Principles of Plant Breeding (2+1)

Mechanism of self incompatibility

• The various phenomena observed in Self Incompatibility is grouped into three categories.

  a) Pollen - Stigma interaction
  b) Pollen tube - Style interaction.
  c) Pollen tube - Ovule interaction.

a) Pollen - Stigma interaction:
This occurs just after the pollen grains reach the stigma and generally prevents pollen from germination. Prviously it was thought that binucleate condition of pollen in gamatophytic system and trinucleate condition in sporophytic system was the reason for self incompatability. But later on it was observed that they are not the reason for S1. Under homomorphic system of incompatability there are differences in the stigmatic surface which prevents pollen germination. In gametophytic system the stigma surface is plumose having elongated receptive cells which is commonly known as wet stigma. The pollen grain germinates on reaching the stigma and incompatability reaction occurs at a later stage.
In the sporophytic system the stigma is papillate and dry and covered with hydrated layer of protein known as pellicle. This pellicle is involved in incompatability reaction. With in few minutes of reaching the stigmatic surface the pollen releases an exinceexcudate which is either protein or glycero protein. This reacts with pellicel and induces callose formation which further prevents the growth of pollen tube.

b) Pollen Tube - Style interaction :
Pollen grains germinate and Pollen tube penetrates the stigmatic surface. But in incompatible combinations the growth of pollen tube is retarded with in the style as in Petunia, Lycopersicon, Lilium. The protein and poly saccharine synthesis in the pollen tube stops resulting in bursting up of pollen tube and leading to death of nuclei.

c)Pollen tube - Ovule interaction :
In Theobroma cacao pollen tube reaches the ovule and fertilisation occurs but the embryo degenerates later due to some biochemical reaction.