25.04.18 11:24

Evolutionary biology: Article in PNAS

Green algae paved the way for land plants

By: Arne Claussen

When plants first went on shore about 510 million years ago, this was a tremendous challenge for their survival. Researchers at Dalhousie University in Canada and Heinrich Heine University Düsseldorf (HHU) have discovered an important prerequisite for this step in “streptophyte algae”, the closest ancestors to plants: These algae already have stress-signalling pathways that were so far only known in plants and through which survival under the environmental conditions on land only becomes possible. The results of their study were announced in April in the scientific journal Proceedings of the National Academy of Sciences (PNAS).

The article was chosen as a PNAS-Coverstory from April 10th, 2018 - a special award for a scientific publication. (Image: PNAS)

Over the course of the Earth's 4.5 billion-year history, the oxygen concentration in its atmosphere has changed several times and with dramatic consequences. For most of the time, it was only about 2 percent; only microscopic life forms can exist at this concentration. The oxygen concentration measured today of about 21 percent results from a unique event in the evolution of life on our planet: the colonisation of land by early plants. With the evolution of land plants about 510 million years ago, the Earth's appearance changed. They released large quantities of oxygen through photosynthesis and thus made life forms possible that were larger than just a few millimetres and could exist on land, since such life forms require oxygen.

So that plants could master the transition to land successfully, it was particularly important that they were able to deal with unfavourable environmental conditions - referred to in biology as stressors. Only in this way could they cope with more intense and unfiltered sunlight as well as large temperature fluctuations on land. A study led by Dr. Jan de Vries at Dalhousie University in Halifax, Canada, together with Dr. Sven Gould from HHU's Institute of Molecular Evolution has now illuminated in greater depth how the earliest land plants might have dealt with these stressors. They were interested in the evolution of regulatory mechanisms that are necessary for land plants' stress response and focussed in this context on those genes induced by strong sunlight and cold.

They discovered that not only the first land plants but also their closest ancestors, streptophyte green algae, already had many of the abilities to cope with stress. This means that the land plants did not have to invent them themselves. In the green alga Zygnema circumcarinatum the researchers even found a potential receptor for the classic plant stress hormone abscisic acid. In land plants, abscisic acid binds to this receptor and in so doing triggers the plant's stress response.

The results now published in PNAS are based on a high-throughput analysis of global gene expression in several of those green algae that are most closely related - from an evolutionary perspective - to land plants. In this process, all those genes are identified that are active in the cell at a specific point in time, for example when the sun is shining or when it is cold. The researchers also found those genes responsible for stress response in land plants.

"Our study alters the picture of how the earliest land plants dealt with stress," says Dr. Gould. "Before their transition to dry land they had already been well prepared by their ancestors, the streptophyte algae," adds Dr. de Vries.

Original publication

J. de Vries, B. A. Curtis, S. B. Gould & J. M. Archibald, Embryophyte stress signaling evolved in the algal progenitors of land plants, PNAS 115 (15), 10. April 2018

DOI: 10.1073/pnas.1719230115

Online: www.pnas.org/content/115/15/E3471

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