| Methyl jasmonate (MeJA) is a signal molecule that plants respond to external injuries and pathogenic bacterium infection and induce expression of resistance gene. Plant epicuticular wax is believed to be the initial defense system to various pathogens. In plant defense mechanisms, however, whether or how wax deposition in Brassica napus is involved in the resistant mechanism induced by MeJA is still unclear. S.sclerotiorum is one of the most important factors severely influencing the yield and quality of Brassica napus in the world. In the current experiment, two B. napus cultivars, resistant cultivar Zhongshuang9and susceptible cultivar Yuyou19, were sprayed with MeJA in their seedling stage. The contents and constituents and the crystal structure of leaf epicuticular wax were analyzed using GC method and scanning electron microscope. The contents of NO and H2O2in leaves were also measured. Antimicrobial activity experiments in vitro were conducted using extracted raw wax and MeJA. The leaves treated with MeJA were inoculated with S. sclerotiorum to analyze the disease resistace and antioxidant enzyme activity of B. napus. The main results were shown as follows:1. MeJA induction increased the contents of total wax, aldehydes, alkanes, secondary alcohols, and ketones in Zhongshuang9; MeJA treament had no significant influence on the contents of total wax and wax constituents in Yuyou19.2. The results of scanning electron microscopy SEM) showed that MeJA treatment significantly influenced the structures of wax crystalloid, particularly on the adaxial leaf surface compared to abaxial leaf surface. The numbers of rod and granular crystalloid reduced and the wax crystalloid fused, covering parts of the stomata under wax. The wax melting of Zhongshuang9was greater than that of Yuyou19, with plate crystalloids stacked, which enlarged the wax coverage area.3. The content of NO and H2O2changed insignificantly after treated with MeJA for both cultivars (Zhongshuang9and Yuyou19), indicating that the disease resistance by MeJA was not improved through increasing the synthesis of cell NO and induction of H2O2burst.4. MeJA induced the resistance of B.napus to S.sclerotiorurn. The inducted resistance effects by MeJA reached60.41%and72.60%for Zhongshuang9and Yuyou19, respectively. The activities of POD and PAL in plants treated with both S.sclerotiorurn inoculation and MeJA were significantly higher than those with inoculation only and control plants. However, no significant difference was observed24h after inoculation for Zhongshuang9and72h after inoculation for Yuyou19.5. The results of the antimicrobial activity experiments in vitro showed that the growth of S. sclerotiorurn mycelia was not inhibited by100μmol L-1MeJA. However, the mycelium growth was significantly inhibited when the concentration of MeJA reached300μmol L-1MeJA, and the inhibition effects increased with increased MeJA concentration.6. The antimicrobial effect of extracted wax on growth of S. sclerotiorurn mycelia was also conducted in vitro. The results showed that extracted wax had great inhibition effect on the growth of mycelia of S. sclerotiorum at the early stage. The inhibition effects of epicuticular wax in Zhongshuang9were positively correlated with the content of epicuticular wax; while no direct correlationship was observed between the content of epicuticular wax and inhibition effect in Yuyou19.In conclusion, MeJA induction alleviated the disease severity of B. napus inoculated with S.sclerotiorum through inhibiting mycelia growth, inducing an increase of antioxidant enzyme activities, and changing crystalloid structure of epicuticular wax. For Zhongshuang9, both the epicuticular wax constituents and crystalloid structure were involved in inducing resistance to S.sclerotiorum; For Yuyou19, only the crystalloid structure might be involved in inducing resistance to S. sclerotiorum. |
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