Biophysics

Facilitated Folding

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Science  14 May 2010:
Vol. 328, Issue 5980, pp. 795
DOI: 10.1126/science.328.5980.795-c

Folding of a lily pollen grain.

CREDIT: KATIFORI ET AL., PROC. NATL. ACAD. SCI. U.S.A. 107, 7635 (2010)

After a pollen grain is released from its anther, it must survive until it arrives at another flower's stigma. To preserve its precious cargo, the wall of the pollen grain folds in on itself to minimize water loss—a process named harmomegathy. The wall consists of a stiff, water-impermeable outer layer—the exine—and a pliant, water-permeable inner layer—the intine—which is exposed only at apertures in the exine. Katifori et al. show how the spatial structure of the pollen wall influences how the grain folds. On the basis of a model in which the bending deformation of the exine is interrupted at apertures and the Gaussian curvature remains constant, the authors numerically simulated pollen grain folding for the single-aperture Lilium longiflorum (lily) and the triple-aperture Euphorbia milii. These folding simulations closely matched those observed via scanning electron microscopy. Varying the aperture design revealed that effective closure to seal the grain required elongated apertures that reached almost to the poles or apertures that spanned the equator. At sufficiently small aperture size, the pollen grain undergoes mirror buckling, which is the lowest-energy solution for folding a closed sphere.

Proc. Natl. Acad. Sci. U.S.A. 107, 7635 (2010).

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