Figure 4.

Different patterns of reactive oxygen species (ROS) accumulation are triggered in non‐host resistance (NHR) and the host defence response (HDR). (a) In situ accumulation of superoxide radicals (O₂ ^•−) detected by dark blue formazan precipitate in Citrus limon leaves inoculated by pressure infiltration with bacterial suspensions of Xanthomonas campestris pv. campestris (Xcc), X. fuscans ssp. aurantifolii (Xfa) strain C and X. citri ssp. citri (X. citri), plus mock. (b) Quantitative reverse transcription‐polymerase chain reaction analysis of copper/zinc superoxide dismutase (SOD2). mRNAs measured at 3 and 24 h post‐inoculation (hpi). Relative gene expression (ΔΔCt) fold change of mRNA levels was performed considering mock‐treated plants as reference sample and histone H4 transcript as an endogenous control. Values are expressed as means ± standard deviation (SD) from three independent biological replicates. Different letters indicate significant differences at P < 0.05 [two‐way analysis of variance (ANOVA), Tukey’s test]. (c) H₂O₂ accumulation at 3 and 24 hpi in C. limon leaves infiltrated with bacterial suspensions of the different Xanthomonas strains [10⁷ colony‐forming units (CFU)/mL]. Leaves were stained with 2′,7′‐dichlorofluorescein diacetate (DCFH‐DA) and observed by fluorescence microscopy. H₂O₂ accumulation in the guard cells is shown enlarged in the top insets. Scale bar, 100 μm. [Colour figure can be viewed at wileyonlinelibrary.com]