Plants are exposed to many different pathogens in the environment. Only a few of these pathogens, however, are able to attack a species of plant and "make it sick". If a particular pathogen is unable to attack a plant, that means that the plant is resistant to it -- in other words, it cannot host the pathogen.
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 inside the compartments. If a fungus tries to invade a plant cell, the peroxisomes are led over to the entry site by the attached PEN2 protein. One or more sugar molecules can be separated from another cell component through the enzyme activities of the PEN2 enzyme, a glycosyl hydrolase. The substance released by it appears to have a fungicidal effect, which kills the pathogen.
The researchers, on the other hand, observed that when PEN2 is missing, the plants become more susceptible not only to grass powdery mildew fungi but also other pests -- for example, the pathogens causing late potato blight. PEN2 is therefore a basic component of the plant's immune system with a broad range of effects.
However if PEN2 is missing, the plant is not completely helpless against fungal diseases. There is still another line of defence which they have to get through. If PEN2 is missing, the plant takes a drastic step: the cell dies together with its attacker, which protects the neighbouring plant tissue from infection.
In this deadly line of defence, very different proteins play a key role -- particularly EDS1, PAD4 and SAG101. They were already known to researchers in other species of plants, which identify molecular traits only present in parasites by using immune receptors both on the cell surface and inside the cell. Only if this second mechanism also fails can the originally non-virulent grass powdery mildew fungus colonise the plant.
The Max Planck research has now demonstrated that the nonhost resistance of plants develops out of a defence system with at least two steps. These steps determine whether a plant is susceptible to a disease or not. The redundancy of the defence layers and the wide-ranging effects of PEN2 explain why, in nature, nonhost resistance is a durable and broadly effective defence mechanism. If a building block is missing from one defence layer, its function will be taken over by components of the next layer.
Until now, scientists had assumed that nonhost resistance is based more on "passive" mechanisms: for example, the structure of the cell wall, poisonous substances on the surface of the plant, or a lack of molecular entry sites for pathogens. But the researchers in Cologne have now shown that active immune responses make a key contribution to nonhost resistance -- for example, the transport of PEN2 to the place of infection.
In further studies, the researchers hope to try to identify materials that are built up via PEN2 at the place of infection. They surmise that these materials could lead to the development of new kinds of "green fungicide" with a broad range of effects in the fight against plant diseases.
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