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Plants interact with their external environment constantly during their growth. Microorganisms,though invisible to the naked eye, are ubiquitous, filling both the air and the soil. Since most plants remain permanently attached to the soil throughout their lives, they have an inseparable relationship with the surrounding microorganisms. Some microorganisms can help plants absorb nutrients, increase their disease resistance, and even help them interact with other organisms; on
the other hand, some microorganisms can harm plants, causing disease or death.There are many types of pathogens that can infect plants, including viruses, bacteria, fungi, and nematodes. Different pathogens may invade through different routes. Like animals, plants have an innate immune system, a self-defense mechanism that activates to fight off external pathogens when they are detected. The plant immune system has two kinds of defense, one on the cell surface and one inside the cell, which are activated by immune receptors that detect pathogen molecules to defend against them. These are PRRs (Pattern Recognition Receptors) and NLRs(Nucleotide-binding domain Leucine-rich repeat receptors). The first , PRRs, is located on the cell membrane and recognizes molecules (PAMPs, Pathogen-Associated Molecular Patterns) on the surface of pathogens. When PRRs bind to PAMPs, they trigger an immune response to resist pathogen invasion. The second line of defense, NLRs, is located inside the cell and activates an immune response after detecting effector proteins secreted by pathogens into the cell.
(文章參考自：解開植物禦敵的秘密：免疫系統的調控與應用_https://research.ipmb.sinica.edu.tw/regulation-and-application-of-plant-immune-system-cn/)