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Moss genome sheds light on plants’ invasion of land

Researchers report the completion of the first genome of a moss. Combined …

Although completing a genome provides science with lots of information, the completion of several genomes provides us with far more than the individual genomes do. Comparisons between the genomes of related organisms can provide us with information about the changes in gene content that accompany major evolutionary transitions. A great example of this is how the sequencing of the Chlamydomonas genome shed light on the origin of plants. Today, Science will be offering up an advanced publication that describes the sequencing of a moss, a relative of Chlamydomonas and descendant of the world's first land plants.

The organism in question is a Bryophyte called Physcomitrella patens. The genome itself is an unassuming 480 Megabases and contains about 36,000 genes. Its significance resides primarily in the fact that Bryophytes are the modern descendants of the first muticellular plants that made their way onto land. If you view Chlamy as lying on the border between algae and animals, you can view Bryophytes as on the border between Chlamy and trees. They are clearly adapted to life on land, but they still need a fairly wet environment, lacking as they do adaptations such as a complex root system and the vascular transport of water.

Physcomitrella itself appears to have only undergone a single whole-genome duplication, in contrast to the multiple rounds of duplications that characterize many of the flowering plants. As a result, there are far fewer duplicated genes and most gene families have fewer members.

Based on its capacity for sending signals between cells, the organism appears partly adapted to the muticellular lifestyle. It contains everything needed to make and use cytokinins, which regulate plant morphology. But it seems to lack other intercellular signaling molecules, such as auxins. It may be able to use ethylene, which flowering plants use to regulate fruit ripening, but the evidence is somewhat sketchy.

It appears to be partly adapted for surviving freezing and desiccation. Like flowering plants, it has a large number of ABC transporters, which reside on cell membranes and help control the flow of material into and out of the cell. It also has enhanced DNA repair capabilities compared to Chlamy, suggesting it can cope with higher exposure to sunlight. In fact, it appears to be well equipped for benefitting from a range of light conditions; the authors say it, "has increased the genetic playground for photosynthesis and connected carbon-based metabolism."

One of the best features of the new genome is that many of the predictions that come out of the genome analysis will be testable. Physcomitrella handles DNA repair in the same way that Yeast does, by homologous recombination. As such, it should be easy to knock out the genes we have now identified and examine the impact that has on its ability to survive in a terrestrial environment.

Science, 2007. DOI: 10.1126/science.1150646

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