This is Amazing - How Plants Defend Themselves Against Pathogens

How Plants Defend Themselves Against Pathogens
Plants have their own unique defense mechanisms in order to protect themselves against a variety of pathogens. This Buzzle article educates you about the defense system that is put in place by plants to protect themselves from these foreign invaders.
Did you know?
The essential oils that make up various spices, condiments, perfumes and seasonings, are in fact toxins produced by plants to repel various insects and pathogens. These substances have a lesser effect on humans, and hence, we are able to use them.
Plants form the base of the food chain. They are the primary source of food and nutrients for many organisms. Hence, they are susceptible to attack from a wide range of pathogens, like viruses, bacteria, fungi, insects, pests, and many other organisms. Even other living organisms and environmental factors are known to affect and cause stress to plants. Hence, to protect themselves from these factors, plants have defense tactics of their own.
This article enlists the defensive measures that plants possess, which resist the entry of these pathogens, and protects them from various infections and diseases.
Passive Defense Mechanisms
Passive Defense Mechanisms
The defenses that exist in the plant structure that serve as a primary defense against antigens are the passive defense mechanisms. It is the presence of the structural features in the plant body that act as a primary defense. The plant cells have cell walls that defend the entry of pathogens.
The outer covering of the plant, like the thick bark, cuticles, thorns, prickles or hairlike structures, lignin, etc., prevent the entry of pathogens. These tough tissues are hard to digest for the infiltrating fungus or bacteria, and hence, penetrating the plant's surface becomes difficult for them.

Also, some substance secreted by the plant like tannins, alkaloids, phenols, etc., act as poison to keep away pests and pathogens. Certain proteins present in the plants have antimicrobial or antifungal properties that repel the pathogens and prevent them from attacking them.
Active Defense Mechanisms
Active Defense Mechanisms
The new immune response that is freshly produced after the infiltration by the pathogens is called active defense mechanism.

First and foremost, the cell wall undergoes many changes as a defense mechanism to oppose the attack by the pathogens. The thickness of the cell wall increases, by the addition of new materials that makes the incursion difficult to the pathogens.
Another interesting phenomenon which occurs is the hypersensitive response (HR). In this response, the cells around the infected site undergo a change and die. They start to accumulate toxins. Their respiration slows down or stops, and their metabolic activities undergo a change.

This response alerts the entire plant about the infection, and rings the alarm bells. These cells also start producing certain new chemicals called phytoalexins, as a reflex action to the attack. This response is called the systemic acquired resistance (SAR), and is produced only under stress or in threatening situations for the plant. It cuts off the supply of essential nutrients and water needed by the infectious pathogens, and usually stops the infection.

The plant may produce some antimicrobial proteins such as defensins, or enzymes like chitinases or peroxidases, that defend the plant against the pathogenic attack.

Plants form symbiotic relationships with many other bacteria and fungi, that protect them from other harmful pathogens. These are called endophytes. These endophytes colonize the plant tissue and create an obstruction to the other harmful pathogens. They can also produce some chemicals which curb the growth of these microorganisms, and thus, act as a defense mechanism for the plant.
Chemical Defenses
There are various types of chemicals that assist plants in different metabolic activities. Primary metabolites are those chemicals produced by plant cells that assist the plant in growth, reproduction, and development. These include sugars, amino acids, proteins, and nucleic acids.
There are secondary metabolites secreted by the plant which are involved in the defense mechanisms. These are usually classified under any of the three major classes of chemical compounds, namely terpenoids, phenolics, and alkaloids.
Terpenoids
Terpenoids
These are the largest class of secondary metabolites consisting of over 22,000 compounds that defend the plants from pathogens. The essential oils that constitute various spices like black pepper, cinnamon, thyme, etc., are produced to repel insects and pathogens. Cotton produces a terpenoid gossypol, that has strong properties which inhibit the growth of bacteria and fungi.
Phenolics
Phenolics
These are the other class of secondary metabolites which are produced by plants to defend themselves against pathogens. Phytoalexins, mentioned above, belong to the phenolic class of compounds, and have strong antifungal and antibacterial properties. Lignin also belongs to this class, which due to its insoluble and rigid nature, creates a barrier against any pathogen attack.
Alkaloids
Alkaloids
These are nitrogen-containing compounds that are bitter in taste. Caffeine present in coffee, tea and cocoa, belong to this class, and are toxic to both insects and fungi. Nicotine in tobacco and capsaicin in chili pepper are alkaloids that are produced to keep pathogens away.
The plant immune system is complicated, and studies are still being done to find out more about this amazing protective mechanism. Plants, being the primary producers, are more vulnerable to attack, and hence, it's important to learn the ways they protect themselves against pathogens.