Revealing the Molecular Network of Pattern-Triggered Immunity (PTI) Signal Transduction

Pattern-triggered immunity (PTI), the first line of defense in plants, is activated when host pattern recognition receptors (PRRs) detect pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs) released during microbial invasion. Over the past decade, major advances have clarified PTI’s complex signaling network, revealing its role as a dynamic regulator of plant immune responses. This review summarizes recent progress in PTI signal transduction, emphasizing the core mechanisms: (1) kinase cascades, including MAPKs and calcium-dependent protein kinases (CDPKs), that amplify defense signals; (2) calcium ion (Ca2+) dynamics acting as rapid secondary messengers; and (3) reactive oxygen species (ROS) bursts that mediate antimicrobial and signaling. The roles of lipid signaling, crosstalk between immune signaling and plant hormones, and scaffold proteins are also discussed. Furthermore, we explore the interplay between PTI and effector-triggered immunity (ETI), highlighting synergistic modules, such as the enhanced disease susceptibility 1–phytoalexin deficient 4–activated disease resistance 1 (EDS1-PAD4-ADR1) which integrate immune signals to enhance pathogen resistance. By linking molecular mechanisms to systemic immune regulation, this review provides a comprehensive framework for understanding plant defense strategies and identifies emerging opportunities to engineer disease-resistant crops through modulation of immune pathways.