Dual Activities Of Serratia Marcescens Pt-3 In P-Solubilizing And Plant Pathogen Control

Phosphorus?P?plays crucial roles in plant growth and development.However,available P to plants is very insufficient in soil,which causes poor crop yield worldwide.The application of industrial fertilizers is a useful solution to overcome this problem.Nevertheless,their continued application induces long-term destructive effects on soil.Additionally,many fungal pathogens that exist in soil can severely damage crop growth and cause severe soil deterioration.Thus,alleviating the deficiency of P in soil and control of fungal pathogens is an urgent global requirement for sustainable food security.Microorganisms as eco-friendly agents in soil have been proved efficient P solubilizing and disease control agents,which can significantly solve the above problem.In the present study,we isolated microbes from rhizospheric soil of a more than 100-year-old tea garden and screened them for the dual activity of P-solubilizing and fungal growth inhibition.For P-solubilization screening,893 bacteria were obtained and cultured in solid medium containing mineral phosphate Ca₃?PO₄?₂ and organic phosphate soy lecithin,respectively.Finally,five strains showed validity in solubilizing organic phosphate,and nine strains could solubilize the mineral form.Among these bacteria,Pt-3 showed valid activity in solubilizing both the organic and mineral phosphate.In solid organic phosphate medium,Pt-3 formed clearing zones of 2.1 cm in diameter,and a clearing zone of 1.8 cm in the mineral phosphate medium.Further,Pt-3 also exhibited efficient P-solubilizing activity in liquid medium and soil conditions.The amount of dissolved phosphate reached 350.9 mg.l^-1 and 4.0 mg.kg^-1 in 24 days,which was significantly higher than 25.0 mg.l^-1and 3.2 mg.kg^-1 for control,respectively.Polymerase chain reaction further revealed that Pt-3P-solubilizing activity relied on gluconic acid production.16S rRNA phylogenetic tree analysis proved that strain Pt-3 was greatly homologous to Serratia marcescens,and was in the same clade with Serratia marcescens ZC06-1?KT438729.1?,Serratia marcescens AG2105?KY 379049.1?,Serratia marcescens SFC20131227?KY992555.1?.Pt-3 also showed the same reaction with S.marcescens strains in biochemical and physiological analyses,including carbon utilization,NaCl tolerance,pH tolerance and antibiotics resistance.Therefore,the strain was identified as Serratia marcescens,Pt-3.In a 30 day maize planting practical,Pt-3 significantly promoted the growth of maize seedlings with leaf number,shoot height,leaf,and root length of maize plants treated with Pt-3 reaching5 leaves per plant,14.00,9.55,and 4.95 cm,respectively,better than that of control at 4leaves per plant,9.40,6.20 and 4.78 cm respectively.Additionally,Pt-3 also exhibited broad-spectrum antifungal activity against seven selected plant pathogens,including Aspergillus flavus,Fusarium graminearum,Alternaria alternata,Magnaporthe oryzae,Aspergillus fumigatus,Colletotrichum graminicola and Botrytis cinerea via face-to-face dual cultural method.Pt-3 inhibition rates on these pathogens ranged from 75.51%to 100%.When active charcoal was introduced into the tests,the antifungal activity of Pt-3 was significantly reduced.In the analysis of antifungal mechanism,A.flavus inoculated on peanut seeds and challenged with Pt-3 showed severe A.flavus structure damage,shrunk conidia,and could not germinate into hyphae as seen under scanning electron microscopy.In contrast,A.flavus not treated with Pt-3 germinated into hyphae and grew normally.Gas chromatography mass spectrometry revealed that disulfide dimethyl was the predominant organic substance in the Pt-3 volatile profile.Thus,based on these findings,we concluded that strain Pt-3 has multiple biological functions:robust phosphorus solubilization,broad-spectrum of antifungal property as well as proficient plant growth-promoting activity,which will provide the critical material for the production of bioactive fertilizers.