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Building and bypassing plant polyspermy barriers

Figure 1 Fertilization-induced programmed cell death of synergids
The gaseous signal ethylene triggers the formation of a pollen tube block

Synergids play an essential role for the attraction of the pollen tube and mediate the discharge of two sperm from the first pollen tube that arrives in the FG (Amien et al. 2010, Huck et al. 2003, Rotman et al. 2003). This process is accompanied by the programmed cell death (PCD) of the first synergid. Notably, PCD of the second synergid and the concomitant termination of pollen tube attraction, requires the fertilization of both female gametes (Beale et al. 2012, Kasahara et al. 2012, Fig. 1)

A, Pollinated gynoeceum containing ovules. B, Magnification of black box in A: micropylar pole of unfertilized female gametophyte containing synergids (s), egg cell (ec), and central cell (cc). C, reception of the pollen tube triggers PCD of the first synergid. D, fertilization of egg and central cell is required for PCD of the second synergid and the establishment of a pollen tube block. Pollen (A), pollen tube (C, D), sperm cells (D), yellow.

Figure 2 Fertilized ein3 eil1 mutants attract supernumerary pollen tubes. Arrowhead, pollen tube, asterisk, embryo.

We have reported on a novel and unprecedented role of the plant hormone ethylene in coupling fertilization and synergid degeneration: EIN3 and EIL1 are transcription factors mediating the plant response to the gaseous signal ethylene (Chao et al. 1997).  We have shown that ethylene hyposensitive ein3 eil1 double and ein3 single mutants attract supernumerary pollen tubes (Fig. 2), which can be traced back to a PCD defect of the second synergid. Notably, gamete fusion is not affected in these mutants, indicating that fertilization and synergid cell death are coupled by virtue of an ethylene response cascade (Völz et al., 2013).

References

Amien S, et al. (2010) Defensin-like ZmES4 mediates pollen tube burst in maize via opening of the potassium channel KZM1. PLoS Biol8(6):e1000388.

Beale KM, Leydon AR, & Johnson MA (2012) Gamete fusion is required to block multiple pollen tubes from entering an Arabidopsis ovule. Curr Biol22(12):1090-1094.

Chao Q, et al. (1997) Activation of the ethylene gas response pathway in Arabidopsis by the nuclear protein ETHYLENE-INSENSITIVE3 and related proteins. Cell 89(7):1133-1144.

Huck N, Moore JM, Federer M, & Grossniklaus U (2003) The Arabidopsis mutant feronia disrupts the female gametophytic control of pollen tube reception. Development 130(10):2149-2159.

Kasahara RD, et al. (2012) Fertilization recovery after defective sperm cell release in Arabidopsis. Curr Biol 22(12):1084-1089

Rotman N, et al. (2003) Female control of male gamete delivery during fertilization in Arabidopsis thaliana. Curr Biol 13(5):432-436.

Volz R, Heydlauff J, Ripper D, von Lyncker L, & Gross-Hardt R (2013) Ethylene signaling is required for synergid degeneration and the establishment of a pollen tube block. Dev Cell 25(3):310-316

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