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Paranoid but Popular
Mutant mouse-ear cress offers insight into natural plant resistance
By Lone Frank

Featured Article.

A paranoid temperament is in the genes—at least when it comes to plants. This is the lesson from studying a dwarf mutant of Arabidopsis thaliana, or mouse-ear cress, which behaves as if it were under constant attack from microbes. Investigating this mutant, a team of Danish researchers and their American and English collaborators have pinpointed a gene with a crucial role in regulating natural resistance against disease. Their findings open a view into the enigmatic immune system of plants and point a way to creating more robust crops.

Wildtype Arabidopsis and the MPK4 knock out.

"Globally, there is obviously significant interest in protecting crops against disease. And a deeper understanding of the genetic basis for resistance will be instrumental in developing and breeding better plants," says plant molecular biologist John Mundy of Copenhagen University's Institute of Molecular Biology who led the Arabidopsis study. Recently published in Cell, the team's major finding is the identification of the gene map kinase 4 (MPK4) as a regulator of resistance to a range of pathogens including bacteria, viruses or fungi. MPK4 controls what is known as systemic acquired resistance, or SAR, which occurs as a response to invasion by microbes. During SAR, plants react by producing and circulating salicylic acid, which acts as an internal signal to trigger expression of a range of anti-microbial proteins. The result is broad and long-lasting immunity. It now turns out that the activity of MPK4 is normally required to suppress SAR, indicating that MPK4 activity must be turned off in the face of attack to allow the development of SAR.

"It is very surprising that the 'inactivation' of a map kinase is used to turn on a plant resistance mechanism," says plant biochemist Dierk Scheel of the Institute of Plant Biochemistry in Halle, Germany. And according to plant geneticist Jérôme Giraudat of the Institute of Plant Sciences in Gif-sur-Yvette, France, "this counter intuitive observation adds important new knowledge of protein kinase signaling." While the new study is a major step forward for basic science, molecular biologist Morten Petersen of Copenhagen University stresses the significant potential for practical application. "Our findings could allow geneticists and plant breeders to purposefully push the right buttons for activating the natural defense mechanisms and turning on plant resistance," he says.

‘It is very surprising that the 'inactivation' of a map kinase is used to turn on a plant resistance mechanism’

Applying the new knowledge of MPK4 could take several directions. "One obvious route is to create transgenic plants in which MPK4 is permanently shut off just as in the paranoid Arabidopsis mutant," says Giraudat. Such inherently resistant plants would not only hold the promise of decreasing agriculture's pesticide load but could prove especially attractive for poor farmers in developing countries who do not have access to pesticides. Peter Brodersen of Copenhagen University points to another interesting avenue, which would appeal to consumers who are wary of genetically manipulated crops and food. "If we could identify harmless chemical compounds able to transiently turn off MPK4 activity, they could be used to spray crops and make them resistant at any time they face a microbial attack."

Originally, the team didn't set out to elucidate the intricacies of plant immunity. Rather, their project is a perfect illustration of how the broad approach of functional genomics, which generally aims at assigning biological function to genetic sequences, can lead anywhere. "One very effective way to identify genes that play essential roles in the normal organism is to knock them out. Then you can get at their function by analyzing the biological effects on the mutant," explains Mundy. His group enlisted nature's own genomic vandalizers—known as transposons—for creating a series of Arabidopsis genetic knock outs. These small, mobile and virus-like DNA elements have been a part of plant genomes for millions of years and most of the time they lie dormant and do no harm. However, under certain natural or experimental conditions they become active in the replicating DNA of forming seeds. This way they wreak havoc in the genome of the next generation by randomly inserting themselves.

"When you observe a phenotype that differs markedly from the wild type, you have hit a functionally important gene," explains Petersen who was immediately alerted by the dwarf Arabidopsis. Subsequent genetic analysis revealed that a transposon had disrupted the MPK4 gene, preventing it from being transcribed. Using a specialized DNA array containing thousands of Arabidopsis genes (an EST microarray), the researchers then analyzed gene expression in their mutant and quickly found that MPK4 somehow controls the expression of anti-microbial genes of the SAR response. "This underscores the power of having access to genetic sequences," says Scheel. He and others in the field applaud that Arabidopsis, which serves as the lab rat and model of choice in plant science, has recently become the first plant to have its genome fully sequenced.

While the Arabidopsis sequence may speed up plant science in general, the new MPK4 study will play into the hands of researchers interested in map kinase signaling, says Giraudat. "By providing the first-ever plant map kinase mutant, it really opens up possibilities for genetic studies of the signaling pathways." And certainly, the interest in map kinases is exploding. These proteins are found on every branch of the evolutionary tree from yeast to humans, and they play crucial roles in controlling the delivery of molecular messages inside cells. The insight into map kinase function in plants is lagging behind, but Scheel predicts it will be of major importance. "Plants have a large number of map kinase cascades suggesting that their signaling networks are very complex," he says. Map kinases are involved in different signaling chains responding to a wide range of environmental stimuli and stresses, and a detailed understanding of the cross talk between them is a prerequisite for accurate manipulation of plant traits. Adds Mundy, "clearly, we must know exactly which processes we affect when we start tinkering with living systems."

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Petersen, M. et al. Arabidopsis MAP kinase 4 negatively regulates systemic acquired resistance. Cell 103, 1111-1120 (December 22, 2000).
Salanoubat, M. et al. Sequence and analysis of chromosome 3 of the plant Arabidopsis thaliana. Nature 408, 820-822 (December 14, 2000).
Tabata, S. et al. Sequence and analysis of chromosome 5 of the plant Arabidopsis thaliana. Nature 408, 823-826 (December 14, 2000).

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