Hypersensitive response

The hypersensitive response (HR) is a mechanism, used by plants, to prevent the spread of infection by microbial pathogens. The HR is characterized by the rapid death of cells in the local region surrounding an infection. The HR serves to restrict the growth and spread of pathogens to other parts of the plant. The HR is analogous to the innate immune system found in animals, and commonly precedes a slower systemic (whole plant) response, which ultimately leads to systemic acquired resistance (SAR).

The HR is triggered by the plant when it recognizes a pathogen. The identification of a pathogen typically occurs when avirulence geneproducts, secreted by a pathogen, bind to, or indirectly interact with the product of a plant resistance (R) gene (gene for gene model). R genes are highly polymorphic, and many plants produce several different types of R gene products, enabling them to recognize virulence products produced by many different pathogens.

In phase one of the HR, the activation of R genes triggers an ion flux, involving an efflux of hydroxide and potassium outside the cells, and an influx of calcium and hydrogen ions into the cell.

In phase two, the cells involved in the HR generate an oxidative burst by producing reactive oxygen species (ROS), superoxide anions, hydrogen peroxide, hydroxyl radicals and nitrous oxide. These compounds affect cellular membrane function, in part by inducing lipid peroxidation and by causing lipid damage.

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