Pollen tubes are slender tubes that emerge from pollen grains following their deposition on a flower’s stigma. These extensions are fundamental to plant reproduction, acting as a direct conduit for male reproductive cells to reach the female ovule. This enables the fertilization process that leads to the development of seeds and fruits.
How Pollen Tubes Form and Grow
The journey of a pollen tube begins when a pollen grain lands on the receptive surface of a stigma. This initial contact triggers the pollen grain to absorb moisture and nutrients from the stigmatic fluid. This absorption causes the pollen grain to swell, and its inner layer, the intine, begins to protrude through a specialized opening in the outer wall, called a germ pore.
The emergence of this protrusion marks the formation of the pollen tube. The pollen tube is an extension of the vegetative cell within the pollen grain. As the tube elongates at its tip, the entire contents of the pollen grain, including the vegetative nucleus and the generative cells, migrate into the tube. The generative cell divides to form two sperm cells. In some plants, a single pollen tube can grow over 12 inches (30 cm) to reach its target.
The Pollen Tube’s Critical Function
The purpose of the pollen tube is to serve as a delivery system for the male gametes, or sperm cells, to the female ovule. These sperm cells are non-motile, making the pollen tube’s directed growth necessary for successful reproduction. Without this tubular pathway, the male gametes would be unable to traverse the distance from the stigma, through the style, to the ovule.
Once the pollen tube reaches the ovule, it penetrates a small opening called the micropyle, and then enters one of the two synergid cells. The pollen tube ruptures, releasing its two sperm cells into the ovule. This release initiates a process unique to flowering plants called double fertilization.
One sperm cell fuses with the egg cell, forming a diploid zygote, which will develop into the embryo. The second sperm cell fuses with two polar nuclei in the central cell of the ovule, forming a triploid cell that develops into the endosperm. The endosperm acts as a nutritive tissue, providing food for the developing embryo. This dual fertilization allows for the formation of seeds and fruits, essential for plant propagation.
How Pollen Tubes Find Their Target
The navigation of the pollen tube to the ovule is orchestrated by chemical signaling. After growing through the style, the pollen tube enters the ovary and begins to respond to specific chemical attractants released by the ovule. These signals originate from the synergid cells within the ovule’s embryo sac.
The chemical signals are small peptides, such as the LURE peptides in Torenia or Zea mays EGG APPARATUS 1 (ZmEA1) in maize, which act as attractants. The pollen tube tip possesses specialized receptors that detect and interpret these chemical cues, guiding its growth towards the micropyle, the entry point of the ovule. This directed growth ensures that the pollen tube successfully navigates through the complex tissues of the pistil, including the funiculus, a stalk-like structure connecting the ovule to the placenta, and finally through the micropyle. Once the pollen tube reaches its target synergid cell, its growth ceases, and the tip ruptures, releasing the sperm cells to accomplish fertilization.