Inhibitory Mechanism And Application Of Lauric Acid And Its Derivatives To Phytophthora Sojae

Phytophthora sojae belongs to Oomycota and is a kind of harmful eukaryotic pathogenic microorganism.Phytophthora sojae only infects a few leguminous plants such as soybean under natural conditions.Phytophthora sojae can harm the roots,stems,leaves and pods of plants.Soybean rhizome rot caused by Phytophthora sojae causes huge losses to soybean industry every year,and is a devastating disease that threatens soybean production and even global food security.With the continuous development of economic and trade globalization and the increasing demand for imported soybeans in China,soybean Phytophthora disease has appeared in some areas of China,which seriously threatens the development of soybean industry in China.In order to control oomycetes disease,the extensive use of chemically synthesized pesticides not only seriously disrupts the ecological balance,but also leads to the emergence of drug-resistant strains,so there is an urgent need to screen new alternatives to control.As an important precursor of green fungicides,phyto-derived antimicrobial substances widely exist in nature and are much better than chemical pesticides in protecting ecological balance.Some plant leaves produce and release volatile organic compounds(VOCs)quickly in response to stress,and these plant chemicals can stimulate a defensive response to protect against pathogen invasion.Therefore,identification and analysis of antimicrobial compounds produced by plants,and further development and modification,so as to make them into commercial new green pesticides,are urgent research subjects at present.Our previous study found that lauric acid,a volatile substance,was detected in the leaves of soybean disease-resistant variety(Williams 82).Compared with the susceptible soybean variety(Williams),the content of lauric acid was significantly increased.It was speculated that lauric acid might play an important role in soybean resistance to Phytophthae soya infection.At the same time,the hydrophilic and lipophilic characteristics of fatty acids may affect their antibacterial activity.In order to test this hypothesis,we selected lauric acid and its three derivatives(glyceryl monolurate,methyl laurate and ethyl laurate)to study the inhibitory mechanism against Phytophthophytes,and to determine whether the antibacterial activity of its precursor(lauric acid)was retained or lost.In this study,four physiological species R2(P6497),R7(P7064),R17(P7074)and R19(P7076)were tested with different concentrations of lauric acid for the radial growth of mycelia.The results showed that lauric acid significantly inhibited the growth of mycelia of four species of Phytophthora sojae,and with the increase of lauric acid concentration,the inhibition effect became more obvious.Among them,lauric acid had the best inhibition effect on radial growth of mycelia of R2 species.On the medium containing different concentrations of lauric acid and its derivatives,it was found that the inhibition effect of lauric acid on the radial growth of R2 mycelia was better with the increase of the concentration of lauric acid and its derivatives,and the inhibition effect of lauric acid on the growth of Phytophthora R2 mycelia was stronger than the other three derivatives.The mycelia of Phytophthora R2 treated with lauric acid were observed under light microscope.Compared with the mycelia of the control group,the mycelia treated with lauric acid showed disorganized growth,significantly increased terminal branches,and significantly inhibited the formation of zoosporangium and release of zoospores.Glyceryl monolaurate had better inhibitory effect on the formation of zoosporangium and the release of zoosporium than lauric acid,methyl laurate and ethyl laurate.The effects of lauric acid and its derivatives on the germination of zoospores showed that lauric acid could significantly inhibit the germination of a large number of zoospores at the concentration of 2 μg/ m L.It was also found that lauric acid had a good inhibiting time to Phytophthora sojae.Lauric acid also had a good inhibitory effect on five soilborne pathogens,including Fusarium oxysporum f.cucumbers,Fusarium oxysporum tomatae,Fusarium oxysporum flavescens,Fusarium oxysporum wheat,and Fusarium oxysporum wheat.Since lauric acid has the ability to merge with cell membrane,it is speculated that lauric acid will cause cell membrane damage and intracellular material leakage of mycelium cells in Phytophthora sp.The mycelium treated with lauric acid was stained with PI,and observed under fluorescence microscope,it was found that the nucleus in the mycelium produced red fluorescence,indicating that the cell membrane was seriously damaged,which proved that lauric acid achieved the antibacterial effect by damaging the cell membrane.The concentrations of DNA,protein and electrical conductivity of mycelia treated with lauric acid and its derivatives were significantly higher than those of the control group.At the same time,compared with glyceryl monolurate,methyl laurate and ethyl laurate,lauric acid can effectively reduce the content of malondialdehyde in plants,so as to reduce the damage degree of soybean plants.The activity of superoxide dismutase(SOD)in plants treated with lauric acid reached a higher level,thus enhancing soybean resistance.The potted control experiment of Phytophthora showed that lauric acid and its derivatives could effectively inhibit the pathogen in soil,and had a good control effect on soybean root rot,and could effectively reduce the incidence of soybean disease.In this experiment,the inhibitory effect of lauric acid on oomycetes was found for the first time and its mechanism was preliminarily analyzed.It is expected to be a precursor of new fungicides and provide new methods and ideas for comprehensive management of soybean Phytophtophella disease.