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.
In a developing pollen grain (1), a material called primexine clumps together into regions with different densities (2). Confined inside a temporary cell wall, that lumpy material creates ripples in the cell membrane (3), which remain after the original cell wall dissolves (4).
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.