Pollen-Pistil Interaction 

Pollen-pistil interaction refers to all the events that occur from the time of pollen deposition on the stigma until the pollen tube enters the ovule. This process is a form of chemical communication that involves recognising and accepting compatible pollen and rejecting incompatible or foreign pollen. Pollen-pistil diagram 

1.0Steps in Pollen- Pistil Interaction

1. Pollen Landing: The process begins when a pollen grain lands on the stigma's receptive surface. This process is called pollination.
2. Recognition: The stigma, through its surface secretions and distinctive proteins, acts as a type of sieve. It can recognise both chemically and genetically compatible pollen (from the right species and genetically suitable) from incompatible pollen. The pollen and stigma recognise each other by chemical components on their surfaces.
3. Hydration: Once recognised as compatible, the pollen grain absorbs water and nutrients from the stigma. This hydration is essential for the pollen grain to germinate.
4. Pollen Germination: The hydrated pollen grain swells, and the intine (the inner wall of the pollen grain) emerges through one of the germ pores to form a pollen tube. The generative cell within the pollen grain divides to form two male gametes.
5. Pollen Tube Development: After pollination, the pollen tube grows down the style, directed toward the ovary. Pollen tube growth is guided by chemical signals emitted by the ovules, a process called chemotropism. 
6. Entering the Ovule: The pollen tube enters the ovule, carrying the two male gametes. Entry into the ovule ordinarily occurs through the micropyle, a small opening at one end.
7. Fertilisation: The male gametes are discharged into the embryo sac, where one male gamete fuses with the egg cell (syngamy) and the second fuses with the central cell (triple fusion), thus achieving double fertilisation.

2.0Mechanism of Pollen Recognition

The stigma and pollen interact through chemical signalling and surface proteins that help determine compatibility.

• Compatible Pollination:
  The stigma recognises pollen of the same species and supports its germination and tube growth.
• Incompatible Pollination:
  The stigma rejects foreign or self-pollen (depending on the plant’s self-incompatibility system) by preventing germination or tube growth.

This selective recognition prevents unwanted fertilisation and promotes genetic diversity.

3.0Types of Compatibility Systems

1. Self-Compatibility:
   Pollen from the same flower or another flower of the same plant can fertilise the ovule.
2. Self-Incompatibility (SI):
   The stigma or style rejects pollen from the same plant.
3. • Gametophytic SI: Compatibility is determined by the genotype of the pollen grain.
   • Sporophytic SI: Compatibility is determined by the genotype of the parent plant that produced the pollen.

4.0Pollen Tube Growth and Guidance

Once the pollen grain germinates:

• The pollen tube grows through the style tissues, absorbing nutrients and guided by chemical cues released by the ovule.
• The vegetative cell controls tube growth, while the generative cell divides to form two male gametes.
• The tube enters the ovule through the micropyle and releases the male gametes into the embryo sac for fertilisation.

5.0Significance of Pollen-Pistil Interaction

• Ensures species-specific fertilisation
• Maintains genetic variation through selective acceptance or rejection
• Plays a key role in hybridisation and plant breeding
• Allows controlled pollination for desired traits

6.0Factors Influencing Pollen-Pistil Interaction

• Environmental conditions (temperature, humidity)
• Pollen viability and maturity
• Stigma receptivity
• Genetic compatibility between pollen and pistil