Pond-dwelling powerhouse's genome points to its biofuel potential

PUBLIC RELEASE DATE:

19-Feb-2014

Contact: David Gilbert degilbert@lbl.gov 925-296-5643 DOE/Joint Genome Institute

Duckweed is a tiny floating plant that's been known to drive people daffy. It's one of the smallest and fastest-growing flowering plants that often becomes a hard-to-control weed in ponds and small lakes. But it's also been exploited to clean contaminated water and as a source to produce pharmaceuticals. Now, the genome of Greater Duckweed (Spirodela polyrhiza) has given this miniscule plant's potential as a biofuel source a big boost. In a paper published February 19, 2014 in the journal Nature Communications, researchers from Rutgers University, the Department of Energy Joint Genome Institute and several other facilities detailed the complete genome of S. polyrhiza and analyzed it in comparison to several other plants, including rice and tomatoes.

Simple and primitive, a duckweed plant consists of a single small kidney-shaped leaf about the size of a pencil-top eraser that floats on the surface of the water with a few thin roots underwater. It grows in almost all geographic areas, at nearly any altitude. Although it's a flowering plant, it only rarely forms small indistinct flowers on the underside of its floating leaves. Most of the time, it reproduces by budding off small leaves that are clones of the parent leaf. It often forms thick mats on the edges of ponds, quiet inlets of lakes and in marshes. It's among the fastest growing plants, able to double its population in a couple of days under ideal conditions.

These and other properties make it an ideal candidate as a biofuel feedstock a raw source for biofuel production. For example, unlike plants on land, duckweeds don't need to hold themselves upright or transport water from distant roots to their leaves, so they're a relatively soft and pliable plant, containing tiny amounts of woody material such as lignin and cellulose. Removing these woody materials from feedstock has been a major challenge in biofuel production. Also, although they are small enough to grow in many environments, unlike biofuel-producing microbes, duckweed plants are large enough to harvest easily.

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.

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Pond-dwelling powerhouse's genome points to its biofuel potential