It is now clear that pollen tubes respond to chemotropic factors responsible for guidance in the pistil. Several phases of guidance have been delineated and multiple signals that act over different spatial ranges guide the pollen tube to the ovule. The sources of some of these signals within the pistil have been defined and chemotropic factors have been identified in a variety of angiosperm species using a combination of genetic, biochemical, and molecular approaches. The discovery of these molecules and their characterization marks tremendous progress in our understanding of guidance signals but many questions remain. Few of these molecules have satisfied all of the experimental criteria for a chemotropic guidance factor and none of them have been associated with pollen tube receptors or with the well-described signaling pathways within the tube that regulate tip growth. In addition, there are likely many more signals to discover and it will be very interesting to determine whether the signals defined thus far function in all plant species.
The field is now poised to define signal transduction pathways that link these extracellular guidance cues to intracellular networks that define, extend, and reorient the pollen tube tip. As these mechanisms are established it will be exciting to determine the extent to which they are conserved among different plant species.
Acknowledgements We thank Rebecca Lesiak for creating the figures, Adisorn Chaibang for the micrograph of pollen tubes growing in an Arabidopsis pistil, and Ravishankar Palanivelu for comments on the manuscript.
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