Screening Of Aluminum-tolerant Sugarcane Rhizosphere Growth-promoting Bacteriaand Their Effects On Plant Growth Under Aluminum Stress

Sugarcane is an important sugar and energy crop.Guangxi,as the largest sugarcane planting area in China,is facing a serious problem of soil acidification,which is often accompanied by aluminum toxicity,seriously inhibiting plant growth and limiting crop yield.It is of great significance to ensure sustainable development of agriculture by finding the effective ways to improve acid soil and understand the tolerance mechanism of plants to aluminum.In this study,the strain resources of sugarcane rhizosphere growth-promoting bacteria with good aluminum resistance were obtained through isolation and screening,and the effects of aluminum-resistant bacteria on sugarcane response to aluminum stress were studied,which provided strain resources and theoretical basis for improving acidic soil by PGPR and exploring ways to improve the aluminum resistance of sugarcane.The main results are as follows:1.Eleven strains of bacteria resistant to 2 mM Al³⁺were isolated from sugarcane rhizosphere soil,and analysis of their tolerance to aluminum stress showed that the strains A1,A17,A23,B25 and X6 can tolerate 4 mM Al³⁺,of which A23 and X6 can tolerate up to 5 mM Al³⁺.Analysis of growth-promoting characteristics of the strains showed that all strains except A1 and A23 produced IAA,and all eleven strains of aluminum-tolerant bacteria have the ability to fix atmospheric nitrogen.Strains A1,A23 and X6 have strong ability to solubilize phosphate,while strains A1 and X6 have outstanding nitrogen fixation and siderophore secretion capability,and A23 and X6 have ACC deaminase activity.Considering the aluminum tolerance and growth-promoting characteristics of bacteria,the dominant strains A1,A23 and X6 were selected for subsequent experiments in this study.2.Morphological characteristics,physiological biochemical indexes and16S rRNA gene sequence analysis discovered,that the strains A1,A23 and X6belonged to Burkholderia sp.,which could grow normally in a strong acid liquid medium with p H 3.8,and effectively increase the p H of its medium in the presence of 0?3 mM Al³⁺.Both of them(A1,A23 and X6)could produce exopolysaccharides with a quantity of 0.27 mg·m L^-1,1.29 mg·m L^-1and 1.68mg·m L^-1respectively.After shaking culture in LB medium with p H of 4.5 and Al³⁺concentration of 1 mM for 2 days,the removal rates of strains A1,A23 and X6 on Al³⁺reached 18.01%,17.13%and 22.42%,respectively,among which strain X6 had the strongest adsorption capacity.3.The mixed bacteria composed of A1,A23 and X6 in the ratio of 1:1:1were used to inoculate barrel sugarcane(30 d).The results showed that the inoculation of aluminum tolerant bacteria could promote the growth of sugarcane.Compared with the control,the plant height,leaf area and aboveground fresh weight increased by 24.95%,41.28%and 40.19%respectively.4.In this study,under 0 mM and 0.5 mM aluminum stress,the alkalization degree of sugarcane rhizosphere in inoculation treatment group was higher than that of none inoculated.Aluminum stress did not affect the chlorophyll content of sugarcane leaves,but inhibited the growth of roots.Under 0.5 mM and 1 mM aluminum stress,the root elongation of sugarcane in the inoculated treatment group was significantly higher than that in the control.5.The results of root tips hematoxylin staining and plant physiological and biochemical analysis showed that inoculating aluminum tolerant bacteria could improve the phosphorus absorption of sugarcane,reduce the accumulation of aluminum in roots,and enhance the antioxidant defense ability of sugarcane plants by significantly increasing the activities of CAT and POD in root tips and reducing the content of MDA.The activities of GO,PEPC,ACO and MDH related to organic acid metabolism in sugarcane root tips were affected by aluminum stress.Inoculation treatment could improve the enzyme activity to a certain extent.6.The expression of aluminum tolerance related genes in sugarcane root tips and leaves under aluminum stress was analyzed by q RT-PCR.The results showed that inoculating aluminum tolerant bacteria could significantly(P<0.05)increase the expression of MAPK and GST genes in sugarcane root tips under aluminum stress,down regulate the expression of PEPC gene in root tips and GST gene in leaves,but increase the expression level of PEPC gene in leaves.Among them,under aluminum stress,sugarcane MAPK gene was activated,and its expression in leaves decreased first and then increased,while its expression in root tips increased significantly,indicating that there may be corresponding MAPK gene in sugarcane involved in aluminum signal transduction.The overexpression of PEPC gene in leaves may increase PEPC activity in leaves and promote the root system to secrete more oxalic acid to improve the tolerance of sugarcane to aluminum,while the high expression of GST gene in root tips may relieve metal toxicity and antioxidation in sugarcane in response to aluminum stress.In conclusion,the aluminum tolerant bacteria screened in this study can promote the growth of sugarcane and participate in the aluminum toxicity resistance process of sugarcane.They mainly enhance the tolerance of sugarcane to aluminum stress by improving the antioxidant defense ability of sugarcane and the expression regulation of aluminum tolerance related genes.These strains have the potential to be developed as biological agents and are expected to be applied to improve acid soil.