| Turnip mosaic virus (TuMV) and Cucumber mosaic virus (CMV) are two worldwide spread plant RNA viruses, whose damages to host plants are utmost serious. Up to now, some progress on pathogenic mechanisms and means of management for plant viruses have been achieved. But the ideal ways to control the most common viral diseases, such as mosaic syndrome caused by CMV infection are still unrealized. Although the inclusion characteristics of Potyvirus in host cells have been well discribed, the pathogenesis of TuMV is not yet fully understood. The present research is aimed to investigate the mechanism of inhibition to hosts' photosynthesis by TuMV infection and to prevent CMV infection by attenuated strains.Influences on host plant cell pathology by TuMV infectionTuMV particles were scattered in cytoplasm area of diseased cells separately or inbundles. The pinwheels, scrolls and laminated aggregates, which were the cross sections of cylindrical inclusion bodies, were observed under transmission electron microscope. Meanwhile, pathological changes of diseased chloroplasts' morphology and structure took place.Influences on chloroplasts' structure and chlorophyll concentration of host plants by TuMV infectionHost plants began to take on systemic mosaic symptoms in the 15th day after having been inoculated with TuMV JC-1. Meanwhile, the chloroplasts' structure and chlorophyll content showed varieties, e.g. the chloroplast cubage became smaller and the volume ofaccumulated starch granules decreased. The total volume of starch granules in per unit chloroplast volume of Chinese cabbage and mustard decreased by 80.1% and 64.6% while their number in per unit volume of chloroplast increased by 2.43 and 1.67 times, respectively. The result implied the average cubage of every starch granule was smaller than the control free of TuMV infection. Moreover, chlorophyll content in Chinese cabbage leaves decreased by 30.3% and that in mustard leaves by 25.5%.Inhibition on photosynthesis of host plants by TuMV infectionAfter Chinese cabbage and mustard having been infected with TuMV, theirphotosynthetic abilities were restrained. Their respective net photosynthetic rates descended by 61.0% and 39.9%, and the difference between the infected and the healthy was significant at 1% level. The leaf stomatal conductance of the infected plants dropped by 79.6% and 29.4%, the intercellular CO2 concentrations by 22.3% and 2.5%. The electron transport rates of chloroplasts were depressed after Chinese cabbage and mustard had been infected with TuMV, whose extents reached 6.4% and 64.6% respectively. The decline of photosystem II (PSII) activities mainly contributed to the drop of electron transport rate, whose degrees reached 56.9% and 34.3%, with the statistical result P<0.01. In addition, each photosystem I (PSI) activity dropped by 25.8% and 24.7%. At the tenth day after the two host plants having been inoculated with TuMV, all of their Fv/Fo, Fv/Fm, Fs, OPSII and q? values became lessened, especially the Fv/Fo, Fv/Fm of Chinese cabbage and OPSII of mustard, which decreased significantly at 1% level. At the same time, the qN values of them were bigger than the according controls. The above results showed that TuMV infection would stress the hosts and do greater harm to PSn than PSI.The detection of coat proteins in host chloroplasts infected with TuMVThe intact chloroplasts were isolated, the proteins attached to the chloroplast surfacesdigested, then the total proteins of chloroplasts extracted. The coat protein was detected in both chloroplasts of Chinese cabbage and mustard infected with TuMV through Western blot. The cytochemical localization with immuno-gold probe also indicated that coat protein did exist in chloroplasts of the two host plants. The conclusion can be drawn thatthe coat proteins will intrude into chloroplasts with TuMV infection, which leads to the modification of chloroplast structure, inhibition of photosynthesis and appearance of mosaic symptoms on leaves.T...
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