Question

1. Explain why and how pollen grains may germinate on a plant stigma but fertilisation may not occur.

Answer 1

In angiosperms, Pollination is defined as the placement or transfer of pollen from the another to the stigma of the same flower or another flower..Upon transfer, the pollen germinates to form the pollen tube and the sperm for fertilizing egg.

In recent decades , incompatibility genes - which prevent pollen from germinationg or growing into the stigma of a flower- have been discovered in many angiosperms species. If plant do not have compatible genes, the pollen tube stops growing . Self incompatibility is controlled by the sterlity locus. pollen tubes have to grow through the tissue of stigma and style before they can enter the ovule. The carpel is selective in the type of pollen it allows to grow inside .The interaction is primarly between the pollen and stigma epidermal cells. In some plants , like cabbage, the pollen is rejected at the surface of the stigma , and the unwanted pollen does not germinate. In other plants, pollen tube germination is arrested after growing one-third the length of the style , leading to pollen tube death. pollen tube death is due to either to apoptosis (programmed cell death ) or to degradation of pollen tube RNA. The degradation results from the activity of ribonuclease encoded by the sterlity locus. The ribonuclease is secreted from the cells of style in the extracellular matrix, which lies alongside growing pollen tube.

in summary, self compatibility is a mechanism that prevents self-fertilization in many flowering plant species . The working of the self in-compatibility mechanism has important consequences for pilant breeder because it inhibits the production of inbread and hybrid plants.