Surprising discovery: green algae already had stress signal pathways that were previously known only in plants
About 510 million years ago, the first plants left the water to gradually conquer the land masses. Researchers from the Canadian Dalhousie University and the Heinrich-Heine-University Düsseldorf (HHU) have discovered streptophytic green algae, the closest predecessors of plants, a crucial condition for the shore leave: They already have stress signal pathways that were previously known only in plants and the enable survival under environmental conditions on land.
The oxygen concentration of the Earth’s atmosphere has changed dramatically over the course of 4.5 billion years of geological history, with dramatic consequences. It was only about 2 percent for much of Earth’s history; at this concentration only microscopic life can exist. The currently measured oxygen concentration of about 21 percent results from a unique event in the evolution of life on our planet: the shore leave of plants. With the emergence of land plants, the face of the earth changed sustainably at the end of the Cambrian. They released large amounts of oxygen via photosynthesis and thus enabled complex life.
Increase in stressors on land
In order for plants to be successful on land, the handling of unfavorable environmental conditions – one speaks in the biology of stress factors – especially important. Only in this way could the higher and unfiltered light intensity as well as the larger temperature fluctuations outside the water be managed. A working group at Dalhousie University in Halifax, around Jan de Vries, together with Sven Gould from the Institute of Molecular Evolution at HHU, has studied the handling of these stress factors in more detail. Above all, she was interested in when the necessary regulatory mechanisms evolved. To do this, they focused on those genes that are activated in high light and cold.
They found that not only land plants, but their nearest relatives, the streptophytic green algae, had many of the appropriate abilities. The land plants did not have to develop these skills themselves. The researchers also found a potential receptor for the classical plant stress hormone abscisic acid in the sternal Zygnema circumcarinatum. The abscisic acid couples to this receptor and thus activates the plant stress response.
The results now published in the “Proceedings of the National Academy of Sciences” (PNAS) are based on a high-throughput analysis of global gene expression in several of those green algae evolutionarily closest to land plants. This determines all genes that are active in the cell at a certain time, for example during light irradiation or cold. Here, the researchers also found the genes that are beneficial to the stress response of land plants. “Our study changes the picture of how the earliest land plants dealt with stress,” Gould said. “They were already well prepared by their ancestors, the green algae, before jumping on dry land,” adds de Vries. (red, 20.4.2018)