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Plants have evolved multiple defense mechanisms to prevent and combat infections caused by bacteria, fungi, viruses, and other pathogens. These defenses can be classified into physical barriers, chemical defenses, and immune responses.
1. Physical Barriers
Plants use structural features to block pathogen entry:
- Cuticle: A waxy layer on leaves and stems that prevents microbial invasion.
- Cell Walls: Made of cellulose, lignin, and pectin, providing strength against pathogens.
- Bark: Acts as a protective shield, preventing pathogens from penetrating stems.
- Stomatal Closure: Stomata (tiny pores on leaves) can close when pathogens try to enter.
2. Chemical Defenses
Plants produce antimicrobial compounds to kill or inhibit pathogens:
- Phytoalexins: Antimicrobial chemicals released in response to infection (e.g., resveratrol in grapes).
- Pathogenesis-Related (PR) Proteins: Enzymes that break down bacterial and fungal cell walls.
- Toxins and Alkaloids: Some plants produce toxic chemicals like nicotine or caffeine to deter pathogens.
3. Immune System & Signaling Responses
Plants have two main immune responses:
A. Basal Immunity (First Line of Defense)
- Pattern-Triggered Immunity (PTI):
- Plants recognize Microbe-Associated Molecular Patterns (MAMPs) like bacterial flagella.
- This triggers defense responses, such as cell wall strengthening and production of antimicrobial compounds.
B. Specific Immunity (Second Line of Defense)
- Effector-Triggered Immunity (ETI):
- Some pathogens release effectors to suppress PTI.
- Plants have Resistance (R) proteins that detect these effectors and activate strong immune responses.
- Leads to Hypersensitive Response (HR)—infected cells undergo programmed cell death to stop the spread of infection.
4. Systemic Acquired Resistance (SAR)
If a plant is attacked in one area, it can "warn" the rest of the plant:
- Salicylic Acid (SA) Signaling: Activates immune responses throughout the plant.
- Jasmonic Acid (JA) & Ethylene (ET) Pathways: Trigger defenses against herbivores and necrotrophic pathogens.
5. Symbiotic Defense
- Beneficial Microbes: Some plants form relationships with helpful bacteria and fungi that outcompete pathogens (e.g., mycorrhizal fungi).
6. Wound Healing & Regeneration
- Callose Deposition: When wounded, plants deposit callose, a carbohydrate that blocks pathogen entry.
- Lignification: Strengthens damaged areas to prevent further infections.
Conclusion
Plants use multiple layers of defense—from physical barriers to complex immune signaling—to prevent infections and survive in challenging environments. Scientists study these defenses to improve crop resistance and sustainable agriculture.
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Potomac River has been the site of several notable aviation accidents:
Eastern Air Lines Flight 537 (November 1, 1949): A mid-air collision between an Eastern Air Lines passenger aircraft and a P-38 Lightning military plane resulted in both aircraft crashing into the Potomac River. All 55 passengers on the Eastern Air Lines flight perished, with the sole survivor being the Bolivian pilot of the P-38.
Capital Airlines Flight 500 (December 12, 1949): A Douglas DC-3 operated by Capital Airlines stalled and crashed into the Potomac River while on approach to Washington National Airport. Six of the 23 passengers and crew on board were killed.
Air Florida Flight 90 (January 13, 1982): A Boeing 737-200 crashed into the 14th Street Bridge over the Potomac River shortly after takeoff from Washington National Airport. The aircraft struck the bridge, hitting seven occupied vehicles, before plunging into the river. Out of the 79 people on the plane, only five survived. Additionally, four motorists on the bridge were killed.
American Eagle Flight 5342 (January 29, 2025): A Bombardier CRJ700 regional jet collided with a U.S. Army Sikorsky H-60 Black Hawk helicopter over the Potomac River near Ronald Reagan Washington National Airport. The collision resulted in the deaths of all 64 individuals aboard the passenger jet and the three crew members of the helicopter.
These incidents highlight the challenges and risks associated with aviation operations in the vicinity of the Potomac River and Washington National Airport.
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