Pathogen avirulence and plant resistance: a key role for recognition

During evolution, planes have developed defence mechanisms to combat pathogenic intruders such as viruses, bacteria and fungi. One of the most commonly encountered phenomena associated with such defence is the hypersensitive response, which involves genetically programmed local death of a few plant cells at the site of invasion - this keeps the pathogen isolated from the rest; of the plant. The hypersensitive response is frequently controlled by a gene-for-gene (avirulence gene-for-resistance gene) interaction. It has long been hypothesized that pathogen avirulence genes encode 'antigenic' molecules, called elicitors, that are recognized by extra- or intra-cellular receptor molecules encoded by the matching resistance genes. There is now experimental evidence that some of these molecules physically interact and subsequently cause an array of defence responses, including the hypersensitive response.