Pollen tube targeting and axon guidance: parallels in tip growth mechanisms

The growth of pollen tubes to plant egg cells and the guidance of axons to neural synapses are classic examples of targeted cell growth. Despite the e volutionary time that separates animals and plants, axon and pollen tube gu idance share remarkable mechanistic similarities. In both instances, extrac ellular cues are transduced by intracellular signal-transduction pathways t hat culminate in directed tip growth. Do the mechanistic similarities exten d to the molecular level? Here, we address this question by a comprehensive review of the molecules and pathways involved in pollen tube targeting and axon guidance. The emerging scenario is Mat similar intracellular molecule s are recruited to control tip growth, while different extracellular molecu les mediate guidance through the distinct plant and animal extracellular ma trices.