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Synergid cells are two specialized cells that lie adjacent to the egg cell in the female gametophyte of angiosperms and play an essential role in pollen tube guidance and function. The term synergid comes from the Greek ''synergos,'' which means ''working together'' and was reportedly coined by Eduard Strasburger, a famous 19th century botanist who was one of the first to note that these cells somehow assist fertilization of the egg (Vesque, 1878; reviewed in Higashiyama, 2002).
After a pollen grain germinates on the surface of the stigma, it forms a pollen tube that grows through the style and is guided through the micropyle (a minute opening at one end of the ovule). The pollen tube grows toward and into one of the synergid cells, which begins to degenerate as the pollen tube enters it (or shortly before). The pollen tube then ceases growth, ruptures, and releases its two sperm cells, which migrate to the egg and central cell, fertilization of which gives rise to the embryo and endosperm, respectively. The synergid cells are necessary for pollen tube guidance in the ovule, as shown by Higashiyama et al. (2001) using a laser ablation technique in Tourenia to selectively remove different cells within the female gametophyte. The synergids are also essential for the cessation of pollen tube growth and release of the sperm cells.
The synergid cell wall forms a highly thickened structure called the filiform apparatus at the micropylar end, consisting of numerous finger-like projections into the synergid cytoplasm. This structure greatly increases the surface area of the plasma membrane in this region, which is also associated with an elaborated endoplasmic reticulum. It is thought that the filiform apparatus mediates the transport of molecules into and out of the synergid cells (Willemse and van Went, 1984; Huang and Russell, 1992).
In this issue of The Plant Cell, Punwani et al. (pages 2557-2568) show that the R2R3 Myb protein MYB98 functions as a transcriptional regulator of genes that are expressed in synergid cells and are required for formation of the filiform apparatus and pollen tube guidance. Kasahara et al. (2005) previously showed that, within the female gametophyte, MYB98 is expressed exclusively in the synergid cells and that...