Rd21a mutants exhibit susceptibility to pathogens and reduced levels of immunity. A) Phenotype of Col-0 and rd21a at 21 d after P. brassicae inoculation. Bar: 1 cm. B) Disease index of Col-0 and rd21a. The percentages of plants in the individual disease classes are shown; Col-0 and rd21a used 35 to 40 plants for analysis. C, D) Quantification of P. brassicae content in diseased roots of Arabidopsis by qPCR, and the vertical coordinates indicate the multiplicity of P. brassicae content in different lines relative to Col-0. The relative biomass of P. brassicae is represented by the content of the P. brassicae ACTIN gene relative to that of the Arabidopsis ACTIN2 gene C) and the content of the P. brassicae DC1 gene relative to that of the Arabidopsis GAPDH gene D). n = 3 biological replicates. A significant difference was determined by Student's t-test (2-tailed). Different letters (a and b) indicate significant differences. Data are shown as mean ± Sd. E) Chitin-induced ROS burst is suppressed in rd21a mutant. The values represent means ± Se. n = 16 biological replicates. F) Induction of salicylate pathway genes was compromised in rd21a mutant. Leaves of 4-wk-old plants were infiltrated with 20-μg/mL chitin for 24 h. Data were normalized to ACTIN2 expression in qPCR analysis. Statistical analysis was performed by 1-way ANOVA with Kruskal–Wallis test (significance set at P ≤ 0.05). Different letters (a, b, and c) indicate significant differences. n = 3 biological replicates; data are shown as mean ± Sd. Experiments in Fig. 6 were repeated 3 times with similar results.