© Lucie RIGLET / RDP / CNRS Images
Pollination of thale cress (arabidopsis thaliana), observed using scanning electron micrsoscopy. Here, the surface of the female reproductive organ, the stigma, which is formed of elongated cells, the stigmatic papillae, which accept all pollen grains that put out a tube to carry male gametes to the ovules, in order to fertilize them and enable seeds to form. Despite encountering hundreds of grains of pollen, the female reproductive organs of plants are able to choose their male partners very quickly. In the case of brassicas (the group that includes cabbages, radishes, rockcress, etc.), this choice depends on a lock-and-key type interaction, the key (a peptide) being introduced by the pollen whereas the lock (a membrane-associated kinase receptor) is present on the plasma membrane of the stigma cells. If the key and the lock are genetically related (being brother and sister, for example), the lock-and-key interaction prompts the stigma to reject the pollen. This mechanism enables the plant to avoid inbreeding.