A complicated way of having sex – and not without risks

Boy meets girl. Boy fertilizes girl. Patter of tiny feet. That’s the story of animal passion. But for flowering plants, it’s different. Pollen meets stigma. Pollen germinates, forms a long pollen tube that grows down into the ovary and releases two sperm cells into the female gametophyte. And that’s where it gets interesting.

One sperm cell fertilizes the gametocyte to form the embryo – a new baby plant. The other fuses with another cell to form a triploid nucleus which develops into the endosperm, a nutrient-rich tissue which feeds the developing embryo. This process is called double fertilization, and it’s the way all flowering plants make seeds. It’s a complicated way of having sex, but it sems to work for angiosperms. At least, it has done for more than 200 million years. But what if it goes wrong?

During pollination and pollen tube growth, the male gametophytes are exposed to environmental stress and mutagens such as ultraviolet light and ionizing radiation. What happens if they are damaged? Can their DNA be repaired?

To find out, researchers used a carbon ion beam to irradiate the bicellular pollen of Cyrtanthus mackenii and induce double-strand breaks in the DNA. The dose of radiation used had no inhibitory effect on pollen tube growth, but the cell cycle of the irraidiated pollen grains arrested at the metaphase step. However, the good news was that double-strand DNA breaks in the damaged pollen could be repaired. This is important when stong sunlight is shining on pollen grains exposed on the surface of anthers, the backs of bees or sitting on a stigma during germination. It’s one reason why flowering plants have been successfully getting it on over the last 200 million years. And now we know a little more about how this essential repair process works.

Hirano, T., Takagi, K., Hoshino, Y., and Abe, T. (2013) DNA damage response in male gametes of Cyrtanthus mackenii during pollen tube growth. AoB Plants. 5: plt004 doi: 10.1093/aobpla/plt004

Abstract:
Male gametophytes of plants are exposed to environmental stress and mutagenic agents during the double fertilization process and therefore need to repair the DNA damage in order to transmit the genomic information to the next generation. However, the DNA damage response in male gametes is still unclear. In the present study, we analysed the response to DNA damage in the generative cells of Cyrtanthus mackenii during pollen tube growth. A carbon ion beam, which can induce DNA double-strand breaks (DSBs), was used to irradiate the bicellular pollen, and then the irradiated pollen grains were cultured in a liquid culture medium. The male gametes were isolated from the cultured pollen tubes and used for immunofluorescence analysis. Although inhibitory effects on pollen tube growth were not observed after irradiation, sperm cell formation decreased significantly after high-dose irradiation. After high-dose irradiation, the cell cycle progression of generative cells was arrested at metaphase in pollen mitosis II, and phosphorylated H2AX (γH2AX) foci, an indicator of DSBs, were detected in the majority of the arrested cells. However, these foci were not detected in cells that were past metaphase. Cell cycle progression in irradiated generative cells is regulated by the spindle assembly checkpoint, and modification of the histones surrounding the DSBs was confirmed. These results indicate that during pollen tube growth generative cells can recognize and manage genomic lesions using DNA damage response pathways. In addition, the number of generative cells with γH2AX foci decreased with culture prolongation, suggesting that the DSBs in the generative cells are repaired.