Natures wonders: how land plant pollen protects DNA (Introduction)

by David Turell , Wednesday, July 20, 2022, 00:01 (647 days ago) @ David Turell

Pollination on land offers dangers to the enclosed DNA for fertilization:

https://www.quantamagazine.org/how-the-diamond-of-the-plant-world-helped-land-plants-ev...

"All seeding plants make pollen; other land plants, such as moss, produce spores. Carrying half the genetic information that the plants need to reproduce, pollen and spores move through the environment on the wind or on a helpful animal, to reach another plant of their species and fertilize its egg cell. But along the way, pollen and spores must contend with dangers that range from dehydration to the sun’s ultraviolet rays to hungry insects. Since plants first found purchase on land around 470 million years ago, keeping the genetic information within pollen and spores safe during their journey to fertilization has been vitally important.

"The main strategy that plants employ to protect that DNA is to encase it in a specialized shell of sporopollenin, which is impervious to the elements and among the toughest materials produced by any living thing. It has been found intact in half-billion-year-old rocks. A 2016 paper found that because of the robustness of sporopollenin, spores maintained their stability in diamond anvils at pressures of 10 gigapascals, or 725 tons per square inch.

***

"Its [sporopollenin] function was well known, and the genes for making it were in every seed- and spore-producing plant, which implied that sporopollenin was a basic adaptation enabling plants to live on land at the very beginning of their escape from the oceans. (Some species of algae also make a sporopollenin-like substance, which suggests that land plants adapted the biosynthesis of that molecule during their evolution.)

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"In conversation, Li used his hands to describe the intricate shape of the structure. With his thumb and forefinger, he showed how aromatic molecules hang off the backbone in alternating L-shapes. He demonstrated how the backbone is bound with the cross-linkages by pointing one flattened hand into the other at an angle, as if engaging in some strange form of prayer. These basic units link together to form the complete exine shell, which takes on radically different shapes in different plants, though the basic molecular subunits are fundamentally similar.

"The structure gave credence to the idea that the hardiness of sporopollenin arises from the varied, braided linkages between the backbones. These ester and ether linkages are resistant, respectively, to basic and acidic conditions; together they resist both. The structure that Li’s group proposed also included several aromatic molecules known to be resistant to ultraviolet light, which accounted for sporopollenin’s ability to guard DNA from the elements.

“'Without these metabolic innovations, plants would not have been able to migrate from water to land in the first place,” Weng wrote in an email to Quanta.

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"Fossils show that plants have been making spores and pollen ever since they first moved to dry land. (my bolds)

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"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?"

Comment: based on all the conditions and dangers affecting pollen, the bolded statements are very pertinent. The plants can't just jump on land without the pollen being protected at the same time. Only design fits.