Explain Pollen Pistil Interaction

Pollen pistil interaction is a significant step that occurs immediately after pollination. This step helps facilitate fertilization. The stigmatic surface of the pistil receives the pollen grains after pollination. 

The pollen grains require an ideal biological and chemical environment for their germination and growing pollen tube. The stigma provides the necessary conditions for pollen germination where there will be several chemical changes or exudates. 

Chemical Changes in Stigma

• Stigmatic ground tissue consists of an abundant amount of carbohydrates that are used as respiratory substrates. They help maintain the osmotic pressure as well.
• Inorganic components such as calcium and boric acid also create pressure inside.
• Enzymes such as pectinase, callose, and cellulose in stigmatic tissue help dissolve the organic substances of pollen.

Role of Pistil

• Stigma exudes water-containing substances in which pollen germination happens.
• The exudates contain lipids and phenolic compounds as well.
• Most plant exudates are wet but cotton plant has a dry stigma with no exudates.
• The cutinase enzyme dissolves the cuticle on the stigmatic surface to allow the penetration of the pollen tube.
• In most plants, pollen germination happens on the stigmatic surface but in some, this process happens in the anther lobe.
• IRBs or Interspecific reproductive barriers in self-incompatible plants such as Solanum lycopersicon are secreted by the pistil to prevent self-pollination.

Thus, the pistil provides the necessary conditions that favor pollen germination. This pollen pistil interaction is essential for the penetration of the pollen tube through the stylar canal and to reach the ovary where fertilization occurs. 

Entry of Pollen Tube into the Ovule

Germination of pollen and the entry of the pollen tube into the ovule are the main events after pollination. Pollen tube germination is a result of the pollen pistil interaction. The pollen pistil interaction involves chemical communications that help with the tube germination and its entry into the ovule. The pollen grain starts germination by absorbing the chemicals secreted by stigma cells.

The germinating tube grows through different surfaces depending on the plant. In Gossypium, it is through the papillae of the stigma. In others, it might happen through the cellulose pectic layers of their walls. 

From the base of the papillae, the pollen tube goes through the intercellular spaces. The next phase of the pollen tube is through the stylar canal where it passes through transmitting tissues. Depending on the transmitting tissues, styles can be open, half-closed, or closed. 

• Open type of style has a wider canal with proper side walls and epidermis that provides nutrition to the germinating pollen tube.
• Half-closed style has a stylar canal partially filled with transmitting tissue in multiple layers., these calls are nutritive in function.
• Closed type of style has its canal filled with glandular cells, through which the pollen tube passes through. In some grasses, the closed styles are filled with tissues other than glandular or transmitting tissues.

Canal Cells

Some open styles have dome-shaped canal cells lining their canal, through which the pollen tube passes. Since these cells are secretory in function this zone is called the secretory zone. The cells of the secretory zone have three regions,

• Outer fibrillar layer
• Middle glandular layer has granules of pectin and proteins
• Irregular interphase lying in between the glandular layer and cytoplasm. This region has plenty of vesicles and tubular called paramular bodies.

Some plants have mucilaginous parenchyma cells beneath the secretory zone that nourishes the pollen tube. 

Later, Strasburger opined that the passage of the pollen tube is facilitated by secretion from the ovule. The three factors that regulate pollen growth are mechanical, chemical, and hydrotropic. It is found that the chemicals secreted by the ovary attract pollen tubes towards the ovary or embryo sac. Thus, the pollen pistil interaction is a significant process that facilitates fertilization.

References

• Sukumaran O R. Pre-Degree Botany. Murali Publications.
• Abraham P C. Anatomy, Embryology & Microtechnique. 1999. St. Mary’s Books & Publications.
• Paper – V – Anatomy and Embryology Unit – 4
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9963197/

Additional Reading

• Types of Embryo Sac in Angiosperms
• Write a Short Note on Synergid Cells