Exine and Aperture Patterns on the Pollen Surface: Their Formation and Roles in Plant Reproduction

Abstract Pollen grains, the male gametophytes of seed plants, surround themselves with a complex pollen wall for protection from various environmental stresses. The deposition and assembly of exine, the outer layer of the pollen wall, lead to the formation of patterns on the pollen surface that are species specific, tremendously diverse, and often very beautiful. These patterns arise due to exine's assembly into various nano‐ and microstructures, and due to the absence of exine deposition at certain areas of the pollen surface. The areas that have reduced exine deposition, or lack it completely, are known as pollen apertures, and their patterns are also species specific and highly variable. Although the intricate patterns of exine and apertures have been drawing attention for centuries, it is still not clear how exactly they develop, what genes are involved in their formation, and what purpose they serve. Here, we review the current state of knowledge about the exine and aperture patterns, their perceived roles in plant reproduction, and the cellular and molecular mechanisms that guide their formation.