Researchers shed new light on how plants became established on Earth’s surface
Researchers from University of Copenhagen have shed new light on the settlement of plants on the surface of our planet. In particular, they showed that two genes are crucial for land plants to protect themselves against fungal attack – a defense mechanism that is 470 million years old. These defense mechanisms most likely paved the way for all plant life on Earth.
About half a billion years ago, plants evolved from aquatic algae to terrestrial plants that laid the foundation for life on land. Mushrooms were one of the obstacles that made this dramatic transition so difficult:
“It is estimated that 100 million years ago, fungi crawled across the surface of the earth in search of food and most likely found them in dead algae washed up from the sea. So if you wanted to establish yourself as a new plant on land and the first thing you encounter is a fungus that would eat you, you needed some kind of defense mechanism,” says Mads Eggert Nielsen, a biologist at the Institute of Plant and Environmental Sciences University of Copenhagen.
According to Mads Eggert Nielsen and his research colleagues from the Department of Plant and Environmental Sciences and the University of Paris-Saclay, the essence of this defense mechanism can be narrowed down to two genes, PEN1 and SYP122. Together they help form a kind of plug in plants that block the entry of fungi and fungus-like organisms.
We have found that when we destroy these two genes in our model plant thale cress (Arabidopsis), we open the door for pathogenic fungi to invade. We found that they are essential for the formation of this cell wall-like plug to defend against fungi. Interestingly, it appears to be a universal defense mechanism found in all land plants,” says Mads Eggert Nielsen, senior author of the study published in the journal eLife.
Formed in a 470 million year old facility
The research team tested the same function in hepatica, a direct descendant of one of Earth’s very first land plants. By incorporating the two corresponding genes from liverworts into the thale cress, the researchers investigated whether they could detect the same effect. The answer was yes.
“Although the two plant families Arabidopsis and Hepatica evolved in different directions 450 million years ago, they continue to share genetic functions. We believe that this family of genes arose with the unique purpose of controlling this defense mechanism and thus was one of the bases for the establishment of plants on land,” says Mads Eggert Nielsen.
A symbiosis between plants and fungi
While fungi were an obstacle for plants in the transition from an algal marine stage to terrestrial plants, they were also a prerequisite. Once plants could survive attacks from fungi trying to eat them on land, the next problem was finding nutrients, explains Mads Eggert Nielsen:
“Dissolved nutrients such as phosphorus and nitrogen are easily accessible to plants in aquatic environments. But 500 million years ago, there was no soil as we know it today – only rock. And nutrients bound in the rock are extremely difficult for plants to get. But not for mushrooms. On the other hand, mushrooms cannot produce carbohydrates – which is why they eat plants. It is believed that a symbiotic relationship between plants and fungi arose here, which then became the basis for the explosion of terrestrial plant life during this period.”
The defense structures that form in a plant do not kill either the plant or the fungus, they only prevent a fungus from entering.
“Because a fungus can only partially penetrate a plant, we believe a tipping point is emerging where both plant and fungus have something to gain. So it was an advantage to keep the relationship as it is. The theory that tamed mushrooms are planted to colonize land is not ours, but we provide fodder that supports that idea,” says Mads Eggert Nielsen.
Can be used in agriculture
The new results add an important piece to the puzzle of plant evolutionary history. More importantly, they could be used to make plants more resistant to fungal attack, which is a big problem for farmers.
“If all plants defend themselves in the same way, it must mean that the microorganisms that can cause diseases – such as powdery mildew, yellow rust and potato mold – have found a way to sneak in, turn off their defenses, or evade their respective host plants.” We want to find out how they do it. We will then try to transfer defense components from resistant plants to diseased plants and achieve resistance there,” says Mads Eggert Nielsen.
Mads Eggert Nielsen is involved in a research project at the Department of Plant and Environmental Sciences, led by Hans Thordal-Christensen, supported by the Novo Nordisk Foundation, focused on making crops more resilient by identifying the defense mechanisms in plants which pathogenic microorganisms seek to shut down.
Researchers have long suspected that the PEN1 and SYP122 genes have a particular function in relation to the transition of plants from their aquatic stage as algae to terrestrial plants, but there is no concrete evidence as to whether they were actually a requirement for the plants’ defense abilities .
Previous studies have shown that by destroying the PEN1 gene, plants lose their ability to defend themselves against powdery mildew fungi. However, if the closely related SYP122 gene is destroyed, nothing happens. The new research results show that the two genes together represent an important key in the plant’s defense mechanism.
Reference: “Plant SYP12 syntaxins mediate an evolutionally conserved general immunity to filamentous pathogens” by Hector M. Rubiato, Mengqi Liu, Richard J. O’Connell and Mads E. Nielsen, February 4, 2022, eLife.
DOI: 10.7554 / eLife.73487