Male–Female Crosstalk during Pollen Germination, Tube Growth and Guidance, and Double Fertilization  Thomas Dresselhaus, Noni Franklin-Tong  Molecular Plant  Volume 6, Issue 4, Pages 1018-1036 (July 2013) DOI: 10.1093/mp/sst061 Copyright © 2013 The Authors. All rights reserved. Terms and Conditions

Figure 1 The Journey of the Pollen Tube and Signaling Events Involved in Tube Reception. The cartoon shows a generalized flower, with the major stages of pollen interactions with the pistil. Pollen grains (yellow) are shed from the anthers and land on a suitable stigma. They adhere, hydrate, and germinate on the stigma (Phase I). Pollen tubes (yellow) invade the stigma (Phase II) and travel during Phase III, using tip growth, through the pistil tissue transmitting tract (TT, pale pink), which secretes the extracellular matrix (ECM). During the final stages, it negotiates various layers of tissues comprising the ovary (pale green) in Phase IV and is guided to the micropyle, helped by signals from the synergid cells (Phase V). It enters the female gametophyte (green) and interacts with the egg apparatus, which comprises the egg (turquoise) and two synergid cells (orange) in most angiosperms. Here, growth is arrested and the pollen tube tip bursts. This releases the two sperm cells, which can perform double fertilization with the egg cell and central cell (dark pink). Sperm cell nuclei are indicated in red. To the right, the colored boxes indicate the various phases (Phases I–V) of the pollen tube pathway, with some of the female and male components identified as being involved in regulating pistil–pollen interactions at particular stages. Adapted from Franklin-Tong (2002). Molecular Plant 2013 6, 1018-1036DOI: (10.1093/mp/sst061) Copyright © 2013 The Authors. All rights reserved. Terms and Conditions

Figure 2 Model Showing Male–Female Crosstalk during the Last Stages of the Pollen Tube Journey. Pollen tubes (in yellow) transport the male germ unit (two sperm cells associated with the vegetative tube nucleus). Sperm nuclei are drawn in red. (A) Ovular pollen tube guidance: small highly diffusible signaling molecules (e.g. GABA, D-serine, and NO; drawn as blue dots) are generated by ovule tissues, especially the tips of the integuments, to trigger exit of pollen tubes from ovary tissues towards the micropylar region of the ovule. Integument tips express corresponding biosynthesis genes like SR1 and POP2 (indicated in blue). (B) Micropylar guidance (ovary tissue removed): less diffusable polymorphic peptides/small proteins (LUREs, EA1; orange dots) are secreted mainly by the synergid cells to guide pollen tubes inside the female gametophyte. (C) Pollen tube perception includes induction of synergid cell death, pollen tube burst (stimulated by ES4; green dots), and sperm delivery at the female gamete boundary. Sperm cells are activated by EC1 (yellow dots) and repulsion signaling is initiated (unknown repulsion signals, small violet dots). Note that receptive synergid and pollen tube cell nuclei degenerate. (D) Prevention of polyspermy: successful fertilization of egg and central cell induces death of the second synergid cell (indicated by arrows to skull and crossbones), which removes the micropylar pollen tube guidance cues. Repulsion signaling superimposes remaining pollen tube attraction and burst molecules, thus preventing attraction of secondary pollen tubes and sperm release. APs, antipodal cells; CC, central cell; dSY, degenerated synergid cell; EC, egg cell; II, inner integument; OI, outer integument; SY, synergid cell; Z, zygote. Molecular Plant 2013 6, 1018-1036DOI: (10.1093/mp/sst061) Copyright © 2013 The Authors. All rights reserved. Terms and Conditions