Blazing New Trails (Pollen Tube Guidance in Flowering Plants).

Higher “plants minimize their exposure to pollen from diverse species by precise regulation of flowering time, development of floral organs, and interactions with specific pollinators. These measures do not guarantee success, however, because of the vast quantities of pollen in the environment. Consequently, to ensure that only the appropriate sperm are delivered to the eggs, many plants have evolved elaborate signaling pathways that monitor the pollen as it grows into the interior of the pistil. Signals are exchanged at every step along this pathway, beginning with the recognition of pollen by cells on the pistil surface and continuing as the pollen grains extend polarized projections (pollen tubes) that invade the pistil and migrate to the eggs. As the pollen tubes race toward their targets, these signals guide the deposition of new membrane and cell wall components and the transportation of sperm and other cellular contents to the tip of the growing tube. Whether the signals that direct pollen tube growth come from female tissues or from the pollen tube itself remains unknown; successful navigation most likely depends on the interplay of both male and female signals. These signals could be structural or chemical in nature, and probably comprise a combination of positive cues that guide pollen tubes to the eggs and negative signals that repel other tubes. Recent investigations into pollen tube guidance have brought the answers to some of these questions within reach.