Pollen-pistil interactions encompass the complex series of events that occur after pollination and lead to successful fertilization in flowering plants. This process involves the recognition and communication between pollen (male gametophyte) and the pistil (female reproductive organ), culminating in the delivery of sperm cells to the ovule for fertilization
Pollen-Pistil interactions
These interactions ensure species-specific fertilization and play a key role in preventing inbreeding and promoting genetic diversity.
Let’s explore this process in detail, from pollination to fertilization:
Pollination
Pollination is the transfer of pollen from the anther to the stigma of a flower, either by biotic (insects, birds, mammals) or abiotic (wind, water) means. The pollen must then undergo a series of steps to achieve fertilization:
Pollen Recognition
Once the pollen lands on the stigma, it must be recognized as compatible with the pistil. The stigma’s surface has receptor molecules that can detect compatible or incompatible pollen, depending on its genetic makeup.
Self-Incompatibility (SI)
If the pollen is incompatible (usually due to genetic similarity), it is rejected, preventing self-fertilization. This mechanism helps promote cross-pollination and genetic diversity.
Pollen Hydration and Germination
If the pollen is compatible, it undergoes the following steps:
Hydration
Pollen grains absorb moisture from the stigma, initiating germination. Hydration is crucial for pollen viability and germination.
Pollen Germination
The hydrated pollen grain begins to germinate, forming a pollen tube that grows through the stigma and style toward the ovary. The tube cell within the pollen grain elongates to form the pollen tube.
Pollen Tube Growth (Pollen-Pistil interactions)
Pollen tube growth is a critical phase of pollen-pistil interactions, during which the pollen tube navigates through the pistil to reach the ovule:
Pollen Tube Guidance (Pollen-Pistil interactions)
The pollen tube grows through the extracellular matrix of the style, guided by chemical signals from the pistil. This guidance ensures the pollen tube follows the correct path to the ovary.
Callose Plugs
As the pollen tube grows, it deposits callose plugs at regular intervals to prevent backflow of cytoplasm and maintain turgor pressure, allowing the tube to extend.
Selective Growth
The pollen tube avoids obstacles and responds to directional cues, ensuring it reaches the ovary in a timely manner. This selective growth is driven by chemotropic signals from the pistil and ovule.
Ovule Penetration
Once the pollen tube reaches the ovary, it needs to find and penetrate the ovule to deliver the sperm cells:
Micropyle Entry
The pollen tube finds the micropyle, the small opening in the ovule through which the tube enters. The synergid cells within the embryo sac guide the pollen tube toward the micropyle through chemical signals.
Tube Rupture and Sperm Release
Upon reaching the embryo sac, the pollen tube ruptures, releasing the sperm cells. This process is facilitated by interactions with synergid cells and specific signalling molecules.
