The Pollen Wall

The most striking feature of the pollen grain is the tough, resistant outer coat termed the exine which is often elaborately sculptured. The exine does not develop over certain regions which define the positions of the germination apertures. Apertures show wide variation including elongate furrows and circular pores or combinations of the two. Aperture number, position and exine ornamentation are useful taxonomic characters and from the fossil record allow the reconstruction of the species composition of past vegetation as well as finding applications in forensic science.

Exine diagram

Exine photographsIn addition to the purely mechanical function of the exine in protecting the reproductive cells from environmental injury, exine sculpturing plays an important role in attachment to insect pollinators and adhesion to the stigmatic surfaces; while wind pollinated species including many grasses and tree species often lack elaborate structure and appear smooth.

Beneath the exine, which is defined by the presence of one or two basal nexine layers, a second major wall layer surrounds the pollen grain protoplasm termed the intine. While the exine is composed of sporopollenin, a complex and highly resistant biopolymer containing fatty acids, phenylpropanoids, phenolics and carotenoids, the intine is largely composed of pectin and cellulose.

The origin and genetic control of the synthesis of the two pollen wall layers is different. The exine is developed through the contribution of the early microspore cytoplasm and the inner anther wall cell layer, the tapetum, which plays a dominant role in the synthesis and deposition of sporopollenin on the pollen grain outer surface. Therefore exine synthesis and patterning are under sporophytic control. In contrast, intine wall synthesis is largely under the control of the microspore cytoplasm and involves gametophytic gene expression from the haploid microspore nucleus.

Surface structure of a pollen grains of Arabidopsis thaliana (thale cress) visualised by scanning electron microscopy. At low magnification (above) the apertural furrows are visible as longitudinal folds in the exine. At high magnification (below) details of the reticulate (tectate) exine patterning is visible.

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