Definition - What does Immune System mean?
In horticulture, an immune system is what plants use to fight off diseases and pathogens. Plants utilize a two-tier immune system. The first tier is responsible for recognizing and responding to pathogens, while the second tier responds to virulence factors of the pathogens in question (either directly or indirectly).
A plant’s two-tier immune system works in different ways. One type of receptor recognizes microbes once they have penetrated cell walls, while the other type of receptor recognizes microbes while they are outside the cell walls. Both receptors will then activate antimicrobial cellular defenses within the damaged or endangered cells, and those cells nearby.
MaximumYield explains Immune System
Like all other living creatures, plants are susceptible to microbes and have developed an immune system to deal with invaders. The terms "disease resistance" and "disease tolerance" are used to describe two different reactions happening in the plant’s immune system.
Disease resistance is an indication that the plant’s immune system has been successfully able to reduce the level of invading microbes within its tissues, mitigating the damage done to its tissues by the pathogen.
Disease tolerance is an indication that the plant is able to survive, and even thrive, with potentially high levels of the pathogen still within its system.
Resistance can be seen as a similar reaction to the human immune system, in which antibodies are created to kill pathogens, while tolerance is more or less a situation in which the pathogen is present, but does little to no damage to the plant even without an immune system response.
A number of mechanisms are used by bacteria, fungi, and insects that invade plant cells. For instance, aphids are capable of penetrating plant cells directly for feeding. Fungi and bacteria can enter through water pores, or through wounds on the plant, compromising cells.
Because plants lack a mammalian-like immune system, a great deal of effort has been spent in breeding plants for resistance to specific diseases. This has resulted in hardier plants that can withstand pathogenic invaders. However, it should be noted that the most successful cultivars are adapted for the most common diseases in specific geographic regions, rather than all types of threats. If you intend to purchase commercial plants for your garden, it’s recommended that you buy through a provider that offers region-specific disease resistant cultivars.