page_title: Hypersensitive response
section_title: Genetics
hierarchical_section_title: Hypersensitive response / Genetics
caption_attribution_description: English: This diagram represents what happens after a virulence factor of a pathogen is sensed by an NLR protein.
context_page_description: Hypersensitive response is a mechanism used by plants to prevent the spread of infection by microbial pathogens. HR is characterized by the rapid death of cells in the local region surrounding an infection and it serves to restrict the growth and spread of pathogens to other parts of the plant. It is analogous to the innate immune system found in animals, and commonly precedes a slower systemic response, which ultimately leads to systemic acquired resistance. HR can be observed in the vast majority of plant species and is induced by a wide range of plant pathogens such as oomycetes, viruses, fungi and even insects.
  HR is commonly thought of as an effective defence strategy against biotrophic plant pathogens, which require living tissue to gain nutrients. In the case of necrotrophic pathogens, HR might even be beneficial to the pathogen, as they require dead plant cells to obtain nutrients. The situation becomes complicated when considering pathogens such as Phytophthora infestans which at the initial stages of the infection act as biotrophs but later switch to a necrotrophic lifestyle.
context_section_description: The first idea of how the hypersensitive response occurs came from Harold Henry Flor's gene-for-gene model. He postulated that for every resistance (R) gene encoded by the plant, there is a corresponding avirulence (Avr) gene encoded by the microbe. The plant is resistant to the pathogen if both the Avr and R genes are present during the plant-pathogen interaction. The genes that are involved in the plant-pathogen interactions tend to evolve at a very rapid rate. 
  Very often, the resistance mediated by R genes is due to them inducing HR, which leads to apoptosis. Most plant R genes encode NOD-like receptor (NLR) proteins. NLR protein domain architecture consists of an NB-ARC domain which is a nucleotide-binding domain, responsible for conformational changes associated with the activation of the NLR protein. In the inactive form, the NB-ARC domain is bound to Adenosine diphosphate (ADP). When a pathogen is sensed, the ADP is exchanged for Adenosine triphosphate (ATP) and this induces a conformational change in the NLR protein, which results in HR. At the N-terminus, the NLR either has a Toll-Interleukin receptor (TIR) domain (also found in mammalian toll-like receptors) or a coiled-coil (CC) motif. Both TIR and CC domains are implicated in causing cell death during HR. The C-terminus of the NLRs consists of a leucine-rich repeat (LRR) motif, which is involved in sensing the pathogen virulence factors.
caption: Mechanism of plant NLR protein activation after pathogen invasion
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