Figure 1.

Network of RALF–FER Signaling in Plant Immunity.

FER and LLGs respond to RALF23, which is cleaved by S1P, and modulate plant immunity, whereas LRXs mediate RALF signaling to maintain cell growth. RALF23-FER-LLGs destabilize the formation of the FLS2–BAK1 complex. Fusarium oxysporum secretes F-RALF peptides that hijack the RALF–FER pathway, leading to the phosphorylation of F. oxysporum MAPK FMK1 and potentially facilitating host infection, together with the suppression of PDF1.2 expression. FER modulates auxin, abscisic acid (ABA), ethylene (ET), Ca²⁺, reactive oxygen species (ROS), MAPK phosphorylation, and pH to affect cell growth and immunity. FER interacts with and phosphorylates RIPK, modulating AHA2 activity. RIPK phosphorylates RIN4, which is detected by RPM1 and RPS2, triggering immune responses. ANX1 also inhibits ETI responses by inhibiting RPM1 and RPS2. PBL13, a homolog of RIPK, interacts with RBOHD to regulate NADPH oxidase-induced ROS production. FER mediates RALF1-induced mRNA alternative splicing via GRP7, which also interacts with FLS2, EFR, and their transcripts. Positive regulatory actions are indicated by arrows, negative regulatory actions are indicated by bars, physical interactions between different proteins are indicated by solid lines and potential physical interactions are indicated by dashedotted lines. Question marks denote mechanisms that remain to be empirically demonstrated.