| In view of the diversity and novelty of microorganisms from mangrove and their secondary metabolites.Our laboratory collected soil samples from the Mangrove Nature Reserve in Mai Po,Hong Kong.In this experiment,a variety of microorganisms were isolated from the soil samples of mangrove rhizosphere by dilution plate method.The isolated strains were identified based on their 16 s rDNA gene sequences and compared with the related database sequence information.In this paper,we classified the genera by their related taxonomic information,and set up a soil sample library with 113 strains for further study.At the same time,two novel strains,XY-J53 and XY-223,which found in mangrove samples were identified in this experiment,and part of the experiment were still under testing.So far,few studies have been done on using mangrove natural products to control agricultural disease.The EA(ethyl acetate)crude extract of the strains were obtained by extraction from the fermentation of these strains.To study the secondary metabolites of these bacteria.We screened 113 strains' crude extracts in order to find if they could inhibit the growth of four phytopathogenic fungi.A total of 20 strains of bacteria were found to have a significant inhibitory effect on the growth of fungi.Selected crude extracts with inhibition to Colletotrichum fructicola and Magnaporthe oryzae growth significantly suppressed anthracnose and rice blast development in pear fruits and rice plants respectively when applied at 50 ?g/mL.Furthermore,10 out of 14 selected crude extracts with good antimicrobial activities had no significant differences in toxicity to Chlorella compared to the control when used at 25 ?g/ mL,while Amistar Top and Mancozeb completely killed the alga under the same concentration.These data illustrate the potential of natural products from mangroverhizosphere bacteria in future agricultural application.L75,a strain of streptomyces with broad-spectrum antifungal bioactivities,was selected from these antifungal strains to do some further research.Its crude extract was obtained by large-scale fermentation(20 L),7g of crude extract was collected.L75 EA extract was significantly more effective in Alternaria solani growth inhibition at 25 ?g/mL or lower compared to Mancozeb,with no observable negative impacts on tomato leaves nor roots development.Furthermore,L75 EA extract had significantly lower aquatic toxicity than Mancozeb under the same concentrations.L75 EA extract targets germ tube elongation of A.solani conidia instead of germination,with a fungistatic mode of action.Liquid chromatography-quadrupole time-of-flight mass spectrometry analysis identified two possible known antifungal compounds,Alteramide A and the Heat-Stable Antifungal Factor,which together contribute partially to the bioactivity of L75 EA extract.On detached tomato leaves,co-inoculation of A.solani with L75 EA extract of 50,25 and 5 ?g/mL reduced diseased areas by approximately 98,90 and 48% respectively in relative to the control after 5 days.This study demonstrates the potential of natural products from mangrove rhizosphere bacteria in agricultural applications.Finally,we randomly selected 115 strains from all the sample bacteria banks for the screening of PGPR(plant growth promoting rhizobacteria)traits.In this assay,we screened 4 traits,and found that 63 strains were positive for at least one PGPR trait.A preliminary understanding of the effects of mangrove strain samples on plant growth traits was carried out.Then,the strain GP3A2,which was positive for four traits was used to test its potential in plant growth promotion.The effect on plant growth was not obvious,which needed to be further explored. |
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