page_title: Pollen tube
section_title: Initiation
hierarchical_section_title: Pollen tube / Mechanism of pollen tube growth / Initiation
caption_attribution_description: Petunias show that the mechanisms behind inbreeding prevention are similar to immune response. This diagram shows non-self pollen fertilizing a petunia and, conversely, self-pollen perishing. The female part of the petunia flower secretes an enzyme that is designed to deter pollen tube growth, thereby preventing fertilization. However, in the cases that the pollen has come from a genetically different plant, the pollen produces its own protein that combats the pistil's enzyme. With the enzyme out of the way, the pollen tube can keep growing and fertilization can occur. To learn more, see the NSF news release. Credit: Zina Deretsky, National Science Foundation Visit NSF’s Multimedia Gallery, at www.nsf.gov/news/mmg, for more images, and for video.
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: Once the pollen grain is recognized and hydrated, the pollen grain germinates to grow a pollen tube. There is competition in this step as many pollen grains may compete to reach the egg. The stigma plays a role in guiding the sperm to a receptive ovule, in the case of many ovules. Only compatible pollen grains are allowed to grow as determined by signaling with the stigma.  
  In the pollen grain, the generative cell gives rise to the sperm, whereas the vegetative cells have a tube cell that grows the pollen tube.  Some plants have mechanisms in place to prevent selfing such as stigma and anther mature at different times or are different lengths, which significantly contributes to increasing genetic diversity of the next generation.
  There is great variation in the rate of growth of pollen tubes and many studies have focused on signaling. The gene expression in the pollen grain has been identified as that of the gametophyte and not of the parental sporophyte, as it expresses its own unique mRNA and own enzymes. In the peach plant, the style environment in which the pollen tube grows through, provides nutrition for the tube's growth to the ovule. Pollen tubes are tolerant and even pollen damaged by X-rays and gamma rays can still grow pollen tubes.
caption: Mechanism to prevent selfing and allowing compatible pollen to grow a pollen tube for fertilization to take place
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