function of abnormal endophytic bacteria in Arabidopsis under La(Ⅲ)exposure.A total of 12 endophytic bacteria were isolated from Arabidopsis exposed to La(Ⅲ).Using 16 s r DNA extraction and PCR amplification,the biological analysis of 12 endophytic strains was carried out.Through morphological,physiological and biochemical and BLAST comparison,it was confirmed that seven of them were Bacillus,and five were Enterobacter.All of them were abnormal endophytes,and five of them were identified as the same category.Bacillus sp NB-01 has the ability to produce iron carriers;Bacillus sp NB-02 has the ability to dissolve phosphorus;Bacillus sp NB-03 has the ability to dissolve phosphorus and fix nitrogen;Bacillus sp NB-04 has the ability to produce iron carriers,IAA,and ACC deaminase(0.0112 U/mg);Bacillus sp NB-05 has the strongest ability to adsorb La(Ⅲ)(15.00%);Enterobacter sp NE-01 has the ability to dissolve phosphorus(12.42 mg/L),fix nitrogen,produce IAA(21.04 mg/L),and produce ACC deaminase.Bacillus sp NB-06 and Bacillus sp NB-07 does not have the aforementioned growth promoting function.(3)The mechanism of effects of abnormal endophytic bacteria on the skotomorphogenesis of Arabidopsis under La(Ⅲ)exposure were revealed.By co-culturing Arabidopsis seedlings exposed to La(Ⅲ)with abnormal endophytic bacteria and measuring the indices of skotomorphogenesis,it was found that after supplementation with six abnormal endophytic bacteria,the effects of different concentrations of La(Ⅲ)exposure on skotomorphogenesis in Arabidopsis decreased.Based on the promoting effect of endophytic bacteria,it was found that endophytic bacteria with the ability to produce IAA and ACC deaminases promoted the elongation of the hypocotyl,reduced the angle of the apical hook,and increase the protochlorophyllin ester content in Arabidopsis;Endophytic bacteria with the ability to dissolve phosphorus promoted the elongation of the hypocotyl of Arabidopsis;Endophytic bacteria that are tolerant to high concentrations of La(Ⅲ)can effectively alleviate the negative effects of high concentration La(Ⅲ)exposure on skotomorphogenesis in Arabidopsis.In summary,La(Ⅲ)regulates skotomorphogenesis in Arabidopsis by inhibiting the growth-promoting ability of endophytic bacteria.This study reveals the role of endophytic bacteria in La(Ⅲ)affecting plant growth and development,as well as the potential for repairing plant growth and development.