| Mango bacterial leaf spot is one of the most important diseases of mango, caused by Xanthomonas campestris pv.mangiferaeindicae, resulting abnormal decomposition on infected leaves and fruit tissue. Also, there have a significant cross-crack black spot on the surface of the infected tissue. In recent years, incidence of this disease showed a rising trend in some major mango growth regions. Efforts have long been made by researchers on this issue, most were about etiology, infection cycle and control methods, but the diseases still can’t be control effectively. One of the main reasons is that the study of the pathogenesis of pathogens remains unknown. The objective of the study was to explore the cell wall degradation enzymes, toxins and extracellular polysaccharide produced by pathogens and their pathogenic effect were studied during the disease occuring. The study aims to speculate epidemic regularity of the disease, providing the basic theory of evidence for effective disease control.(1) The3,5-dinitrosalicylic acid method (DNS method) was used to determine the activity of cell wall degrading enzymes(CWDEs) during pathogens infection. The results show that CWDEs were produced in improving Marcus’s medium, as well as in the infected leaves. Among six CWDEs, carboxymethyl cellulase(Cx) is the highest, followed by polygalacturonase (PG), pectin methylgalactuionase (PMG) and β-glucosidase, polygalacturonic acid transeliminase (PGTE) and pectinmethyltrans-eliminase(PMTE). However, all these six kinds of enzymes increased significantly in their activity after the invasion of pathogen. In the early stage of invasion (0to4th), β-glucosidase and PMG reached their maximum, Cx, PG, PMTE and PGTE reached its peak in later stage of4th to14th. However, the control remained a lower level in all time. In order to optimize the reaction conditions for enzyme activity of the pathogen, some chemicals were introduced to take a further test of cell wall degrading enzymes activity from the pathogens. It was found that1.0%sodium carboxymethyl cellulose (CMC) was the most suitable substrates for induction of Cx, whereas1.0%pectin for the induction of PG and PMG. In regarding to reaction temperature, it was found that60℃was perfect for Cx, and50℃for PG. In regarding to reaction time, it was found that60min was more suitable for Cx, whereas20min was more suitable for PG In regarding to pH value, the result indicated that the enzyme activities of Cx and PG were the highest when the pH value reached4.6and6.6, respectively.(2) With the method of extraction and rotating evaporation, the filtrate liquid was extracted in Watanabe culture. The assay of biological activity of the liquid showed it could induce the symptoms of mango bacterial leaf spot, and could inhibit rice seed germination. The optimum conditions of producing toxin were as followed:Watanabe filtrate culture, pH value7to8,24℃,120h. The toxin was not a kind of protein substance with thermostability by the determination of basic properties of crude toxin. In order to figure out the physiological and biochemical indexes of the crude toxin, mango leaves were inoculated by crude toxin. The results show the activities of peroxidase (POD), polyphenoloxidase (PPO), cate-lase (CAT) and phenylalanineammonialyase (PAL) were increased in the inoculated leaves. And the soluble protein and sugar content were significantly increased, while the content of chlorophyll decreased. The results indicated the crude toxin is harmful to mango tissue.(3) With the method of High Performance Liquid Chromatography (HPLC), we got the white powder purification of extracellular polysaccharide which was determined to be the lactose. When it was inoculated in the mango leaves, the symptoms showed the same as the inoculation by the pathogen.The results above show the cell wall degradation enzymes, toxins and extracellular polysaccharide were the important pathogenic factors, which play the important role for diseases in invasion, degradation, toxin, and further destroying the plant tissue. |
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