Pollen Tube - Pollen Tube Guidance

Pollen Tube Guidance

Extensive work has been dedicated to comprehend how the pollen tube responds to extracellular guidance signals to achieve fertilization. It is believed that pollen tubes react to a combination of chemical, electrical, and mechanical cues during its journey through the pistil. However, it is not clear how these external cues work or how they are processed internally. Moreover, sensory receptors for any external cue have not been identified yet. Nevertheless, several aspects have already been identified as central in the process of pollen tube growth. The actin filaments in the cytoskeleton, the peculiar cell wall, secretory vesicle dynamics, and the flux of ions, to name a few, are some of the fundamental features readily identified as crucial, but whose role has not yet been completely elucidated.

Pollen tubes, as most biological systems, are influenced by electrical stimulus. Efforts have already been made to clarify the mechanisms of intra- and extracellular electrical signaling in pollen tubes. However, our understanding of how pollen tubes react to electric fields and how the electric cue is related to the internal dynamics of pollen tube growth remains limited. For instance, pollen tubes have been reported to grow towards the negative electrode, positive electrode, and nearest electrode under constant electric fields. Another report states that pollen tubes do not change growth direction under AC electric fields. Although it is believed that the behavior under electric fields may depend on the species, it is not clear how electric fields influence pollen tube growth.

There are also microsystem-based assays to assess and quantify pollen tube behavior. It has been shown that a microsystem-based assay can mimics the in vivo micro-environment of ovule fertilization by pollen tubes in Arabidopsis thaliana. In this microdevice, the pollen tube growth rate, length and ovule targeting frequencies were similar to those obtained using a semi in vivo plate assay. Pollen tubes preferentially enter chambers with unfertilized ovules, suggesting that the pollen tubes sense the concentration gradient and respond to the chemoattractants secreted by unfertilized ovules.