Extracellular guidance cues and intracellular signaling pathways that direct pollen tube growth

Abstract

Fertilization in flowering plants requires that a pollen tube deliver two sperm to the female gametes, which develop in ovules buried deep within floral tissues. The tube germinates on a receptive stigma and enters the style where it grows rapidly in a nutrient-rich extracellular matrix secreted by cells of the transmitting tract (Lord 2003). Subsequently, it enters the ovary where it continues to grow on the surface of cells while targeting an individual ovule. Inside the ovule, the pollen tube immediately encounters the haploid synergid cells and continues to grow through the filiform apparatus, a spe-cialized cell wall that forms at the basal junction of the two synergids. The journey ends when the tip enters one of the two synergids and bursts. How does the pollen tube navigate these diverse environments within the pistil to reach a precise cellular target? Recently a great deal of progress has been made toward defining the sources of signals that direct specific stages of the pollen tube journey and toward identifying molecules that direct tube growth. However, our understanding of how the tube changes direction of growth in response to signals presented by floral cells along its path is still limited. For example, no pollen tube receptors have been identified for any of the extracellular guidance cues identified thus far and consequently, it has not been possible to assign specific signal transduction pathways linking the floral environment to changes within the pollen tube that cause reorientation of the tip. Here we review the recent progress toward identification of extracellular guidance cues and highlight efforts to understand how the tube perceives and transduces these signals into changes in the direction of its growth. © Springer-Verlag Berlin Heidelberg 2006.