Pollination - Pollen Pistil Interaction

Pollen - Pistil Interaction: 

1) Pollen Grain Structure and Development: 

• Pollen grains are the male gametophytes of seed plants that are required for sexual reproduction.  

• They form in flower anthers by microsporogenesis, in which diploid microsporocytes undergo meiosis to produce haploid microspores.  

• These microspores evolve into adult pollen grains, which have two primary layers: an exterior exine and an inner intine.  

• The mature pollen grain usually contains a generative cell, which divides to produce sperm cells, and a tube cell, which develops into the pollen tube upon germination. This structure enables pollen to successfully transport sperm cells to the ovule for fertilisation. 

2) Pistil Structure and Function: 

The pistil, or female reproductive portion of a flower, is usually found in the centre. It is made up of three primary parts: the stigma, the style, and the ovary. 

Stigma: It is the sticky or feathery top portion of the pistil that captures and holds pollen grains. 

Style: A slender stalk that joins the stigma and the ovary, allowing pollen tubes to grow towards the ovary. 

Ovary: The expanded basal region that contain ovules, which mature into seeds when fertilized. 

Function: The pistil enables pollination and fertilization. The stigma receives pollen, which germinates and produces a pollen tube that goes through the style and into the ovary. Once the pollen tube reaches an ovule, sperm cells are released to fertilize the egg, resulting in seed growth and, eventually, fruit creation. 

3) Pollination Mechanisms: 

• There are many techniques of pollination. These techniques include anemophily, zoophily, hydrophily, etc. Each mechanism promotes effective fertilization by facilitating the transfer of pollen to receptive stigmas. 

4) Artificial hybridization: It is one of the major approaches used in crop improvement. Only the desired pollen grains are hand pollinated and used for fertilization. This is accomplished through emasculation and bagging procedure. 

5) Recognition and Rejection Mechanisms: 

• The recognition and rejection processes in pollen-pistil interaction ensure that only suitable pollen fertilizes the ovules. The pistil recognizes suitable pollen using unique biochemical signals, resulting in pollen tube expansion.  

• In incompatible relationships, the pistil activates rejection mechanisms, such as pollen tube inhibition or destruction, which prevents fertilization. These systems maintain species integrity and inhibit self-fertilization, hence increasing genetic variety. 

6) Self-Incompatibility and Pollen-Pistil Compatibility: 

• Self-incompatibility is a genetic process in flowering plants that prevents self-fertilization while promoting cross-pollination, which increases genetic variety. The pistil recognizes and rejects pollen from the same plant or individuals with similar genetics. 
• Pollen-pistil compatibility ensures that only compatible, genetically distinct pollen germinates and fertilizes the ovule. This contact is regulated by genes in both the pollen and the pistil that recognize and accept or reject pollen. Successful pollen-pistil compatibility promotes pollen tube growth and fertilization, whereas incompatibility inhibits these activities  

Entry of pollen tube into the ovule