This durable type of immunity of a plant to parasites is called nonhost resistance. Although, in nature, nonhost resistance stops almost all parasite attacks, it has been the subject of little research. Scientists from the Max Planck Institute for Plant Breeding Research in Cologne, working with Volker Lipka, Jan Dittgen, and Paul Schulze-Lefert, and in co-operation with colleagues from the Carnegie Institution in the US, have uncovered the molecular components of nonhost resistance and described this system of defence in the current edition of the journal Science (November 18, 2005). In their findings, they draw parallels between the immune systems of plants and animals. This research could be central to the development of new "green" fungicides.
The Max Planck researchers were able to identify the gene known as PEN (penetration) as an important component of nonhost resistance. They isolated arabidopsis mutations, which are partially susceptible to powdery mildews. If these genes are defective, or if the protein they code is missing in the plant cells, the fungus can invade the leaf epidermis cells more frequently. For that reason the scientists looked particularly at the question of exactly which function the PEN2 protein has in the defence against pathogens.
PEN2 is an enzyme located in the membrane of what are called peroxisomes. These are spatially separated cell compartments, in which metabolic reactions often take place that would be dangerous for the organism at any place other than