Pollen: A great leap for plants gets its hooks in people, too
By Lawrence J. Winship for the Gazette
July 8, 2022
Every spring our car windows, decks and sidewalks are blanketed by layer upon layer of yellow powder. A seemingly unending rain of tiny particles filters down from birches, oaks, pines and other trees, sticking to every horizontal surface — and making about 25% of the human population sneeze.
Spring is the onset, but not the end, of pollen season — the ever-present reminder that a lot of plant sex is going on around us, above our heads, along roads and in our gardens. Yes, without pollen, flowering plants would set no seed. So, beginning with spring-flowering trees, and continuing into summer with grasses and ragweed, our need for antihistamines, neti pots and air filters is part of life here in the Pioneer Valley.
Pollen from a variety of common plants: sunflower (small spiky sphericals), morning glory (big sphericals with hexagonal cavities), hollyhock (big spiky sphericals), lily (bean-shaped), primrose (tripod-shaped) and castor bean (small smooth sphericals). The bean-shaped grain in the bottom left corner is about 50 microns long. One micron is one thousandth of a millimeter. DARTMOUTH COLLEGE ELECTRON MICROSCOPE FACILITY
Of course, pollen is not just there to inconvenience and debilitate us; it plays a central role in the adaptation of plants to life on dry land. Non-flowering plants, such as mosses, liverworts and ferns, still use a system of sexual reproduction evolved millions of years ago in aquatic habitats. Liquid water is essential as the medium through which motile “male” gametes swim, propelled by tiny flagella, to find and fertilize an immobile “female” gamete.
This mechanism is great as long as there is sufficient rain and fog to wet the fertile parts of plants during reproduction, but is much chancier in the dry uplands. The evolution of pollen grains made it possible for the fertile male part of the plant to travel long distances in dry air, dried up in a form of suspended animation, until the pollen grain lands on the receptive surface of a flower.
There, the pollen takes up moisture and begins to grow and divide, producing a structure called the pollen tube that allows the sperm cell to make its way to the egg. The trillions of pollen grains coursing through the skies are actually tiny dormant plants.
The first flowers likely evolved in concert with insects that carried the pollen to another flower, perhaps initially as pollen feeders. Wind pollination seems to have evolved later and is what is called a “derived” character, meaning that other closely related plants still rely upon insects or other animals.
Wind pollination shows up in at least 65 separate plant lineages, suggesting that this is a very important adaptation that provides significant evolutionary value over and over again.Charles Darwin was puzzled by this adaptation. He wondered how such an apparently wasteful use of plant energy and nutrients could provide enough value to be retained as a trait. This was in the light of his study of orchids, where very tight connections between bees and orchid species are common, and pollen is clumped into small pollinia rather than strewn by the wind. Yet over 10% of angiosperm species are wind-pollinated, which probably reflects the limited numbers of pollinators in drier steppes and prairies.
Wide dispersal of pollen is also a means of increasing genetic diversity. Clearly, wind pollination works very well for a large number of ecologically important plant types — but not so well for allergy sufferers.
So why does pollen stimulate allergic reactions that can sometimes be quite severe? When pollen lands on the inside of your nose, the pollen is quite dry. Pollen coats are spiky and menacing looking — but it is not the shape that does the damage. While there may be a few waxy or fatty molecules on the surface of the pollen, the main proteins and lipids known to stimulate our immune system are actually still inside the pollen grain.
These chemicals have important functions in the growing pollen tube but cannot be released until the grain takes up water. Studies on cypress pollen showed that its pollen grains absorb moisture in the nose within minutes and then disintegrate, releasing their contents.
PAUL KNUT/VIA PEXELS PAUL KNUT/VIA PEXELS
This helps explain why pollen can stimulate an asthmatic response even though pollen grains themselves are too large to make it down in the very small passages in the lungs where asthma originates. It is the broken bits of the pollen that make their way deep into the lungs.
Another explanation is called the “thunderstorm” hypothesis. Pollen is carried up into thunderstorms by rising air currents, gets wet and then explodes, releasing small particles that are later carried down into human lungs. It makes sense that pollen absorbing pure water might explode, because the cell contents of the growing pollen tube are filled with sugars and salts that cause osmotic uptake of water, so much that in the absence of balancing salts and sugars on the outside, they burst.
But as I pondered the fate of pollen in our noses and bronchial tubes, I began to wonder about a somewhat more intimate relationship between humans and pollen. Perhaps our mucous membranes are not that different from the receptive surfaces of the flower. Could pollen actually be taking up moisture and begin to germinate in our noses?
How long would a pollen tube get before it bursts? Then the allergenic proteins and fats would be within easy reach of the white blood cells protecting us from invaders. I searched the internet for images and found none. Perhaps this thought is so off the wall that no one has looked?
The next time I’m sitting at a microscope, perhaps I’ll take a peek. When we sniff up pollen, are we being pollinated?
Lawrence J. Winship is emeritus professor of botany at Hampshire College and a former board member of the Hitchcock Center for the Environment.
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