Immune system of plants

 The immune system of plants, also known as plant immunity or plant defense mechanisms, is a complex network of molecular and cellular processes that enable plants to defend themselves against various pathogens, pests, and environmental stresses. 

Unlike animals, plants lack specialized immune cells but have evolved sophisticated mechanisms to recognize and respond to threats.

One crucial aspect of plant immunity is the recognition of pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs) by pattern recognition receptors (PRRs) located on the plant cell surface. 

Upon recognition, PRR-mediated signaling pathways are activated, leading to the induction of defense responses, such as the production of antimicrobial compounds, reinforcement of cell walls, and activation of signaling cascades.

Another vital component of plant immunity involves the deployment of resistance (R) proteins, which specifically recognize pathogen effector molecules. 

Plants possess inherited genes known as resistance (R) genes, which play a crucial role in their defense against pathogens. 

These genes exhibit high levels of polymorphism and belong to a gene family with very similar sequences. 

They arise through mutations and errors in genetic recombination, leading to the formation of extra copies of the same gene or the creation of new ones. 

This process results in a wide range of alleles and, consequently, a greater diversity of protein products for recognizing infections.

This recognition triggers a robust immune response, often leading to localized cell death known as the hypersensitive response (HR), which restricts pathogen spread.

The gene-for-gene hypothesis was proposed by H. H. Flor, who discovered that the products of R genes form receptors that interact with pathogen gene products associated with virulence (avr). 

These protein products act as ligands that match with the receptor, enabling the immune system to recognize the intruder and trigger a hypersensitive response (HR). 

If a mutation produces a new ligand that lacks a specific receptor recognized by the immune system, the infection progresses, and the plant succumbs to it.

Plants also employ chemical defenses to combat pathogens and herbivores. 

These include the production of secondary metabolites, such as phytoalexins, terpenoids, and alkaloids, which have antimicrobial and deterrent properties.

Furthermore, plants can communicate internally and systemically to coordinate defense responses throughout their tissues. 

The HR sets off a series of events. Initially, the production of oxygenated compounds, known as reactive oxygen intermediates (ROIs), such as hydrogen peroxide and superoxide ions, occurs. 

These compounds thicken cell walls and destroy adjacent cells to contain the infection. Subsequently, antibacterial and antimicrobial compounds called phytoalexins act against the pathogen. 

Through a hormone called salicylic acid (SA), which travels from infected tissues to healthy ones, the systemic acquired response (SAR) is activated, preparing the root and shoot systems for infection.

This systemic acquired resistance (SAR) involves the transmission of signals, such as salicylic acid, jasmonic acid, and ethylene, to activate defense mechanisms in distant parts of the plant upon pathogen attack.

Additionally, beneficial microbes, such as rhizobacteria and mycorrhizal fungi, can establish symbiotic relationships with plants, enhancing their immunity through mechanisms like induced systemic resistance (ISR) and priming.

Overall, the immune system of plants is a multifaceted and dynamic defense network that enables them to survive and thrive in challenging environments by effectively recognizing and responding to diverse threats.