A guided tour: Pollen tube orientation in flowering plants

Sexual reproduction in flowering plants requires that two sperm cells are delivered to the embryo sac where double fertilization of an egg cell and of a central cell results in the formation of a diploid embryo and of the triploid nutritional endosperm tissue. The immobile male gametes are delivered to the immobile female gametophyte by a single cell, the pollen tube. The pollen tube must be able to germinate on a genetically appropriate stigma and it must be directed through the transmitting tract of the style from where it must target an ovule. Moreover, the pollen tube must enter the ovule at a defined opening, the micropyle, grow toward one of the two synergids and release the two sperm cells upon contact. This complex process requires recognition events with chemically based or physically supported cell-cell communication as well as directional cues for the growing pollen tube. A number of molecules and mechanisms have been implicated in pollen tube guidance which are summarized in this review.