| Plant growth promoting rhizobacteria(PGPR)are very important for plant growth and development.It has the prospect of the development and application of biological fertilizers or agricultural soil improvers,which is the focus of many studies.Bacillus is a gram-positive strain of plant growth promoting rhizobacteria,which can promote plant growth,prevent and control various soil-borne fungal diseases of rice and other crops.It can also control powdery mildew,downy mildew,blight,gray mould and other bacterial and fungal diseases of vegetables and fruits.However,the chemical growth promoting factors of this bacterium are rarely studied.There are some reports on the volatile pro-biogenic factors produced by PGPR,but few studies on the non-volatile pro-biogenic factors.In this study,the biological activity of seven Bacillus were tested,and it was found that Bacillus velezensis WRN031 had a significant promoting effect on maize roots,which could promote the increase of primary root length and weight on maize seedlings,and the relative increase varied were 36.9 ± 2.2% and 78.4 ± 18.4% for root length and weight(fresh)over the un-inoculated control.By serial dilution the B.velezensis WRN031 was identified from rhizosphere soil of maize after seven days co-inoculation,indicating B.velezensis WRN031 was a plant growth promoting rhizobacteria.In order to explore the colonization position of this bacterium in maize roots,a GFP labeled B.velezensis WRNA031 was constructed by electrical transformation.Fluorescent microscopy observation GFP-labeled B.velezensis WRN031 on maize roots found that the strain was accumulated in maturation zone of both primary and lateral root after one and three days co-inoculation,respectively.The fluorescence intensity was further quantified.With the days increasing,the results showed that the number of GFP-labeled B.velezensis WRN031 increased in maize roots,and reached the highest on the 7th day.By chromatography technique and spectroscopic methods,such as NMR,MS,UV and IR,two non-volatile meso stereoisomers 3S,4R-acetylbutanediol(3S,4R-ABD,AQ-1)and 3R,4R-acetylbutanediol(3R,4R-ABD,AQ-2)were isolated and identified from acetate extract of B.velezensis WRN031.Quantitative analysis found that two compounds could be secreted into the rhizosphere soil of maize with a ratio of 1:2 after seven days inoculation.The contents were 0.76 ± 0.34,1.47 ± 0.66 μg/g,respectively.Further,compounds AQ-1 and AQ-2,and their mixtures(v/v=1:2)were tested for plant growth-promoting activity,antibacterial activity,antifeedant activity,and nematode toxicological activity.The antibacterial activity test found that the compounds AQ-1 and AQ-2 showed weak antibacterial activity.At a concentration of 512 μg/m L,the inhibitory rates of the compounds AQ-1 and AQ-2 on the Exserohilum turcicum were 26.12 ± 6.75%,28.11 ± 6.57%,and the antibacterial activity against Curvuaria lunata were 26.52 ± 6.21 %,36.29 ± 5.39%.Bioactivity screening exhibited that maize seedlings were treated with the compounds AQ-1 and AQ-2 and their mixture(v/v=1:2)for 7 days showed certain growth promoting activity of maize seedling roots compared with the control group.The treatment of compound AQ-1 increased root length up to 26.4 ± 2.1% over control at the treatment of 0.08 ?g per plant.At the treatment of 0.8 ?g per plant,the mixtures of compounds AQ-1 and AQ-2 increased the root length up to 35.3 ± 1.1%,the treatment of compound AQ-2 increased root length and weight up to 47.6 ± 14.8% and 62.4 ± 23.8% over control.The activity of rice seedlings was further screened by the compounds AQ-1 and AQ-2 and their mixture(v/v=1:2).The results indicate that in the presence of compounds inoculation the growth of plants was higher compared to uninoculated plants,but there were no obvious differences in root weight(fresh).At the treatment of 0.4 μg/plant,the root length of rice treated with compound AQ-1 up to 17.4 ± 3.5%;when the treatment concentration was 0.8 μg/plant,the rice root length increased by 20.7 ± 2.6% compared with the control;at the treatment of 4 μg/plant,the root length of rice treated with compound AQ-2 increased by 11.3 ± 2.8% compared with the control.In conclusion,B.velezensis WRN031 is a plant growth promoting rhizobacteria that can colonize the rhizosphere maturity zone and promote the growth of plant roots.Non-volatile substance 3S,4R-ABD and 3R,4R-ABD produced by B.velezensis WRN031 can promote the growth of plant roots,and these two compounds have been detected in rhizosphere soil.It shows that 3S,4R-ABD and 3R,4R-ABD are rhizosphere-promoting substances of B.velezensis WRN031.This paper reveals the relationship between rhizosphere promoters and host plants from the perspective of non-volatile substances,which provides a basis for the development and utilization of plant growth regulators. |
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