S. polyrhiza turns out to have one of the smallest known plant genomes, at about 158 million base pairs and fewer than 20,000 protein-encoding genes. That's 27 percent fewer than Arabidopsis thaliana which, until recently, was believed to be the smallest plant genome and nearly half as many as rice plants.
"The most surprising find was insight into the molecular basis for genes involved in maturation a forever-young lifestyle," said senior author Joachim Messing, director of the Waksman Institute of Microbiology at Rutgers University.
S. polyrhiza leaves resemble cotyledons, embryonic leaves inside plant seeds that become the first leaves after germination. But where other plants develop other kinds of leaves as they mature, S. polyrhiza's never progresses and continuously produces cotyledon leaves. This prolonging of juvenile traits is called "neoteny." S. polyrhiza had fewer genes to promote and more genes to repress the switch from juvenile to mature growth.
"Because of the reduction in neoteny, there is an arrest in development and differentiation of organs. So this arrest allowed us to uncover regulatory networks that are required for differentiation and development," Messing said.
Also intriguing to the research team were which genes were preserved over time and which were not. Many of the genes responsible for cellulose and lignin production in land dwelling plants were missing, and there were fewer copies of those that were present. Genes for another compound related to cell walls called "expansins" which are involved with cell wall and root growth were also reduced.
Genes for starch production, on the other hand, were retained and are probably used for creating starch-filled turions, specialized buds produced by aquatic plants for overwintering, enabling them
|Contact: David Gilbert|
DOE/Joint Genome Institute