A large portion of plant seeds is endosperm. It has the important task of nourishing the plant embryo during the early stages of its development. In flowering plants, there is a complicated double-fertilisation mechanism that arises among embryos and endosperm. They develop together into mature seeds. The exact process, and the communication between the two parts of the seeds, has been unclear to scientists. Researchers at the Max Planck Institute for Plant Breeding Research and the University of Cologne have, however, isolated a mutant where there is only one single fertilisation. In a recent online edition of the journal Nature Genetics (November 28, 2005) they explain that this single fertilisation, which creates an embryo, also triggers the development of endosperm, even when the central cell where endosperm develops is not fertilised. The ovules of flowering plants are housed in a carpel. Pollen lands on the flower's stigma and forms a pollen tube. It then uses each one of its two sperm cells to fertilise the egg cell, from which the embryo hatches, and the central cell, where the endosperm grows. This double fertilisation is what is special to all flowering plants.
Scientists in Cologne, working with Arp Schnittger, have found a mutant of the plant Arabidopsis thaliana called cdc2. It has an altered pollen. Because of a failed cell division, the cdc2-plants develop pollen that has only one sperm cell instead of two. The researchers have now been exploring the question if whether, under these conditions, fertilisation is possible at all. It turned out that the mutated pollen can survive and even grow into a female partner. Once it has arrived there, the single sperm cell of the cdc2 pollen merges only with the egg cell and not with the central cell. This shows a hierarchy, never before discovered, in the fertilisation process of Arabidopsis.
The scientists made another astounding observation: although the central cell remained unfertilised, it began to develop endosperm. The researchers deduced that shortly after the egg cell was fertilised, a positive signal was sent out to its environment, which appears to be necessary for normal growth of an endosperm. Because the double fertilisation process can be genetically dissected, the existence of this mutant offers new possibilities to learn about the development of endosperm and the embryo in seeds. In the next few months, the researchers hope above all to find out how exactly the signal functions and what chemical reactions are behind it.
"Explaining the mechanism behind double fertilisation in flowering plants and early seed development is particularly interesting in the context of plant breeding," says Arp Schnittger, "because reproduction without fertilisation would be advantageous for many different kinds of breeding."
Stained nucleus of germinating pollen in the wild type (normal case) and in the cdc2 mutant of Arabidopsis thaliana. The pollen lands on the stigma and builds a tube (visible here). The growth of the pollen tube is controlled by what is called the vegetative nucleus (in picture, somewhat larger and more diffuse). The pollen tube then transfers, in the normal case, two sperm cells (bright, smaller nuclei) to the female partner for fertilisation. In the mutant, the pollen is created with only one sperm cell. Nonetheless, the pollen is able to germinate (see picture), grow into the female partner, and eventually effect a single, but equally exact, fertilisation.
Image: Max Planck Institute for Plant Breeding Research/Art Schnittger
 Schnittger research group, University of Cologne
Moritz K Nowack, Paul E Grini, Marc J Jakoby, Marcel Lafos, Csaba Koncz and Arp Schnittger
A positive signal from the fertilization of the egg cell sets off endosperm proliferation in angiosperm embryogenesis
Nature Genetics, online edition, December 2005