Plant Reproduction & the Pollen Tube Journey-How the Females Lure the Males
Lorbiecke, Rene, The American Biology Teacher
Sexual reproduction is one of the most important features in the life cycle of higher plants. In contrast to animals, plant fertilization is based on the interplay between multicellular haploid gametophytes. The life cycle of land plants alternates between a diploid sporophytic generation and a haploid gametophytic generation. The sporophyte of flowering plants consists of what is generally recognized as "the plant." By contrast, the gametophytic generation arises by meiosis and is tremendously reduced to only a few sex-specific cells that develop in specialized reproductive organs of the flower.
The male gametophyte equals the mature pollen grain that arises in the anther and consists of only three cells (i.e., two sperm cells and a vegetative cell). The female gametophyte corresponds to the embryo sac and covers an egg cell, a central cell, two synergid cells, and several antipodal cells. Embryo sacs are sheltered inside the ovules, which are, in turn, enclosed by the ovary of the pistil (Figure 1). Because embryo sacs are embedded quite deep inside the female reproductive structures, direct fertilization of the egg cell by the immotile sperm cells is prevented. Therefore, flowering plants have evolved a unique and complex fertilization procedure that culminates in double fertilization of egg and central cell and is followed by seed development.
Engagement in flowering plants starts when the pollen is delivered to the stigma of the style. This is achieved by a wide range of mechanisms, such as wind, insects, or self-pollination. After a pollen grain lands on a receptive stigma of a pistil, a pollen tube germinates from the hydrated pollen grain and delivers the immotile sperm cells over a long distance toward the ovule. A pollen tube is a tip-growing cell that grows surprisingly fast. In maize, pollen tubes can grow through a style of as much as 50 cm length at rates close to 1 cm/hour (Barnabas & Fridvalszky, 1984). Similar to axons in the development of the nervous system, the directional growth of pollen tubes is controlled by multifarious interactions with the sporophytic pistil and the female gametophyte (Okuda & Higashiyama, 2010). On its way, the pollen tube penetrates the stigmatic surface, is then guided through the stigma and style and placenta, grows onto the funiculus, and finally enters the micropyle of the ovule for double fertilization (Figures 1 and 3A). This process is known as pollen tube guidance and is based on astonishing molecular communication systems between the pollen tube and the female reproductive structures (reviewed in Shimizu & Okada, 2000; Chapman & Goring, 2010; Chae & Lord, 2011).
* Learning Objectives
This activity aims to prompt students' curiosity toward plant reproduction and provide greater insight into the "real in vivo situation" of this elementary and fascinating process. Though a number of existing teaching activities deal with in vitro pollen tube growth on medium, didactically prepared visualization of pollen tube growth in vivo is more ambitious and usually needs fixed material and advanced equipment (e.g., fluorescence microscopy).
The students set up a semi-in vivo experiment with explants of Fast Plant flowers. The students themselves prepare the growth medium and the plates, dissect and manipulate the flower organs, and see how the pollen tubes penetrate the female pistil. The aim of doing the experiment and evaluating the results is to
* increase students' knowledge about flower anatomy and sexual reproduction of higher plants,
* clarify the beneficial effect of maternal sporophytic tissue and ovules to trigger pollen tube growth,
* demonstrate pollen tube guidance as an example of molecular cell-to-cell communication, and
* improve students' ability to interpret and discuss the outcomes of a biological experiment.
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The extentions of the activity can be used to teach soft-skill competences and the understanding of science process skills such as inquiry and experimental design. …