What are the main mechanisms of plant disease resistance?

Plants have a sophisticated, multi-layered immune system.These defenses can be broadly divided into pre-formed (passive) barriers and induced (active) immune responses.

A. Pre-formed (Constitutive) Defenses

These are the plant's "first line of defense," always present regardless of a pathogen attack.

• Physical Barriers: The waxy cuticle on the leaf surface prevents water from pooling (which fungi need to germinate) and is difficult for pathogens to penetrate. The rigid cell wall (made of cellulose, lignin, and suberin) is a formidable physical barrier.
• Chemical Barriers: Some plants store pre-formed antimicrobial compounds (like saponins or phenols) in their cells, which are released when the cell is damaged by a pathogen.

B. Induced (Active) Defenses

If a pathogen breaches the pre-formed defenses, the plant activates a two-tiered immune system.

Tier 1: PAMP-Triggered Immunity (PTI)This is the general-purpose immune system that recognizes broad categories of microbes.

1. Recognition: Plant cells have receptors on their surface called Pattern Recognition Receptors (PRRs).
2. Detection: These PRRs detect common, conserved molecules found in many microbes, known as Pathogen-Associated Molecular Patterns (PAMPs). Examples include flagellin (a protein from bacterial flagella) or chitin (a component of fungal cell walls).
3. Response: This detection triggers a broad, relatively weak defense response. This includes strengthening the cell wall, producing antimicrobial chemicals, and generating Reactive Oxygen Species (ROS)—toxic molecules that can damage the pathogen.

Tier 2: Effector-Triggered Immunity (ETI)This is a highly specific and powerful immune response.Pathogens have evolved to defeat PTI by injecting "effector" proteins into the plant cell to suppress its defenses. ETI is the plant's counter-move.

1. Recognition: Inside the plant cell, specialized proteins called Resistance (R) proteins are on patrol.
2. Detection: These R proteins are designed to detect specific pathogen effectors (also called Avirulence, or Avr, proteins).This detection can be direct (R protein binds to the effector) or indirect (R protein monitors a plant protein that the effector targets).
3. Response: This "gene-for-gene" recognition triggers a very strong, rapid defense.The hallmark of ETI is the Hypersensitive Response (HR), where the plant cells surrounding the infection site deliberately commit suicide. This "scorched earth" tactic traps the pathogen, preventing its spread to the rest of the plant.

This local HR also triggers a plant-wide "alarm" called Systemic Acquired Resistance (SAR), which pre-activates the defenses in distant, uninfected leaves, making them more resistant to future attacks.