Pollen Pistil interaction , Double fertilization

 

• When pollen is landed on stigma by Pollination pistil gives a response to pollen for the germination of the pollen tube that is entered into the ovule.

• All the events from deposition of pollen on  stigma to entry of the pollen tube into the ovule is referred to as pollen pistil interaction.

• Pollen pistil interaction is a dynamic and chemical dialogue.

• The ability of pistil to recognise the pollen and than its acceptance and rejection is a continous dialogue between pollen and pistil.

• The Dialogue between pollen grain and pistill is mediated by chemical component.

• It is not mandatory that the right pollen grain always landed on stigma. So pistil has the ability to recognize the right type or compatible pollen grains and reject the wrong type or incompatibility of pollen grains .

• If the right type of pollen grains is landed on stigma, pistil gives  response and allows pollen grains for germination.

• Generally when pollen grains are released from anther, it assumes two celled stages having a generative cell and vegetative cell.

• Once pollen grain is landed on stigma, the generative cell divides and form two male gametes.

• Pollen grain which release at 3 celled stage has two male gamete since beginning.

• On stigma, pollen grain  absorbs the water and pistil releases sucrose solution.

• This sucrose solution reaches pollen grains through style and proceeds the process of pollen germination.

• All the events since deposition of pollen grains to entry of pollen tube into ovule is referred as pollen pistil interaction.

Entry of Pollen tube into Embryo sac

• The pollen tube penetrates into stigma by the activity of an enzyme cutinase.

• This enzyme is present in the pollen tube. The pollen tube with two male gametes makes entry into the ovule.

• Pollen tube enters into the ovule through the three routes.

• When a pollen tube makes entry through the micropyle into the ovule, it is called porogamy.

• Porogamy is the common method of entry of pollen tubes.

• In Betula, Casuarina, a pollen tube makes entry through the chalaza into the ovule, it is called chalazogamy.

• In cucurbita and populus, a pollen tube makes entry through the integuments into the ovule, it is called mesogamy.

• Filiform appratus near the synergids guides the entry of pollen tube into embryo sac.

Double fertilization

• After the entry of the pollen tube into the ovule, the tip of the pollen tube dissolves and the content of the pollen tube or Two male gametes, are discharged into cytoplasm of synergids at Embryo sac.

• One male gamete is fused with the egg and forms a diploid zygote.

• This fusion is called syngamy. The other male gamete is fused with two polar nuclei and form a triploid primary endosperm nucleus.

• This is called triple fusion because three haploid nuclei are fused.

• Syngamy and triple fusion both are collectively known as Double fertilization.

• On the other hand, the type of fertilization in which one male gamete is fused with egg and second male gamete is fused with polar nuclei called double fertilization.

• This unique method is observed in Angiosperm and first discovered by the S.G. Nawaschin.