Two Genes Crucial for Plants Colonizing the Earth 470 Million Ago Have Been Identified

New information about the origins of plant life on Earth's surface was revealed by researchers.

University of Copenhagen researchers have provided new insight into how plant life evolved on Earth's surface. They particularly showed that two genes, which have been used as a defense strategy by terrestrial plants for 470 million years, are essential for defending themselves against fungal invasion. All terrestrial plant life most likely evolved as a result of these protections.

About half a billion years ago, plants evolved from watery algae to be able to exist on land, establishing the foundation for life on land. One of the challenges that made this abrupt change so challenging was fungi: 

“It is estimated that 100 million years prior, fungi crept across Earth’s surface in search of nourishment and most likely found it in dead algae washed up from the sea. So, if you, as a new plant, were going to establish yourself on land, and the first thing you encountered is a fungus that would eat you, you needed some sort of defense mechanism,” declares Mads Eggert Nielsen, a biologist at the Department of Plant and Environmental Sciences at the University of Copenhagen.

The key components of this defensive system may be boiled down to two genes, PEN1 and SYP122, according to Mads Eggert Nielsen and his research colleagues from the Department of Plant and Environmental Sciences and the University of Paris-Saclay. They work as a type of stopper in plants that prevents the invasion of fungi and creatures that resemble fungi.

“We found out that if we destroy these two genes in our model plant thale cress (Arabidopsis), we open the door for pathogenic fungi to penetrate. We found that they are essential to form this cell wall-like plug that defends against fungi. Interestingly, it appears to be a universal defense mechanism that is found in all terrestrial plants," explains Mads Eggert Nielsen, the study's senior author and author of the article in the journal eLife.

Originated in a 470-million-year-old plant

The same function has been examined by the study team in liverwort, a direct ancestor of one of the first terrestrial plants on Earth. The researchers tested whether they could detect the same impact by transferring the two matching genes from liverwort into thale cress. Yes, was the response.

“Even though the two plant families that Arabidopsis and liverwort belong to evolved in divergent directions 450 million years ago, they continue to share genetic functions. We believe that this gene family emerged with the unique purpose of managing this defense mechanism and has thus been one of the foundations for plants to establish themselves on land,” according to Mads Eggert Nielsen.

A symbiosis between plants and fungi

Fungi were both a requirement and a barrier for plants as they evolved from an algal sea stage to become terrestrial plants. The next challenge for plants was to get nutrition once they had learned to withstand attacks by fungus that wanted to devour them on land.

“Dissolved nutrients like phosphorus and nitrogen are easily accessed by plants in aquatic environments. But 500 million years ago, soil as we know it today did not exist – only rocks. And, nutrients bound in rocks are extremely difficult for plants to get a hold of. But not for fungi. On the other hand, fungi cannot produce carbohydrates – which is why they consume plants. This is where a symbiotic relationship between plants and fungi is believed to have arisen, which then became the basis for the explosion of terrestrial plant life during this period.”

The defensive mechanisms that develop in a plant just prevent a fungus from entering; they do not really kill the plant or the fungus.

“Since a fungus can only gain partial entry into a plant, we believe that a tipping point arises where both plant and fungus have something to gain. Therefore, it has been an advantage to maintain the relationship as is. The theory that plants tamed fungi to colonize land is not ours, but we are providing fodder that supports this idea,” says Mads Eggert Nielsen.

Can be applied in agriculture

The new findings add a crucial component to the puzzle of plant evolution. More crucially, they might be used to improve crop resistance to fungal infections, which is a huge issue for farmers.

“If all plants defend themselves in the same way, it must mean that the microorganisms capable of causing diseases – such as powdery mildew, yellow rust, and potato mold – have found a way to sneak in, turn off or evade the defenses of their respective host plants. We want to find out how they do it. We will then attempt to transfer defensive components from resistant plants to those plants that become diseased, and thereby achieve resistance,” Mads Eggert Nielsen agrees.

Mads Eggert Nielsen is working on a project at the Department of Plant and Environmental Sciences led by Hans Thordal-Christensen and funded by the Novo Nordisk Foundation that aims to make crops more resistant by identifying the defense mechanisms in plants that pathogenic microorganisms are attempting to shut down.

Additional Facts

Researchers have long assumed that the PEN1 and SYP122 genes had a specific role in the transition of plants from their watery stage as algae to land plants, but there is no definitive proof that they were required for the plants' defensive capacities.

Previous research has revealed that deleting the PEN1 gene reduces plants' capacity to protect themselves against powdery mildew fungus. However, when the similarly related gene SYP122 is deleted, nothing happens. The new findings show that the two genes work together to form a crucial part of the plant's defensive mechanism.

Reference: “Plant SYP12 syntaxins mediate an evolutionarily conserved general immunity to filamentous pathogens” by Hector M Rubiato, Mengqi Liu, Richard J O’Connell and Mads E Nielsen, 4 February 2022, eLife.