Fig. 1. FERONIA is required for elicitor release in the ccr1-3 mutant.

(A) Simplified model of the signaling hypothesis. Altered lignin is sensed by a cell wall receptor kinase to activate wall remodeling enzymes that solubilize pectin to release elicitor-active oligosaccharides that are sensed by other receptor kinases to activate defense responses. Lignin in ccr1-3 has reductions in G and S units (see Fig. 2, A and B). Colored circles represent pectin units, with galacturonic acid in blue. The three classes of pectins are rhamnogalacturonan II (left), homogalacturonan (middle), and rhamnogalacturonan I (right). (B) Strategy for identification of the two types of receptors in (A). ccr1-3 and cell wall receptor kinase mutants were crossed to screen for suppressors of ccr1-mediated PR expression in inflorescence stems. CWEs from stems were tested for the presence of elicitors by injection into WT Col-0 leaves and subsequent measurement of PR transcripts. (C) Vegetative phenotypes of Col-0 and the ccr1, fer-4, and ccr1/fer-4 mutants. Scale bar, 10 cm. (D) PR1, PR2, and PR5 transcript levels in inflorescence stems of ccr1-3, fer-4, and ccr1-3/fer-4 relative to Col-0. Transcript levels in Col-0 were set as 1. (E) PR1 transcript levels in Col-0 leaves after injection with CWEs from Col-0, ccr1-3, or ccr1-3/fer-4 (0.1 mg/ml uronic acid equivalents). Water and PGA (0.1 mg/ml) were injected as negative and positive controls, respectively. (F) PR1 transcript levels in Col-0 and fer-4 leaves after injection with CWEs from Col-0 or ccr1-3, with water and PGA as controls. Transcript level of the water control was set as 1. Bars represent means ± SD. n = 3 (D), n = 6 (E), and n = 4 (F). Hollow dots represent individual data points. Letters and asterisks indicate statistically significant differences according to Student’s t test, ***P < 0.001, in (E) and (F) and one-way ANOVA followed by Tukey’s honestly significant difference (HSD) test, α = 0.05, in (D).