Chracterization Of The Identity And Biocontrol Potential Of A Strain Of Streptomyces F-1 Isolated From Rice

Rice sheath blight,caused by Rhizoctonia solani is one of the major diseases of rice, the study on biological control of it is hot in recent years.There are many microorganism population in the sphere of plants naturally,among them,some species such as actinomycetes have antagonism to the phytopathogen,and can be used as biocontrol agents.Evaluate the potential ability of antistrains is the precondition to use them for biocontrol,and explore the antagonism mechanism of them is the base to improve their ability for biocontrol.In this study,a Streptomyces strain F-1 which has significant antagonism to Rhizoctonia solani,Botrytis cinerea,Sclerotinia sclerotiorum and so on phytopathogenic fungi was isolated from healthy rice plant.Based on this,the classified appraisal,biocontrol potential and mechanism of the strain F-1 were studied in this research.The results are as follows:1.Three hundred and eighty five bacterial strains and two Streptomyces strains were isolated from the healthy and diseased(Rhizoctonia solani) rice plants,which were collected from 13 different areas of Hubei province,Hunan province,Shandong province. Though the dual-cultural tests and the in vitro bioassay for inhibition of infection by R. solani on rice leaf segments,the Streptomyces strain F-1 was selected.2.The strain F-1 was classified as Streptomyces platensis by observation of its morphological,cultural and physiological characteristics,and by analysis of its 16S rDNA sequence(GenBank accession number EF583557).3.The potential of the antifungal substances(AFS) produced by Streptomyces F-1 to control rice sheath blight was determined on rice leaf segments(in vitro),rice seedlings (in vivo) and rice plants(in vivo).The results show that AFS-containing cultural filtrate of Streptomyces F-1(AFS_PDB) can inhibit infection of rice tissues by Rhizoctonia solani.For example,in the bioassay on rice leaf segments in vitro,the disease severity on rice leaf segments was 2.2 for the treatment of water(control),was 0.9 for the treatment of AFS_PDB. Hyphal growth by R.solani was significantly reduced in the presence of AFS of Streptomyces F-1 and the affected hyphae showed abnormal characteristics including swelling,crooking and collapsing. 4.Production and antifungal activity of AFS ofStreptomyces F-1 were studied.The results indicated that the mycelial biomass and AFS production by Streptomyces F-1 could reach the highest level after shake-incubation for 3 days in potato dextrose broth at 28℃and 150 rpm.The pH growth and AFS production by Streptomyces F-1 ranged from 5 to 8 with the optimum pH of 6-7.Streptomyces F-1 could utilize each of the 8 carbon sources investigated(sucrose,glucose,maltose,mannitol,lactose,mannitol,sorbitol and insitol) for growth,and glucose and maltose were the optimum carbon sources for AFS production.Streptomyces F-1 could utilize each of the 6 nitrogen sources investigated (ammonium sulfate,ammonium nitrate,urea,ammonium carbonate,sodium nitrate, potassium nitrate) for growth,and ammonium sulfate and ammonium nitrate were the optimum nitrogen sources for production of AFS.AFS produced by Streptomyces F-1 is sensitive to heat and chloroform;stable under the acidic condition and to proteinase K, and resistant to ultraviolet radiation.It could be salted out by ammonium sulfate.Besides R.solani,AFS of Streptomyces could inhibit Botrytis cinerea,Sclerotinia sclerotiorum, Pyricularia grisea and Colletorichum gloeosporioides.5.By the use of ammonium sulfate-mediated precipitation/salt dialysis and chromatographic separation with Sephadex G-100,an antifungal protein with molecular weight of about 33 kDa was purified.The water-washing extracts of the colonies,the mycelial extracts and the cultural filtrates of Streptomyces F-1 showed antifungal activity against R.solani.6.It was found that the volatile substances produced by Streptomyces F-1 could inhibit infection of rice leaves by R.solani,rapeseed leaves by S.sclerotiorum and strawberry fruits by Botrytis cinerea.Sixteen volatile compounds were tentatively identified from the volatile phase evaporated from the cultures of Streptomyces F-1 by the use of GC/MS.They could be chemically grouped into alcohols,esters,acids,alkanes, ketones and alkenes,including geosmins,an earthy-muddy-smelling compound and two antifungai compounds,phenylethyl alcohol and(+)-epi-bicyclesesquiphellandrene.