Paper of the month (August 2006)

A positive signal from the fertilization of the egg cell sets off endosperm proliferation in angiosperm embryogenesis

Nowack MK, Grini PE, Jakobyl MJ, Lafos M, Koncz C and

Schnittger A*

* Unigruppe am Max-Planck-Institu fur zuchtungsforschung, Max- Delbruck-Laboratorium, Lehrstuhl fur Botanik III, Universitat Koln, Carl-von-Linne-Weg 10, D-50829 Koln, Germany.

Nature Genetics 38: 63 - 67.

Abstract: Double fertilization of the egg cell and the central cell by one sperm cell each produces the diploid embryo and the typically triploid endosperm and is one of the defining characteristics of flowering plants (angiosperms). Endosperm and embryo develop in parallel to form the mature seed, but little is known about the coordination between these two organisms. We characterized a mutation of the Arabidopsis thaliana Cdc2 homolog CDC2A (also called CDKA;1), which has a paternal effect. In cdc2a mutant pollen, only one sperm cell, instead of two, is produced. Mutant pollen is viable but can fertilize only one cell in the embryo sac, allowing for a genetic dissection of the double fertilization process. We observed exclusive fertilization of the egg cell by cdc2a sperm cells. Moreover, we found that unfertilized endosperm developed, suggesting that a previously unrecognized positive signal from the fertilization of the egg cell initiates proliferation of the central cell.

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My comments: This fascinating article reports on the effects of cdc2a mutation in Arabidopsis. The mutation causes mature pollen grain to contain only one sperm (see image from the article which was also our "image of the month") and suprisingly this "one sperm" containing pollen is not only viable, it germinates properly, reaches the embryo sac and fertilizes the egg cell. Based on the ratios reported in the paper, it seems that the mutant pollen germination, growth and guidance is comparable to that of a wild type.

When there is only one sperm, at the time of double fertilization, it raises an interesting situation: which cell would it fertilize–the egg cell or the central cell? The authors performed some elegant experiments using CDC2A promoter driven GUS expression constructs to show that in a wild type condition, both sperm show CDC2A expression and after fertilization CDC2A expression is seen in both the egg and central cells. However, in the mutant the GUS reporter activity was exclusively seen in the egg cell. Based on this they conclude that in nearly every case the single mutant sperm always fertilized the egg cell, probably because the egg cell is closer to sperm delivery (in synergid cell) than the central cell.

The second unique and interersting phenotype was that even though only one fertilization event occurs (zygote formation by fusion of egg and sperm cell), the endosperm begins to undergo cell division and proliferates even though it was not fertilized. This reveals for the first time that a positive signal from the egg gives the OK signal for central cell (in case of wild type it will be endosperm) to proliferate. The next question is what happens to the fertilized egg? Does it go on to become a fuctional embryo? The authors show that the embryo undergoes few more mitotic divisions but then arrests at the globular stage leading to a dead seed, explaining how they could not obtain a cdc2a homozygote to begin with. Their results also raises some other interesting questions and answers for which only further studies will be able to provide. First, is there no surviellance mechanism to ensure central cell is actually fertilized before it is allowed to proliferate? Second, is this positive signal from a fertilized egg acts upstream of relieving the suppression of central cell proliferation (for example, FIS group of genes)?

The authors report a final set of elegant and clever experiments to show that an underdeveloped endosperm is the reason why the cdc2a/cdc2a embryo aborts. For this they show that a fis1 mutation (which leads to premature endosperm development independent of fertilization) can partially rescue cdc2a phenotype. This also seem to suggest, I think, that FIS genes act downstream of the positive signal from egg.

This mutation now offers a new entry point to dissect the double fertilization process, a hallmark and defining event in angiosperms. The elegant experiments, clever use of the cdc2a mutation phenotype (ie. instead of focusing on what CDC2A does, they use it as a tool to dissect double fertilization) to reveal several seminal observations about a novel signal that coordinate egg and endosperm development, made this our paper of the month.

-Ravi Palanivelu, 08/02/2006.

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