01 Reproduction in Lower and Higher Plants - part 07 - Pollen - Pistil Interaction and Double Fertilization

01 Reproduction in Lower and Higher Plants - part 07 - Pollen - Pistil Interaction and Double Fertilization

Pollen - Pistil Interaction : 

• It is the interaction of pollen grains with sporophytic tissue (stigma). 
• It begins with pollination and ends with fertilization. 
• All the events from the deposition of pollen grain on stigma to the entry of pollen tube in the ovule (synergid) are referred as pollen - pistil interaction.
•  Pollination does not guarantee the transfer of right type of pollen, often wrong type also land on stigma. 
• The pistil has the ability to recognise and accept the right or compatible pollen of the same species. Thus wrong type of pollen is discarded by pistil. 
• Compatibility and incompatibility of the pollen-pistil is determined by special proteins. 
• This process involves pollen recognition followed by promotion or inhibition of pollen. 
• The stigmatic surface of flower refuse other wrong type or incompatible pollen grains.
•  A physiologial mechanism operates to ensure that only intraspecific pollen germinate successfully. 
• The compatible pollen absorbs water and nutrients from the surface of stigma, germinates and produces pollen tube.
•  Its growth through the style is determined by specific chemicals. 
• The stigmatic surface provides the essential prerequisites for a successful germination, which are absent in the pollen. 
• The pollen tube is finally pushed through the ovule and reaches the embryo sac. 
• The tip of the pollen tube enters in one of the synergids and then ruptures to release the contents. 
• Due to pollen pistil interaction, intense competition develops even in the compatible pollen grains (gametes). 
• It also plays important role in sexual reproduction and seed formation. 
• Pollen grain can also be induced to germinate in a synthetic medium. 
• Sucrose induces pollen germination and tube growth in vitro. 
• Addition of boric acid facilitates and accelarates pollen germination.
  

Artificial hybridization : 

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

Double Fertilization :

• Double fertilization is a complex  fertilization mechanism in flowering (angiospermic) plants. 
• It was discovered by Nawaschin in the liliaceous plants like Lilium  and Fritillaria.
• After a pollen grain has reached the surface of the stigma, it germinates and forms a pollen . tube, which penetrates the stigma, style, ovary chamber and then enters ovule. 
• The growth of pollen tube is guided by the chemicals secreted by the synergids. It usually enters ovule through  micropyle. It is termed as porogamy. 
• But in some cases, it is found to enter through chalaza,known as chalazogamy and in some plants by piercing the integuments, called mesogamy.
• Finally, it penetrates embryo sac of ovule  through its micropylar end.
• The pollen tube carrying male gametes penetrates in one of the synergids. 
• Watery contents of synergid are absorbed by pollen tube which then ruptures and release the contents, including the two non-motile male gametes. 
  
• As non motile male gametes are carried through hollow pollen tube, it is known as siphonogamy that ensures fertilization to  take place.
• Syngamy and triple fusion are two  events of sexual reproduction in angiospermic flowering plants. 
• Syngamy is the fusion of  haploid male gamete with haploid female  gamete (egg) to produce a diploid zygote.
• Triple fusion, second haploid male gamete fuses with diploid secondary nucleus  producing primary endosperm nucleus (PEN)that developes into triploid endosperm. 
• The zygote develops into an embryo. 
• Syngamy is  a type of generative fertilization whereas triple  fusion is a type of vegetative fertilization. 
• Here, both the male gametes participate  and therefore, it is described as or called double  fertilization

Significance of Double Fertilization : · 

• It is a unique feature of angiosperms. 
• It  ensures that the parent plant invests a  seed with a food store, only if the egg is  fertilized.· 
• The diploid zygote develops into an embryo which consequently develops into a new  plant.
• The triploid PEN develops into nutritive endosperm tissue. 
• It restores the diploid condition by fusion of haploid male gamete with haploid female gamete (i.e. through syngamy). 
• It also helps to avoid polyembryony.