Study On Agricultural Antifungal Activity Of Syzygium Infrarubiginosum Extracts

The research on biological activity and mechanism of natural products is a challenging task in the development of green pesticides.Hainan Island is a tropical region with abundant plant resources.It is an ideal plant treasure house for exploring and discovering green pesticides.In this paper,by evaluating the agricultural biological activity of tropical plant extracts,the Syzygium infrarubiginosum extract and its active components with high antifungal activity were investigated,and the preliminary antifungal mechanism of active compound was explored.The main research contents were as follows:（1）Screening of plant extracts for agricultural biological activity.The methanol extracts of 40 kinds of plants collected from Changjiang and Wenchang areas of Hainan Province were evaluated on antifungal activities,it was found that methanol extracts of S.infrarubiginosum had broad-spectrum antifungal activity,and the inhibition rates against seven plant fungi were more than 65%.（2）Bioassay-guided isolation of active antifungal compounds.Three active compounds were isolated from the petroleum ether section.Antifungal activity studies showed that compounds 1-3 have broad-spectrum antifungal activity against seven plant pathogen fungi(EC₅₀ values were 4.89～16.84μg m L^-1,6.52～17.65μg m L^-1,6.88～14.90μg m L^-1,respectively),and compound 1 had the most significant antifungal effect on C.musae(EC₅₀=4.89μgm L^-1).（3）In vivo antifungal effects of S.infrarubiginosum extracts and compound 1-3.The in vivo antifungal experiments against cowpea powdery mildew（Podosphaera xanthii）showed that both petroleum ether（PE）extract exhibited highly effective activity against powdery mildew diseases at concentrations of 6.25μg m L^-1,12.5μg m L^-1,25μg m L^-1,and the antifungal activities were 62.4%,78.7%,97.8%,respectively,which were significantly higher than that of osthole and propiconazole.At the same time,compound 1-3（94.6%）showed similar inhibitory effect with that of propiconazole（90.3%）at 25μg m L^-1.At the concentration of 100μg m L^-1,control effects of PE extract on C.musae and mango postharvest disease were 82.3%and91.2%,respectively,which were better than azoxystrobin.Furthermore,field experiment results showed that the 5%microemulsion of methanol extracts had obvious control effects on cucumber powdery mildew,and the control effects were70.8%and 80.7%after 6 days and 14 days,respectively.（4）Inhibiton effects of compound 1 on spore germination of C.musae.The results showed that compound 1 had significant inhibitory effects on spore germination,with an inhibition rate of 95.4%at 10μg m L^-1.（5）Effects of compound 1 on C.musae mycelium and cellular structure.It was found that C.musae mycelia treated with compound 1 became shrunk,deformed and bent observed by optical microscope,at the same time,mycelium cell wall became significantly thinner and large vacuoles were present in the cytoplasm observed by transmission electron microscopy,indicating that compound 1 may destroy the integrity and rigid structure of the cell wall of C.musae mycelium.（6）Effects of compound 1 on the cell membrane permeability of C.musae.The results showed that with the increase of time treated with compound 1(5μg m L^-1and10μg m L^-1),permeability of C.musae mycelium rised gradually,and the relative seepage ratios in treatment group were 45.2%and 45.5%respectively at 9 h,while permeability increased slower and had no obvious changes in the control group,so compound 1 might destroy the cytoderm and plasma membrane integrity of C.musae,and led to the leakage of cytoplasm contents.（7）Effects of compound 1 on the biochemical index of C.musae.The experimental results showed that compound 1 had no obvious effect on the contents of soluble protein and reducing sugar,but it had a significant effect on the mycelial chitosan content and chitinase activity,and the changes were more than 40%.Therefore,compound 1 significantly affected chitin synthesis and decomposition of C.musae mycelium,leading to the hyphal tip growth and branching,cell wall divisions and other morphological changes.（7）Effects on C.musae cell membrane.Compared with the control treatment,the relative seepage ratios were much higher in the groups treated with compound 1 and the damages increased with the increase of treatment time,and the respiratory suppression rates of the treatment group reached more than 80%at 7 h,and the inhibition effect was more obvious than that of azoxystrobin,along with the significant change of pyruvic acid contents（40.2%）.In the above experiments,the biological activity of the extract and compound 1-3of S.infrarubiginosum and the antibacterial mechanism of compound 1 against C.musae were investigated from two aspects of biological activity and mechanism of action.The results showed that the extract and compound 1-3 had good inhibitory effects on a variety of plant diseases,and compound 1 had the most obvious effects on the chitin and chitinase of the cell membrane of C.musae,and also affected the energy metabolism process of the target bacteria,leading to cell apoptosis,so as to achieve the antibacterial effect.