Science - April 28, 2010

Anniversary for Mouse-ear cress

It comes up through the cracks in the pavements on every street corner in the Netherlands: Arabidopsis thaliana, commonly known as mouse-ear cress or thale cress. And this year this weed celebrates its 25th anniversary as a model plant for plant scientists, whose research has produced a detailed knowledge of almost all the molecular processes in plants.

Arabidopsis thaliana
The date is somewhat arbitrary, admits Wageningen geneticist Maarten Koornneef. Together with his American colleague David Meinke, Koornhof published an overview of Arabidopsis's career as a model plant in Plant Journal last month. This career began 25 years ago.
Koornneef should know, because he was there from the start. In 1983, he published the first genetic map of mouse-ear cress, with 76 genes. 'It was hard to get the article published', he recalls. 'Arabidopsis was not 'in', because it had not fulfilled 1960s expectations of it as a model plant.'
But after a scientific conference in 1985, molecular biologists in the US began to work with Arabidopsis and top genetic researchers became interested in the plant. Then research funding became available in the US and the EU; 'and the rest is history.' Ten years ago, the complete genome of the mouse-ear cress was published - a first for a plant. An estimated 12,000 plant researchers are now doing research on this little plant. 'We have about 100,000 different mutations, in each of which one gene is switched on or off', says Koornneef. 'You can just order them for further research.' In short, we know more about the mouse-ear cress than about any other plant.
Plant sciences in Wageningen have benefited enormously from it, says Ton Bisseling, molecular biologist and director of the Experimental Plant Sciences (EPS) Research School. 'Forty to fifty percent of EPS's publications are about Arabidopsis. And another 30 percent are on subjects related to Arabidopsis - knowledge about Arabidopsis can often be applied to other crops. Thirty years ago, individual researchers looked for model plants for their own research questions. Everyone worked on their own. Now Arabidopsis is the link; the different disciplines adjust to the model system.'
 
What makes this plant so attractive?
Koornneef: 'It has a small genome, with just five chromosomes and 25,000 genes. The genome in other crops, like peas and barley, can be at least 30 times larger. But those other crops do not have more functional genes, they just have more non-coding DNA, also known as junk DNA. So the genome of A. Thaliana is very functional. Almost all the basic processes in Arabidopsis are ones that other plants have too. And Arabidopsis is self-fertilizing, which is an advantage when you want to study the way characteristics are passed on. The plant is small too, and has a short life cycle, which helps to keep research costs down. And lastly, it is handy that it's easy to introduce foreign DNA.'

What have we learned from the mouse-ear cress?
Koornneef: 'The way plant hormones function at molecular level, for example. They play a role in all the development processes in the plant. Our knowledge about biosynthesis and the working of these hormones at biochemical and molecular levels was limited. Now we know which processes almost every hormone is involved in, and how. On top of that, new plant hormones have been discovered through Arabidopsis. For example, the flowering of plants is regulated by florigen, a protein hormone that is transported from the leaf to the top of the plant. The gene that codes for this flowering hormone was isolated in Arabidopsis. Further research has shown that the gene plays this role in all plant species. The nice thing is that this discovery is based on a mutation that was discovered 30 years ago by my former boss Jaap van der Veen.
'Our knowledge about the development of the plant embryo is also based on research on Arabidopsis, just like knowledge about seed germination and root formation. All of them fundamental processes. A table has been made with the major discoveries regarding the influence of light on the development of plants. The first breakthroughs came from research on all kinds of crops, but after 1988 nearly all the significant discoveries were made in work on Arabidopsis.'

What can plant breeders do with these discoveries?
Koornneef: 'Plant breeding companies want to improve the taste of tomatoes, for example. Which genes are responsible for this? Researchers can look at the factors in Arabidopsis which regulate the ripening hormone ethylene. A mutation was found in Arabidopsis in which this hormone didn't function, and then the relevant gene for ripening was isolated. After that, researchers look at the tomato to see if this gene is responsible for ripening there too. They can make a 'calculated guess' without having to screen 20,000 genes. They can do their work with fewer than ten genes.'
'You can also use knowledge about mouse-ear cress to make transgene plants. Colleagues in the US have found a set of genes in Arabidopsis that code for drought resistance. They have patented this set and made a proposal to a biotechnology company: do you want to build this set into maize? There are transgene applications of Arabidopsis research in the pipeline in the US.'
'The knowledge is also applied in creating resistance to diseases. Disease resistance in Arabidopsis is quite subtle, and we have a profound knowledge of it. Researchers in Cologne who were studying Arabidopsis found a certain degree of resistance to the potato disease Phytophthora infestans, and transferred this knowledge to the potato.

Who gains from this knowledge?
Bisseling: 'All the knowledge is accessible to the public. The Arabidopsis community is very open. That's not altruism though. Openness is good for scientific progress. Occasionally Arabidopsis genes are patented, but much less often than with other crops. With a crop like tomato, the applications are much more important and people are a bit more secretive.'
Koornneef: 'I still remember the Arabidopsis conference in 1987, with lots of new people, some of whom had come from research on the fruit fly. Just like Arabidopsis, the fruit fly provided geneticists with a simple model organism with thousands of mutations. That group of people brought a sense of openness with them, and a tradition that sees the rapid exchange of knowledge as the way to make progress. That helped to make Arabidopsis so successful. Looking back it was a lucky choice.'
Mouse-ear cress in Wageningen
-Arabidopsis research was brought to Wageningen from the US 50 years ago by Wil Feenstra. When Feenstra became professor in Groningen, the Wageningen Professor of Heredity Jaap van der Veen continued to work on mouse-ear cress.
-From 1976, Maarten Koornneef was in at the start of the rise of mouse-ear cress as a model plant, through his description of new types of mutations. He is seen by colleagues as a world authority on Arabidopsis.
-Now hundreds of Wageningen researchers are working on mouse-ear cress. The Wageningen geneticist Hans de Jong even goes as far as to say that the Radix building should be called Arabidopsis, as nearly all the researchers in the building are working on this model plant.
-The Experimental Plant Sciences research school produces more publications on Arabidopsis than on major food crops such as potato and tomato.


Top 3 Wageningen Arabidopsis articles
1 Analysis of the genome sequence of the flowering plant Arabidopsis thaliana. Hundreds of authors including M. Koornneef; Nature 2000, cited 3219 times
2 Construction of integrated genetic-linkage maps by means of a new computer package: Join Map. P. Stam; Plant Journal 1993, cited 603 times
3 Genetic control of light-inhibited hypocotyl elongation in Arabidopsis thaliana. L. Heynh, M. Koornneef, E. Rolff, C.J.P. Spruit; Zeitschrift für Pflanzenphysiologie 1980, cited 535 times

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