Forging the pollen fortress: Cell biological mechanisms of exine formation

During its development, pollen becomes surrounded by a complex cell wall known as the exine. Exine is preceded by the primexine-a thin, transient extracellular structure essential for the formation of a well-developed exine but challenging to visualize and study. Exine formation requires a partnership between the developing pollen and the inner sporophytic anther layer, the tapetum. The tapetum produces enzymes and materials necessary for exine development, which are delivered to the surface of developing pollen and become assembled into the distinct layers and patterns of exine. However, how exine materials are transported, and how the events occurring in the tapetum and in developing pollen are coordinated, remains poorly understood. This review highlights recent advances in understanding primexine structure and composition, the trafficking of exine materials toward the pollen surface, and the recently discovered communication mechanism involving the tapetum, developing pollen, and the middle layer of the anther.