Plants have a sophisticated immune system to protect against pathogens. They use pattern recognition receptors to detect conserved pathogen molecules, triggering initial defenses. If pathogens bypass this, plants employ resistance genes to recognize specific pathogen proteins and mount a stronger response.

Plants defend themselves through various mechanisms. These include the hypersensitive response, which involves localized cell death, and cell wall reinforcement to create physical barriers. Plants also produce antimicrobial compounds and phytoalexins to directly combat invading pathogens.

Pathogen Recognition and Immune Responses

Pattern Recognition Receptors and PAMPs

• Plants have evolved pattern recognition receptors (PRRs) to detect conserved pathogen-associated molecular patterns (PAMPs) and trigger immune responses
• PAMPs consist of essential microbial structures such as bacterial flagellin, lipopolysaccharides, and fungal chitin
• PRRs are typically receptor-like kinases or receptor-like proteins located on the plant cell surface
• Examples of well-characterized PRRs include FLS2 (recognizes bacterial flagellin) and CERK1 (recognizes fungal chitin)
• Recognition of PAMPs by PRRs activates PAMP-triggered immunity (PTI), the first layer of plant defense

Effector-Triggered Immunity and R Genes

• Pathogens secrete effector proteins to suppress PTI and promote virulence
• Plants have evolved resistance (R) genes that encode intracellular nucleotide-binding leucine-rich repeat (NB-LRR) proteins to recognize specific pathogen effectors
• Recognition of effectors by R proteins activates effector-triggered immunity (ETI), a more rapid and robust defense response compared to PTI
• ETI often leads to a hypersensitive response, characterized by localized cell death at the infection site to limit pathogen spread
• Examples of R genes include RPM1 (recognizes Pseudomonas syringae effector AvrRpm1) and N (recognizes tobacco mosaic virus replicase)

Defense Mechanisms

Hypersensitive Response and Cell Wall Reinforcement

• The hypersensitive response is a form of programmed cell death that occurs at the site of pathogen infection to limit pathogen spread
• It involves the rapid accumulation of reactive oxygen species, activation of defense genes, and localized cell death
• Plants reinforce their cell walls by depositing callose, lignin, and other structural proteins to create a physical barrier against pathogen invasion
• Callose deposition is triggered by the activation of callose synthase enzymes and can occlude plasmodesmata to restrict pathogen movement between cells

Antimicrobial Compounds and Phytoalexins

• Plants produce a diverse array of antimicrobial compounds to defend against pathogens
• These include pathogenesis-related (PR) proteins such as chitinases and glucanases that degrade fungal cell walls
• Plants also synthesize small molecule antimicrobial compounds called phytoalexins in response to pathogen attack
• Phytoalexins are low molecular weight, lipophilic compounds that accumulate rapidly at the infection site and exhibit broad-spectrum antimicrobial activity
• Examples of phytoalexins include camalexin (Arabidopsis), resveratrol (grapes), and glyceollin (soybeans)