Tolerance to pathogen infection is associated with hypersensitive reaction. Discuss how the induced synthesis of pathogenesis-related proteins promote tolerance to pathogen infection
Induced synthesis of pathogenesis-related (PR) proteins is a key defense mechanism in plants against pathogen infection. These PR proteins are involved in various biochemical pathways, such as the production of antimicrobial compounds, signal transduction, and reinforcement of cell walls. While hypersensitive reaction (HR) is commonly associated with plant defense, it is important to note that tolerance to pathogen infection is a distinct phenomenon.
Tolerance refers to the ability of a plant to minimize the adverse effects of pathogen infection, often by reducing disease symptoms and maintaining normal physiological functions. It is different from resistance, where the plant actively prevents pathogen invasion or limits its spread. Tolerance is often achieved through physiological and biochemical adaptations that allow the plant to withstand the presence and detrimental impacts of pathogens, without necessarily eradicating them.
The induction of PR proteins plays a significant role in promoting tolerance to pathogen infection by aiding in these physiological and biochemical adaptations. Here are a few ways in which PR proteins contribute to tolerance:
1. Reinforcement of cell walls: PR proteins, such as PR-1, PR-2, and PR-3, are involved in strengthening the plant cell walls. These proteins contribute to the deposition of callose, lignin, and other compounds that reinforce the cell wall, making it harder for pathogens to penetrate and spread within the plant tissues. Strengthening the cell walls helps prevent tissue damage and limits the spread of the pathogen.
2. Antimicrobial activity: Some PR proteins exhibit antimicrobial activity against a broad range of pathogens. For example, PR-5 proteins, also known as thaumatin-like proteins, possess antifungal properties. PR proteins with antimicrobial activity directly inhibit the growth and development of pathogens, thereby reducing the severity of infection and promoting tolerance.
3. Signal transduction: PR proteins often participate in signal transduction pathways, enabling the plant to mount an effective defense response. These proteins can interact with other defense-related proteins and modulate the expression of genes involved in defense responses. By coordinating signaling pathways, PR proteins help regulate the plant's immune response to achieve a balance between defense and tolerance.
4. Defense priming: PR proteins can also play a role in defense priming, which refers to the pre-activation of defense responses in the absence of pathogens. When exposed to certain stress conditions or elicitors, plants can produce and accumulate higher levels of PR proteins. This pre-exposure primes the plant's defense mechanisms, making them more efficient and responsive upon encountering an actual pathogen attack. Priming allows the plant to mount a quicker and stronger defense response, leading to enhanced tolerance.
In summary, the induced synthesis of PR proteins is associated with both hypersensitive reaction and tolerance to pathogen infection. While HR is a rapid and localized defense response, tolerance involves a range of physiological and biochemical adaptations to minimize the adverse effects of pathogen attack. PR proteins promote tolerance by reinforcing cell walls, exhibiting antimicrobial activity, participating in signal transduction, and contributing to defense priming. These mechanisms collectively contribute to the plant's ability to withstand pathogen infection and maintain normal physiological functions.