Breeding of Fruit and Plantation Crops

Mechanism of self incompatibility

• Based on the various phenomenon observed during pollination and fertilization it can be grouped into three:
 
1. Pollen stigma interaction
2. Pollen tube style interaction
3. Pollen tube ovule interaction
4. Pollen-stigma interaction
• This interaction occurs just after the pollen grains reach the stigma and generally it prevents pollen germination. In the gametophytic system, stigma surface is plumose having elongated receptive cells and is commonly known as wet stigma. Incompatibility reaction occurs at a later stage.
• There are clear cut serological differences among the pollen grains with different S genotypes and such differences have not been observed in sporophytic system.
• In sporophytic system, stigma is papillate and dry covered with a hydrated layer of proteins known as pellicle.
• There is evidence that the pellicle is involved in incompatibility reaction. There are striking differences in the stigma antigens related to the S allele composition. Within few minutes of reaching the stigmatic surface, the pollen releases exine exudates which are either protein or glycoprotein in nature.
• This exudate induces immediate callose formation in papillae (which are in direct contact with the pollen) of incompatible stigma. Often callose is also deposited on the young protruding pollen tubes preventing any further germination of the pollen. Thus, in the sporophytic system, stigma is the site of incompatibility reaction.
• The incompatibility reaction of pollen is probably due to the deposition of some compounds from anther tapetum on to the pollen exine.

Pollen tube - style interaction

• In most of the gametophytic system, pollen grains germinate and pollen tubes penetrate the stigmatic surface. But, in the incompatible combinations, the growth of pollen tube is retarded within the stigma.

Pollen tube - ovule interaction

• In some cases, pollen tube reaches the ovule and affects fertilization. However, in incompatible combinations, embryo degenerates at early stage of development.