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Are self-replicating DNA elements helping plants adapt?
By Lone Frank

Researchers suggest that self-replicating regions of DNA can help plants adapt to their local environment.

In a new study just published in the Proceedings of the National Academy of Sciences USA, Alan H. Schulman, of the University of Helsinki's Institute of Biotechnology, and his team have investigated the BARE-1 retrotransposon, a small stretch of DNA that inserts itself into the plant genome, which is found in wild barley, Hordeum spontaneum. BARE-1 has the potential to replicate and multiply in the cell. The research team suggests that creating more copies of BARE-1 is part of the plant's response to climatic stress.

"In the barley genus as a whole, the BARE-1 copy number seems to vary with the climate, and we wanted to test if this could be seen with a very small geographical area," explains Schulman whose testing ground was Israel's Evolution Canyon. Here the slopes differ dramatically with respect to microclimate; the north side represents Eurasian conditions, while the south is Afro-Asian. The higher the site the more sun and the less water. Sampling in different locations revealed that on average BARE-1 makes up 3 percent of the barley genome, but the copy number varies by 3-fold between individual plants and increases with aridity and exposure to sun. "This indicates that selection on the level of the plant is operating to increase the number of BARE-1 elements," says University of Georgia plant geneticist Susan Wessler, who applauds the study for being the first to identify a molecular mechanism for genome enlargement.

But what could be in it for the plant? For reasons unknown, bulking up on DNA seems to alter cell physiology and increase cell size. And one hypothesis states that cell expansion is an efficient way for plants in the Mediterranean basin to cope with the climate. According to Schulman, this means that "plants growing in drier sites would experience a selection pressure to develop larger cells." Which fits nicely with the observation that in barley a larger genome was correlated with a higher BARE-1 copy number and with a dry and sunny growth site. To substantiate their field study the researchers are now going back to the lab to investigate directly if climatic stress causes BARE-1 to replicate inside the cell. "It is still speculative whether we are dealing with an adaptive mechanism and how it could be adaptive," says Wessler. "But this group's work is important in raising the bar on investigating the whole area of genome dynamics."

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Kalendar, R., Tanskanen, J., Immonen, S., Nevo, E. & Schulman, A.H. Genome evolution of wild barley (Hordeum spontaneum) by BARE-1 retrotransposon dynamics in response to sharp microclimatic divergence. Proc Natl Acad Sci USA 97, 6603-6607 (June 6, 2000).

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