The plant immune system is a two-branched innate immune system that recognizes molecules common to many microbes, including non-pathogens, and responds to pathogen virulence factors. This system provides insights into molecular recognition, cell biology, and evolution across biological kingdoms. Plants lack mobile defender cells and a somatic adaptive immune system, relying instead on innate immunity at the cellular level and systemic signals from infection sites. The system includes two branches: one using transmembrane pattern recognition receptors (PRRs) to detect microbial-associated molecular patterns (MAMPs/PAMPs), and another using NB-LRR proteins to recognize pathogen effectors. These proteins are named after their nucleotide binding (NB) and leucine-rich repeat (LRR) domains. NB-LRR proteins recognize effectors from diverse kingdoms and activate similar defense responses. They are effective against obligate biotrophs and hemi-biotrophic pathogens but not necrotrophs. The plant immune system can be represented as a four-phase "zigzag" model, where phase 1 involves PAMP-triggered immunity (PTI), phase 2 involves effector-triggered susceptibility (ETS), phase 3 involves effector-triggered immunity (ETI), and phase 4 involves pathogen adaptation to avoid ETI. ETI is a faster and stronger version of PTI that often results in hypersensitive cell death (HR). The system also includes indirect and direct recognition of pathogen effectors by NB-LRR proteins, which can be activated by pathogen-induced modified self. The plant immune system is influenced by various factors, including the evolution of NB-LRR proteins and pathogen effectors, and the balance between different plant hormones. Understanding the plant immune system is crucial for crop improvement in food, fiber, and biofuels production.The plant immune system is a two-branched innate immune system that recognizes molecules common to many microbes, including non-pathogens, and responds to pathogen virulence factors. This system provides insights into molecular recognition, cell biology, and evolution across biological kingdoms. Plants lack mobile defender cells and a somatic adaptive immune system, relying instead on innate immunity at the cellular level and systemic signals from infection sites. The system includes two branches: one using transmembrane pattern recognition receptors (PRRs) to detect microbial-associated molecular patterns (MAMPs/PAMPs), and another using NB-LRR proteins to recognize pathogen effectors. These proteins are named after their nucleotide binding (NB) and leucine-rich repeat (LRR) domains. NB-LRR proteins recognize effectors from diverse kingdoms and activate similar defense responses. They are effective against obligate biotrophs and hemi-biotrophic pathogens but not necrotrophs. The plant immune system can be represented as a four-phase "zigzag" model, where phase 1 involves PAMP-triggered immunity (PTI), phase 2 involves effector-triggered susceptibility (ETS), phase 3 involves effector-triggered immunity (ETI), and phase 4 involves pathogen adaptation to avoid ETI. ETI is a faster and stronger version of PTI that often results in hypersensitive cell death (HR). The system also includes indirect and direct recognition of pathogen effectors by NB-LRR proteins, which can be activated by pathogen-induced modified self. The plant immune system is influenced by various factors, including the evolution of NB-LRR proteins and pathogen effectors, and the balance between different plant hormones. Understanding the plant immune system is crucial for crop improvement in food, fiber, and biofuels production.