Recently, Li and his colleagues used their method to characterize sporopollenin from more than 100 diverse land plant species collected from botanic gardens around 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 plant types in a curious pattern.
They found that 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, similar sporopollenins. This makes sense because these plants disseminate their pollen willy-nilly on the wind; they need long-chain sporopollenin to protect it.
But among angiosperms, or flowering plants, the situation is more complex. Their flowers shade their pollen from sun and desiccation, and insects efficiently move pollen from flower to flower, minimizing the exposure to other risks. Consequently, angiosperms don’t need their sporopollenin to be so uniformly robust.
And making long-chain sporopollenin is an energy-intensive process, Li said, so “when flowers evolved, they did not want to produce pine-like sporopollenin anymore.” According to Li and Weng, significant differences seem to have evolved between the sporopollenins produced by the two major categories of angiosperms, monocots and dicots, which diverge in the structures of their embryos, vasculature, stems, roots and flowers.
Of course, the distinctions aren’t absolute. Some flowering plants do produce sporopollenin with a pine-like structure, said Li. “Maybe if we had another 6 million years, they may lose the function of those,” or maybe there are other ecological checks and balances at play preserving that sporopollenin structure for certain groups of plants.
“Evolution is not a line,” said Li. “Like the whales. At one point they lived on land; now they live in the ocean.” Yet whales still have some land animal characteristics. Perhaps some flower pollens retain obsolete traces of their own history.
The Mysterious Polymer
Other plant researchers agree that Li and Weng’s structural work on sporopollenin has improved our knowledge of the molecule. But not all of them are persuaded that their proposal is correct or that it concludes the century-long search for the structure of sporopollenin.
“It was much clearer than before,” said Zhong-Nan Yang, a biologist who studies sporopollenin at Shanghai Normal University. “But it needs to be verified.” He said Li and his colleagues still must identify the genes responsible for the enzymes needed to make certain features of pine sporopollenin.
A 2020 study aimed at “demystifying and unravelling” the molecular structure of sporopollenin posed a more direct challenge. Using a bevy of methods and working on sporopollenin from club moss rather than pine, Banoub’s group at Memorial University arrived at a structure that differed in several important ways from the one proposed by Li and Weng. Most importantly, Banoub said, “We have proved there are no aromatic compounds within the sporopollenin.” The disparity, he thinks, might be explained by differences between sporopollenin in pine and club moss.
“My personal view is they are not correct,” said Li, but he prefers not to comment further until some relevant results from his lab are ready for publication.
“It is still quite the mysterious polymer,” commented Teagen Quilichini, a plant biologist at Canada’s National Research Council who has studied sporopollenin, in an email. “Despite what some reports suggest.”
Tough but Still Edible?
Notwithstanding the controversies over their structure for sporopollenin, Li and others in the Weng lab have moved on to another evolutionary question: Has nature figured out how to take apart this nearly indestructible material it put together?
As he hiked around Walden Pond in search of other pollen-coated inlets, Li compared sporopollenin to lignin, the plant polymer that strengthens wood and bark. After woody plants first evolved about 360 million years ago, the geological record shows an abundance of fossilized lignin in strata for tens of millions of years. Then suddenly about 300 million years ago, the lignin vanishes. Its disappearance marks the moment when a fungus called white rot evolved enzymes capable of degrading lignin and ate much of it before it could fossilize.
Sporopollenin, Li reasoned, must also have a fungus or other microbe capable of breaking it down. Otherwise we’d be drowning in the stuff. Li’s back-of-the-envelope calculations are that 100 million tons of sporopollenin are produced in forests every year. That doesn’t even account for the sporopollenin produced by grasses. If nothing is eating it, where does it all go?
This is why, as a source for his latest sample of pollen, Li opted to forgo Amazon Prime in favor of a day at Walden Pond. Observations by his team suggest that some microorganisms grown in petri dishes can survive when fed nothing but sporopollenin and nitrogen. Samples from Walden, which are naturally full of lake microbial communities, should help Li determine whether populations of fungi and other microbes in the wild can unlock the nutrients in sporopollenin’s seemingly unbreakable molecules.
As we snacked on seaweed and granola bars by the pond’s edge, it was easy to see the whole situation from the fungi’s perspective. Nature hates to waste a meal — even one so tough to chew.
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