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Leaves of two peach cultivars, Venture and BabyGold 5, with distinct degrees of resistance and susceptibility, respectively, were used for a comparative molecular analysis of defense responses to bacterial spot caused by ' Xanthomonas campestris' pv. 'pruni' ('Xcp'). After inoculation with 'Xcp', expression of five pathogenesis-related (PR) and five ethylene-response factor (ERF) genes was induced to a significantly higher degree in the resistant compared to the susceptible cultivar. There were two major phases of gene induction by 24 h post-inoculation. One was a rapid but transient induction of genes and the second was a late but sustained induction. These two phases may illustrate the immunity responses of epidermal and mesophyll cells, respectively. The induction of most defense-inducible genes by the exogenous application of MeJA or ethephon more than salicylic acid (SA) signified the positive role of jasmonic acid (JA) and ethylene (ET) signalling pathways in resistance to 'Xcp', which is consistent with the potential hemibiotrophic nature of this bacterium. JAZ proteins, which act as repressors for the JA signalling pathway, may interfere with defense against bacterial pathogens. While these proteins are degraded within a few minutes of JA treatment in ' Arabidopsis', JAZ1 from peach (Pp-JAZ1) was not degraded even after 1h of MeJA application. Overexpression of this gene in tobacco suppressed known JA-mediated activities, such as the inhibition of root-growth and the induction of JA-biosynthesis genes after wounding. 'Pp-JAZ1' overexpression also affected flower development and converted tobacco flowers to be cleistogamous rather than chasmogamous. Transgenic plants were more resistant than wild-type plants to the infection by the tobacco pathogenic bacterium, 'Pseudomonas syringae' pv. 'tabaci'. This resistance response is mediated partially through the regulation of JA to stomatal opening during bacterial infection. |
