How the ‘Diamond of the Plant World’ Helped Land Plants Evolve

Fu-Shuang Li knew exactly where to go when he needed some pollen for his research. The clouds of golden pollen that coat the water and gather in the swirls against the shore are released by the pitch pine trees ringing Walden Pond. In his famous account of living beside the pond in the 1840s, Henry David Thoreau described the amount of pollen that could have been collected.

At the pond's edge, Li dipped in a test tube, drawing out a few hundred million of liters of water, which was filled with pollen and other things. It wasn't a lot, but it was enough for Li to study the outer shell of pollen. The material that makes up the shell is known as the diamond of the plant world.

Scientists have been trying to understand the chemical basis of sporopollenin for more than 100 years. Light, heat, cold and desiccation can damage the genes in pollen. Plants can't live on land without it. It was difficult to study the structure of the basic plant polymers after decades of research. Li said that nature was able to resist any attack. Including by people who work in science.

It is possible that sporopollenin's defenses have been overcome recently. The first complete structure of sporopollenin was published by Li and others. Subsequent work by the team, some of it not yet published, has filled in more details about how different groups of plants fine tune their structure to better meet their needs. The proposed structure and improved view of sporopollenin is not without controversy, but it has clarified the molecule's role in helping plants conquer the land.

The Inert Enigma

Other land plants, such as moss, are capable of producing spores. Carrying half the genetic information that the plants need to reproduce, pollen and spores move through the environment on the wind or on an animal to reach another plant of their species and fertilize its egg cell Along the way, the risks include dehydration, the sun's ultraviolet rays, and hungry insects. Plants have been keeping their genetic information safe since they first found land around 470 million years ago.

Plants use a special shell of sporopollenin, which is impervious to the elements and among the toughest materials produced by any living thing, to protect their genetic material. It was found in half a billion-year-old rocks. The paper found that the stability of the spores was maintained because of the strength of sporopollenin.

Since at least 1814, researchers have known about the drug. The strange substance always remained even after the rest of the pollen grain was removed. For most of the next century, those studying it in the form of spores and pollen only referred to it as sporonin or pollenin. In 1931, it was called sporopollenin to appease both communities.

Knowledge about the molecule ended with the name. Researchers wanted to use the material for everything from coating the hull of ships to protecting fragile proteins in oral vaccines, because they realized that sporopollenin could be key to understanding how plants conquered nearly every habitat on Earth. Every effort was made to get the structure and chemical composition of sporopollenin.

The structure of a complex molecule can be determined by breaking it down into its parts and finding the structure of those. The chemical agents that are used to digest it were not able to digest it. In the 1960s, new biochemical methods and mass spectrometry made some progress on the structure and chemical composition, and biologists inferred some details from knowledge of the genes and enzymatic processes that synthesise sporopollenin

The methods couldn't give a full picture of the molecule. The sugars in the double helix of DNA are made from two parallel backbones. The links of different types appeared to connect these backbones. Some of the findings from the genetic and biochemical methods were not in this sketch.

According to Joseph Banoub, a professor of chemistry and biochemistry at Memorial University of Newfoundland in Canada, the only thing everyone agreed on was the formula for the composition of carbon, hydrogen, oxygen.

Pitch Pine Perfect

After he joined Weng's lab as a post-doc, Li began to work on sporopollenin. One of the few places where people study plants is in the Cambridge neighborhood of Kendall Square, which is home to the primary obsession of medical research.

Li was drawn to the challenge of stroopollenin. Its function was well known, and the genes for making it were in every plant, which meant that it was a basic adaptation that allowed plants to live on land at the beginning of their escape from the ocean. It's possible that land plants adapted the biosynthesis of that molecule during their evolution. The chemistry behind that ability wasn't fully understood.

It would have been poetic if Li had used the pollen from the waters of the pond. His team ordered the pollen from Amazon. It is widely sold as a health supplement. The rest came from the other side of the country.

Li and his team ran trial-and-error testing on compounds that can degrade other tough biopolymers. They developed a process that could take samples of pollen, crush them in a ball milling machine, and break them down. Mass spectrometry could be used to determine which pieces of the molecule were different.

When mixed with another dissolving agent, the other half of the molecule broke down. The process degraded other features of the molecule, so Li's group used a more exotic technology to study it.

Flowers Made a Difference

A paper published in Nature Plants in December of last year proposed the most complete structure of sporopollenin to date.

Li described the structure with his hands. He showed how aromatic molecule hang off the backbone in different L-shapes. He pointed a flattened hand into the other at an angle to show how the cross-linkages bind the backbone. The basic units link together to form the complete exine shell, which is vastly different in shape in different plants.

The structure gave credence to the idea that there are different braided links between the backbones. The ester and ether linkage are resistant to basic and acidic conditions. The structure proposed by Li's group included a number of aromatic molecules that were resistant to ultraviolet light.

Plants wouldn't have been able to migrate from water to land without these innovations.

More than 100 diverse land plant species were collected from botanic gardens in the northeastern United States. According to Li, who is preparing to submit the results of the study for publication, the structure of sporopollenin varies across plants.

Gymnosperms, the land plant group that includes cycads and conifers like pitch pine, and the so-called lower land plants like mosses and ferns tend to have long sporopollenins, according to a new study. The plants need long-chain sporopollenin to protect them from the wind.

The situation is more complex for angiosperms. Their flowers shade their pollen from the sun and desiccation, and insects move it from flower to flower, so they don't expose themselves to other risks. angiosperms don't need to be so strong.

When flowers evolved, they didn't want to make pine-like sporopollenin anymore, because it's an energy intensive process. The two major categories of angiosperms, monocots and dicots, which differ in the structures of their embryo, vasculature, stems, roots and flowers, have evolved significant differences according to Li and Weng.

The differences aren't absolute. Some flowering plants have pine-like structures. If we had another 6 million years, the function of those may be lost.

Li stated that evolution is not a line. It was like the whales. They used to live on land, but now live in the ocean. Some land animals have characteristics. Some flower pollens have their own histories.

The Mysterious Polymer

The structural work done by Li and Weng has improved our knowledge of the molecule. Some of them aren't convinced that the proposal is correct or that it ends the century-long search for the structure.

The scientist who studies sporopollenin at the Normal University said that it was much clearer than it had been previously. It needs to be confirmed. He said that Li and his colleagues still need to identify the genes that make the pine sporopollenin work.

The 2020 study on the structure of sporopollenin posed a more direct challenge than the previous one. The structure proposed by Li and Weng was different in several important ways. Banoub said, "We have proven there are no aromatic compounds in the sporopollenin." He thinks there are differences between pine and club moss.

Li doesn't want to comment further until the results from his lab are ready for publication.

Teagen Quilichini, a plant Biologist at Canada's National Research Council, said in an email that it is still a mystery. Despite what some reports say.

Tough but Still Edible?

Despite the controversy over the structure of sporopollenin, Li and others in the Weng lab have moved on to another evolutionary question.

Li compared sporopollenin, a plant that strengthens wood and bark, to a plant called Lignin, which strengthens wood and bark. For tens of millions of years, the geological record shows an abundance of fossilized Lignin in the ground. The lignin disappeared about 300 million years ago. It's disappearance marks the moment when a fungus called white rot was able to degrade lignin and then eat it before it could fossilize.

It must have a microbe that can break it down. We would be overwhelmed by it. 100 million tons of sporopollenin are produced in the forest every year. That doesn't account for the grasses. Where does it all go?

As a source for his latest sample of pollen, Li decided to forgo Amazon Prime in favor of a day at the park. Some of the organisms grown in petri dishes can live if they are fed sporopollenin and nitrogen. Li wants to know if populations of fungi and other organisms in the wild can open up the sporopollenin molecule.

It was easy to see the situation from the fungis perspective as we snacked on seaweed and bars. Nature doesn't like to waste a meal.