Physics explains how pollen gets its stunning diversity of shapes

The varied patterns can all be explained by a process called phase separation

pollen shapes

PATTERNS APLENTY  Pollen from flowering plants comes in myriad shapes (as seen in these scanning electron microscope images, gray). Computer simulations based on a physics process called phase separation reproduced the grains’ shapes (orange).

SEM images: PalDat.org; Simulations: Asja Radja

Pollen grains sport a variety of snazzy shapes, from golf ball–like divots to prickly knobs or swirls that evoke a peppermint candy. But these myriad patterns may all be due to one simple trick of physics, scientists report in the Feb. 7 Cell.

That trick: phase separation, in which a mixture naturally breaks up into separate parts, like cream floating to the top of milk (SN: 7/21/18, p. 14). As pollen develops in a flowering plant, a material called primexine is deposited at the grain’s surface, inside a temporary cell wall. Formed from a mixture of materials including cellulose and pectin — the stuff that makes jam set — the primexine clumps together in denser and less dense regions “like bad gravy,” says biophysicist Alison Sweeney of the University of Pennsylvania.

That lumpiness generates wiggles in the pollen’s cell membrane, Sweeney and her colleagues found. Finally, the temporary cell wall dissolves, and a woody material called sporopollenin reinforces the wiggly pattern. The resulting shape can vary depending on the composition of the primexine.

Using computer simulations of the process, scientists reproduced the shapes of lumpy, patterned pollen, which make up roughly 10 percent of the pollen from cataloged flowering plant species. The remaining 90 percent sport smooth surfaces or have a foamy appearance. The simulation could explain those patterns, too: They arose if the phase separation process stopped before the primexine fully separated.

SMOOTH OVER Pollen grains from flowering plants can be relatively smooth (one shown in this scanning electron microscope image at left). Computer simulations of pollen formation show that halting a process called phase separation produces pollen that’s smooth rather than bumpy (simulation, right).A. Radja et al/Cell 2019

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