page_title: Pollen tube
section_title: Growth and signaling
hierarchical_section_title: Pollen tube / Mechanism of pollen tube growth / Growth and signaling
caption_attribution_description: English: Schematic representation of an Angiosperm ovule with embryo sac: Egg cell (yellow) Synergids (orange) Central cell with two polar nuclei (bright green) Antipodals (dark green)
context_page_description: A pollen tube is a tubular structure produced by the male gametophyte of seed plants when it germinates. Pollen tube elongation is an integral stage in the plant life cycle. The pollen tube acts as a conduit to transport the male gamete cells from the pollen grain—either from the stigma to the ovules at the base of the pistil or directly through ovule tissue in some gymnosperms. In maize, this single cell can grow longer than 12 inches to traverse the length of the pistil.
  Pollen tubes were first discovered by Giovanni Battista Amici in the 19th century.
  They are used as a model for understanding plant cell behavior. Research is ongoing to comprehend how the pollen tube responds to extracellular guidance signals to achieve fertilization.
context_section_description: Pollen tube growth is influenced by the interaction between the stigma-style and the pollen grain.  The elongation of the tube is achieved with elongation of the cytoskeleton and it extends from the tip, which is regulated by high levels of calcium in the cytosol.  The calcium levels help the synaptic vesicles in the membranes grow and extend at the tip. Polypeptides found in the style also regulate growth of tube and specific peptides that play a role in signaling for growth have been identified.
  The LURE peptides that are secreted from the synergids, which occupy the space adjacent to the egg cell, can use attractants. In mutant Arabidopsis plant embryos, specifically in those without the synergids, the pollen tubes were unable to grow. Pollen tube growth is toward eggs of the same species as the pollen.  Intraspecific signaling helps fertilize egg and sperm of the same species. The signaling in the style is important as pollen tubes can grow without the presence of an embryo sac with just interaction with the style. Other parts in the ovary include cytoplasmic factors like miRNA and chemical gradients that attract the pollen tube to grow toward the synergids.
  Calcium and ethylene in Arabidopsis thaliana  were involved in termination of the pollen tube when it grows near the ovary. The increase in calcium allowed release of the two sperm cells from the tube as well as degeneration of a synergid cell. The chemical gradient of calcium can also contribute to termination early on in tube growth or at the appropriate time.
  The length of the pollen tube varies by species.  It grows in an oscillating fashion until it is ready to release the sperm near the egg for fertilization to take place. Some fast-growing pollen tubes have been observed in lily, tobacco, and Impatiens sultanii. The rate of growth confers advantage to the organism but it is not clear whether the variation in growth rate exists in the population or it has been selected for over generations due to increased fitness.
caption: angiosperm ovule
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