Synergid cells are a pair of small specialized cells seen next to the egg cell in the embryo sac. They are a part of the egg apparatus. They help with the pollen tube guidance and fertilization.
After pollination, the pollen tube enters the ovule and grows into the embryo sac with the help of chemicals on the stigmatic surface. These chemicals are secreted by the filiform apparatus in the synergids. Thus synergid cells physiologically support pollen tube germination leading to fertilization.
Regardless of the path of pollen germination, its ultimate entry into the embryo sac happens through the micropylar end. Here, it could enter in three different ways.
- Between the egg and synergids
- Between the embryosac wall and one of the synergids
- Direct penetration through one of the synergids
Structure of Synergid Cells
- The two synergid cells in the embryosac have uneven thickening on their walls.
- They either lack a chalazal wall or will eventually lose this wall. Such uneven thickening of the wall facilitates the entry of the pollen tube.
- Synergid cells have a large vacuole near the chalazal end. This causes the nucleus and cytoplasm to migrate towards the micropylar region.
- Synergid cells are biologically active cells having abundant ribosomes, Golgi bodies, mitochondria, etc.
- The microplylar end has finger-like projections called filiform apparatus.
- This filiform apparatus helps move materials in and out of synergids.
- The filiform apparatus is the receptor of the pollen tube from where the tube enters synergids.
- Moreover, mutations in the genes of the filiform apparatus
Mechanism of Pollen Tube Entry
The pollen tube is attracted towards the embryosac by the chemicals secreted by synergids. When the pollen tube comes in contact with the synergid, its growth is stopped. It has reached the filiform apparatus of synergids and passes through it to enter the cytoplasm.
The synergid through which the pollen tube passes will degenerate. This triggers the rupture of the pollen tube and it releases its nucleus into the embryo sac and eventually fertilizes the egg.
This disintegrated synergid loses its vacuole, nuclear membrane, nucleus, and nucleolus and is flattened. Its cell organelles are disorganized and the cytoplasm forms crystals. Such synergid cytoplasm contains some darkly staining bodies called X-bodies. These are formed by the vegetative and synergid nucleus, nucellar cells, and degenerative megaspores.
Functions of Synergid Cells
- Guiding pollen tube towards embryo sac: The high Ca content in the filiform apparatus of synergid attracts the pollen tube towards it. The finger-like projections of filiform increase the membrane surface area generate more Ca+ ions and thicken the wall. The increase in thickness increases the Ca content.
- Induction of synergid degeneration: Pollen tube entry is related to the degeneration of synergids. In some plants, the pollen tube enters a degenerated synergid and in others, the entry of the pollen tube leads to the degeneration of the synergid. In some rare cases, the direct contact of the pollen tube triggers synergid degeneration. In other words, the pollen tube entry leads to the programmed cell death of synergids.
- Regulation of pollen tube growth: The pollen tube that enters the synergid should stop its growth and release the male gametes. The degenerating synergids regulate the growth of the pollen tube and cease it after its entry. Studies suggest that the high Ca content in the synergid cytoplasm is unfavorable for the further growth of pollen tubes.
- Tube aperture formation: The opening in the pollen tube to release the gametes is formed as a result of cytoplasmic components of degenerated synergids.
- Degeneration of male germ unit: The released two male gametes are in contact with each other. To facilitate fertilization the contact between them must be broken. The degenerating synergids aid in the degeneration of this male germ unit as well. It leads to the conclusion that the degenerating synergid causes physiological transformations to ensure successful fertilization.
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