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Duckweed: tiny plant with big biofuel potential
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Duckweed, a ‘forever young’ tiny floating plant with one of the smallest known plant genomes, may be about to become big in the field of biofuel production. The complete genome of Greater Duckweed (Spirodela polyrhiza) has this week been detailed and analysed in comparison to other plants such as rice and tomatoes, in a publication from researchers in institutions in the USA and Germany. The study was published in the journal Nature Communications.

Duckweed, a tiny floating plant with a few thin, underwater roots and a single small kidney-shaped leaf, is generally considered to be an annoyingly difficult-to-control weed, growing in ponds and small lakes. It is very fast-growing and can double its population over the course of a couple of days if conditions are favourable. However, it has also been commercially exploited to clean contaminated water and as a pharmaceutical source to produce pharmaceuticals. Its rapid growth, the lack of any need to hold itself upright or transport water from its roots to its leaves and its small content of woody materials like lignin and cellulose make it an ideal candidate for biofuel production. Added to that is its ease of harvesting compared to biofuel-producing microbes.

The sequencing of the Spirodela polyrhiza genome has revealed it to have one of the smallest known plant genomes. There are about 158 million base pairs containing fewer than 20,000 protein-encoding genes. The plant has what senior author Joachim Messing of Rutgers University describes as “a forever-young lifestyle” in which as they mature, they continuously produce cotyledon leaves, that is embryonic leaves inside plant seeds. This prolonging of juvenile traits is called "neoteny." The genome sequencing project revealed that S. polyrhiza had fewer genes to promote and more genes to repress the switch from juvenile to mature growth. This arrest in development allowed the research team to “uncover regulatory networks that are required for differentiation and development," according to Dr Messing.

Importantly in terms of biofuel production, the genome analysis also revealed the molecular basis of the low woody material content of duckweed. The plant lacked many of the genes responsible for cellulose and lignin production in land dwelling plants and had fewer copies of the ones that were present. Meanwhile, genes for compounds associated with cell wall and root growth, called "expansins", were also reduced. Among genes which were retained were those for starch production, probably used for creation of starch-filled turions. These are buds which aquatic plants use to allow them to dwell at the bottom of ponds during winter and revive in warmer weather. Interestingly, S. polyrhiza has more copies of genes for enzymes involved in nitrogen absorption and metabolism than in other plants despite its overall tiny genome. This would contribute to the plant's ability to clean up contaminated water by utilising excess nitrogen.

Overall, the genome analysis will contribute to thorough understanding of the genome and cellular mechanisms of S. polyrhiza and hence to current efforts to recruit duckweed as a biofuel source. Dr Messing estimates that duckweed will be a viable biofuel source as soon as in the next five years. Indeed, he points out that a New Jersey company called Ceres Energy Group is already producing electricity from duckweed. Genome analysis will allow exploitation of traits that are advantageous in biofuel production, such as reduced cellulose or increased starch, in development of new duckweed varieties.

Dr Messing concludes that: "The sequencing of this genome opens new frontiers in the molecular biology of aquatic plants….This publication represents the single largest advance in this field and a new milestone in plant molecular biology and evolution, as previous studies were either classical botany or biochemistry of photosynthesis. The placement of the Spirodela genome as a basal monocot species will serve as a new reference for all flowering plants."

Sources

http://www.eurekalert.org/pub_releases/2...021414.php [DOE/Joint Genome Institute press release; accessed 19 February 2014].

WANG,W., HABERER, G., GUNDLACH, H., GLÄßER, C., NUSSBAUMER, T., LUO, M.C., LOMSADZE, A., BORODOVSKY, M., KERSTETTER, R.A., SHANKLIN, J., BYRANT, D.W., MOCKLER, T.C., APPENROTH, K.J., GRIMWOOD, J., JENKINS, J., CHOW, J., CHOI, C., ADAM, C., CAO, X.H., FUCHS, J., SCHUBERT, I., ROKHSAR, D., SCHMUTZ, J., MICHAEL, T.P., MAYER, K.F.X. and MESSING, J., 2014. The Spirodela polyrhiza genome reveals insights into its neotenous reduction fast growth and aquatic lifestyle. Nature Communications, Vol. 5 (19 February 2014), doi:10.1038/ncomms4311

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Duckweed: tiny plant with big biofuel potential51