A complicated way of having sex — and not without risks

Cyrtanthus mackenii 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

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.

AJ Cann. ORCID 0000-0002-9014-3720

Alan Cann is a Senior Lecturer in the School of Biological Sciences at the University of Leicester and formerly Internet Consulting Editor for AoB.