Describe the sequence of cellular events that occurs in the pollen tube after it has emerged from the stigma but before it reaches the ovule, including the key signals and structures involved.
Answer
During pollen tube growth, the tube emerges from the stigma, guided by chemical signals from the style, traverses the style with the help of cytoskeletal elements and cell wall remodeling enzymes, then penetrates the transmitting tract, navigates towards the ovule guided by guidance cues such as arabinogalactan proteins and peptides from the ovule, and finally reaches the micropyle where it releases two sperm cells.
Step-by-step solution
- Emergence: The pollen tube buds from the pollen grain at the stigma surface after hydration and germination.
- Guidance in the style: The tube is attracted by chemical signals (e.g., peptide signals) secreted by the style cells, which direct its growth direction.
- Cell wall remodeling: Enzymes such as pectin methylesterases and cellulases modify the cell wall to allow elongation, while actin-myosin machinery provides internal pressure.
- Transmitting tract traversal: The tube enters the transmitting tract where it moves rapidly towards the ovary.
- Ovule guidance: The tube senses ovular cues such as peptides from the synergid cells and responds by altering its growth path toward the micropyle.
- Penetration: Upon reaching the micropyle, the tube penetrates the integument and delivers two sperm cells to the embryo sac.
- Key structures: style, transmitting tract, micropyle, synergids, and the tube itself. Key signals: chemical guidance cues, cytoskeletal elements, cell wall-modifying enzymes.
Common mistake: Overlooking the role of chemical guidance cues from ovule tissues
Concept tested: Pollen tube guidance and fertilization
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