Pollen Pistil Interaction

The recognition and acceptance or rejection of pollen by the pistil, from pollen deposition on the stigma to pollen tube entry into the ovule, is called pollen–pistil interaction.

• Pollen grains arrive on the receptive stigma by wind, insects, water, or other agents.
• Pollen grains may be dry or wet. Attachment depends on matching pollen coat / pellicle layers.

• The stigma surface has special proteins and chemical secretions that act as recognition molecules.
• These interact with chemical components on the surface of the pollen grain.
• Compatible pollen → accepted; pistil promotes post-pollination events.
• Incompatible pollen → rejected; pollen germination is prevented, or pollen tube growth in the style is blocked.

• Compatible pollen absorbs water and nutrients from the stigma surface — it hydrates, swells, and germinates.
• The intine (inner wall; cellulose + pectin) grows out through a germ pore to form the pollen tube.
• Contents — tube nucleus (vegetative nucleus) and generative cell — move into the pollen tube.

• The pollen tube grows downward through the style tissues toward the ovary.
• Growth is guided by chemical signals (chemotropism) — secretions from the ovule attract the tube.
• If pollen was shed in two-celled condition (vegetative cell + generative cell): the generative cell divides inside the tube to form two male gametes during tube growth.
• If pollen was shed in three-celled condition: the two male gametes are already inside the tube from the start.

The pollen tube reaches the ovary. Based on how it enters the ovule, there are three types:

Entry of pollen tube into the ovule

• Once in the ovule, the pollen tube enters one of the two synergids through the filiform apparatus.
• The filiform apparatus is a highly thickened, finger-like cell wall structure at the micropylar end of the synergid.
• It guides the entry of the pollen tube into the synergid by secreting chemical substances.
• The penetrated synergid immediately begins to degenerate.

• The tip of the pollen tube enlarges and ruptures, releasing its contents into the embryo sac.
• Contents released: two male gametes + vegetative (tube) nucleus.
• Male Gamete 1 + Egg cell → Zygote (2n) → Syngamy
• Male Gamete 2 + Two Polar Nuclei → Primary Endosperm Nucleus (3n) → Triple Fusion
• Syngamy + Triple Fusion together = Double Fertilisation - a hallmark unique to angiosperms.

• Pollen-pistil interaction is the series of events from pollen landing on the stigma to fertilisation.
• The pistil recognises compatible pollen and rejects incompatible or foreign pollen using specific proteins.
• Compatible pollen germinates on the stigma and forms a pollen tube by absorbing nutrients.
• The pollen tube grows through the style and reaches the ovule, entering through a synergid in the embryo sac.
• The pollen tube releases male gametes, enabling fertilisation and seed formation.