How the plant immune system can drive the formation of n...( Plant geneticists know that not all...)

Plant geneticists know that not all plants from the same species can be successfully bred. Apparently, there are reproductive barriers that not only prevent the exchange of genes between well-established species (which goes to the very definition of a species), but also between varieties of one and the same species. How these barriers arise is of central importance if one wants to understand the origin of biodiversity. A research team led by Detlef Weigel from the Max Planck Institute of Developmental Biology in Germany and Jeff Dangl from the University of North Carolina has now shown that a mis-regulated plant immune system can establish reproductive barriers and might be a first step toward speciation.

The international collaboration studied a genetic incompatibility known as hybrid necrosis, using thale cress, Arabidopsis thaliana. The new study, reported in the latest edition of the open-access journal PLoS Biology, was based on the observation that unfit hybrids arising in different plant species are very similar. Their growth is retarded, the leaves become yellow and necrotic, the tissue collapses, and they often do not survive to make flowers; the syndrome is generally known as hybrid necrosis. We suspected that hybrid necrosis is always caused by the same biochemical mechanism, explains Weigel, director at the Max Planck Institute.

To test this hypothesis, the scientists took 280 genetically different strains of Arabidopsis from all over the world, which they crossed in 861 different combinations. Most of the hybrid plants were strong and grew normally, but 20or two percentof the crosses produced only small necrotic and unhealthy plants. Genomics-based experiments showed that these hybrids all had a comparable profile of gene activity: A common group of some 1000 genes were either more strongly or more weakly active in the hybrids than in their healthy parents. Moreover, this pattern was very similar to what is seen with a stron
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