Max Planck Gesellschaft researchers in Cologne, Germany demonstrate that a multi-step defence system underlies the durable resistance of plants to fungal parasites.

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.

Stanford, California

Source: The Carnegie Institution

Genetic defenders protect crops from fungal disease

Like waves of soldiers guarding a castle gate, multiple genetic defenders cooperate to protect plant cells against powdery mildew disease, according to a new study. Powdery mildew is a common fungal infection in plants that attacks more than 9,000 species, including many crops such as barley and wheat, and horticultural plants such as roses and cucumbers. The researchers, including Shauna Somerville and Mónica Stein of the Carnegie Institution's Department of Plant Biology, are the first to document how these defense genes team up in plants. The discovery could help combat fungal parasites that devastate crops and cost growers billions of dollars in pesticides every year.

The study, published in the November 18 issue of the journal Science, describes powdery mildew infection in the mustard relative Arabidopsis thaliana. Each species of mildew is host-specific, meaning it can infect some plant species, but not others. By disabling protective genes in Arabidopsis, the researchers were able to infect the plants with species of powdery mildew that normally attack peas or barley, revealing much about how plants use genes to fight infection.

"Most plants are resistant to the majority of pathogens they encounter, but the basis for this resistance was unknown," Somerville said. "Identifying these genes has provided us with the first insight into how plants defend against multiple pathogens."

Once a powdery mildew infection takes hold, it covers the plant with fuzzy splotches, while sapping precious nutrients. At the cellular level, the fungal spores invade healthy plant cells and form root-like feeding structures called haustoria. The plant cell wall is the primary barrier to this invasion and one of the defense genes described in the current study, called PEN2, prevents the fungus from penetrating cell walls in the first place.

If this first line of defense breaks down, as it does in about 5 to 25 percent of normal Arabidopsis plants (depending on the mildew species), a second set of genes jumps into the fray. These genes, called EDS1, PAD4, and SAG101, work together in a complex inside the cell, and can signal infected cells to die. By sacrificing these fallen cells, the defense genes can spare healthy ones from infection.

Somerville, Stein, and colleagues at the Max Planck Institute for Plant Breeding in Köln disabled the protective genes in Arabidopsis by introducing mutations, one at a time and in various combinations. They infected these mutants with one of two species of powdery mildew: Blumeria graminis hordei, a species that attacks barley, and Erysiphe pisi, one that thrives on the leaves and pods of pea plants.

"Disabling just three genes allowed the pea powdery mildew to reproduce as well on Arabidopsis as it does on its normal host," Somerville remarked. "Thus, the resistance barriers limiting the growth of inappropriate pathogens are much less complex than expected, relying on just a limited number of genes."

The EDS1, PAD4, and SAG101 gene complex's ability to signal cell death is relatively well known to scientists. However, very little is known about how PEN2 behaves in the cell. The current study demonstrates that the PEN2 protein is a catabolic enzyme--a protein that breaks down other molecules--though its specific target remains unknown.

The study expands on the researchers' previous work with a gene called PEN1. As its name suggests, PEN1 and PEN2 seem to share a common purpose. However, they seem to act via different mechanisms, and PEN2 protects against a wider range of fungal pathogens. For example, Arabidopsis plants with a disabled PEN2 gene are also more susceptible to Phytopthora infestans, the fungus responsible for the notorious Irish Potato Famine of the mid-19th century.

"The resistance mechanisms operating at the cell wall seem to be surprisingly simple," Somerville said. "This suggests it might be possible to reverse engineer crops like wheat with Arabidopsis PEN genes to help control powdery mildew and other destructive diseases, thus minimizing the need for pesticides."

The Carnegie Institution of Washington has been a pioneering force in basic scientific research since 1902. It is a private, nonprofit organization with six research departments throughout the U.S. Carnegie scientists are leaders in plant biology, developmental biology, astronomy, materials science, global ecology, and Earth and planetary science. The Department of Plant Biology is located at 260 Panama St., Stanford, CA 94305.