Comparison of pectic enzymes in resistant and susceptible mints infected by Verticillium dahliae Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ns064891v

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  • Verticillium dahliae Kleb. incites a vascular wilt disease of mints. Susceptible Mentha piperita L. , resistant M. crispa L., and intermediate hybrid 148 were used to study 1) the site of differential resistance and 2) the role of polygalacturonase (PG) in the disease syndrome. Many more V. dahliae propagules were recovered from surface sterilized, finely fragmented, susceptible mint roots than from resistant roots. Therefore, the vascular system of susceptible mint was more extensively invaded than resistant mint. Increase of Verticillium in the stems was dependent on the extent of root invasion; severity of foliar symptoms, in turn, was related to pathogen proliferation in stems. These results indicate that the roots are the first important site of differential resistance. Inoculation through excised shoots showed that mint stems differ in their resistance to Verticillium. Although Verticillium increased similarly in stems of shoot inoculated resistant and susceptible mints, severe symptoms developed only in susceptible M. piperita; resistant M. crispa developed only slight symptoms. PG activity was detected only by the reducing group assay in extracts from diseased mint prepared without a phenolic adsorbent, polyvinylpyrrolidone (PVP). In extracts prepared with PVP, PG was detected by both the viscosity reduction and reducing group assays. Enzyme extracts prepared late in disease when phenol oxidase activity was high showed considerable decrease in viscosity reducing power but not a decrease in reducing groups released from substrate. These results indicate that Verticillium produced an exo -PG in mint that was less affected by phenol oxidation products than the endo -PG. Increase of Verticillium and PG production was similar in shoot inoculated resistant and susceptible mints. Foliar symptoms developed only in susceptible M. piperita and intermediate resistant hybrid 148. No foliar symptoms occurred in resistant M. crispa. Resistance of mint to Verticillium apparently is not due to inactivation of fungal PG. PG production in mints during disease development showed no relationship between symptom severity and PG activity in infected mint stems. A nonpathogen of mint produced PG activity in mint similar to that produced by Verticillium but no wilt symptoms developed. Hypotheses regarding the contribution of PG to wilt symptoms depend on the formation of PGs in diseased plants that hydrolyze pectic substances to large fragments. Enzyme extracts prepared from plants as symptoms increased, liberated a high number of reducing groups from substrate while the viscosity reducing power decreased. This indicated that large pectic fragments are not formed in plants. It was proposed that the role of PG in the Verticillium wilt of mint disease is to provide an additional carbon source for the pathogen.
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