The plant defense system is a complex network of biochemical pathways that help plants to cope with various biotic and abiotic stresses, such as pathogens, herbivores, drought, salinity, and temperature. There is no single common pathway for plant defense, but rather multiple pathways that are activated depending on the type of stress and the plant species. However, some general features of plant defense pathways are:
They often involve the production of *secondary metabolites*, which are chemical compounds that are not essential for plant growth and development, but have various functions in plant defense, such as signaling, deterrence, toxicity, or attraction of beneficial organisms¹.
They often involve the generation of **reactive oxygen species (ROS)**, which are highly reactive molecules that can damage the pathogens or induce cell death in infected tissues. ROS also act as signaling molecules that trigger other defense responses².
They often involve the expression of *pathogenesis-related (PR) proteins*, which are proteins that have antimicrobial or antifungal activities, such as chitinases, glucanases, peroxidases, or defensins².
They often involve the activation of *mitogen-activated protein kinase (MAPK) cascades*, which are signaling pathways that transmit signals from the cell surface to the nucleus and regulate gene expression, enzyme activity, and cell division⁴.
They often involve the induction of *systemic acquired resistance (SAR)* or **induced systemic resistance (ISR)**, which are mechanisms that confer long-lasting and broad-spectrum resistance to the whole plant after a local infection by a pathogen. SAR and ISR involve different signaling molecules, such as salicylic acid, jasmonic acid, ethylene, or volatile organic compounds².
One example of a specific pathway for plant defense is the *phenylpropanoid pathway*, which is triggered by pathogens hitting the plant cell wall. This pathway produces a large variety of compounds based on the shikimate pathway intermediates, such as lignin, flavonoids, coumarins, stilbenes, and phytoalexins. These compounds have various roles in plant defense, such as strengthening the cell wall, scavenging ROS, inhibiting pathogen growth, or attracting beneficial organisms³.
I hope this answer helps you to understand the plant defense system better.
Source:
(1) How do plants defend themselves against pathogens ... - Springer. https://link.springer.com/article/10.1007/s12298-022-01146-y.
(2) 30.25: Plant Defense Mechanisms - Against Pathogens. https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_%28Boundless%29/30%3A_Plant_Form_and_Physiology/30.25%3A_Plant_Defense_Mechanisms_-_Against_Pathogens.
(3) An overview of plant resistance to plant-pathogenic bacteria. https://link.springer.com/article/10.1007/s40858-023-00560-1.
(4) Phenylpropanoid Pathway Engineering: An Emerging Approach towards Plant .... https://www.mdpi.com/2076-0817/9/4/312.
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