MYB98Is Required for Pollen Tube Guidance and Synergid Cell Differentiation inArabidopsis 

Abstract

The synergid cells of the female gametophyte play a role in many steps of the angiosperm fertilization process, including guidance of pollen tube growth to the female gametophyte. However, the mechanisms by which the synergid cells become specified and develop their unique features during female gametophyte development are not understood. We identified MYB98 in a screen for Arabidopsis thaliana genes expressed in the female gametophyte. MYB98 is a member of the R2R3-MYB gene family, the members of which likely encode transcription factors. In the context of the ovule, MYB98 is expressed exclusively in the synergid cells, and mutations in this gene affect the female gametophyte specifically. myb98 female gametophytes are affected in two unique features of the synergid cell, pollen tube guidance and the filiform apparatus, but are otherwise normal. MYB98 also is expressed in trichomes and endosperm. Homozygous myb98 mutants exhibit no sporophytic defects, including trichome and endosperm defects. Together, these data suggest that MYB98 controls the development of specific features within the synergid cell during female gametophyte development.

Species Preferentiality of the Pollen Tube Attractant Derived from the Synergid Cell ofTorenia fournieri

“…Analysis of genes specifically expressed in the synergid cell will provide insight into the molecular mechanism of chemoattraction, including the synthesis and secretion of attractants. For example, the MYB98 gene of Arabidopsis is specifically expressed in the synergid cell and governs pollen tube guidance and the formation of the filiform apparatus, which appears to be important for secretion of the chemoattractant (Kasahara et al, 2005).…”

“…Once attracted, pollen tubes of T. fournieri never leave the embryo sac and form narrow coils on the surface of the micropylar end of the embryo sac before entering the sac, suggesting that the pollen tube is trapped at the highest concentration of the chemoattractant. In Arabidopsis, the target embryo sac itself guides the final steps of the pollen tube into the ovary (Hü lskamp et al, 1995;Ray et al, 1997;Shimizu and Okada, 2000), and use of the mutant line myb98, which is defective in synergid cell development, revealed that the last guidance toward the micropyle is provided by the synergid cell (Kasahara et al, 2005).…”

NUCLEAR FUSION DEFECTIVE1 Encodes the Arabidopsis RPL21M Protein and Is Required for Karyogamy during Female Gametophyte Development and Fertilization

“…TEM analysis was performed as described previously (Kasahara et al, 2005). Three nfd1 female gametophytes at the terminal developmental stage were analyzed.…”

Pollen Tube Guidance by Attractant Molecules: LUREs

“…Micropylar guidance, however, has been suggested to be governed by the synergid cell. MYB98, a synergid cellspecific transcription factor, was shown to be necessary for both the organization of the filiform apparatus of the synergid cell, which is a cell-wall structure positioned as a gate, and micropylar pollen-tube guidance (Kasahara et al, 2005;Punwani and Drews, 2008). The synergid cell of Arabidopsis is likely to emit some diffusible signal because, in an in vitro system, pollen tubes cultured on medium are attracted to the micropyle of the ovule (Palanivelu and Preuss, 2006).…”

“…A semi-in vitro system using Torenia fournieri, in which the embryo sac protrudes from the micropyle of the ovule, showed that the female gametophyte clearly attracted the pollen tubes using a diffusible signal (Higashiyama et al, 1998;Higashiyama and Hamamura, 2008). The two synergid cells, which are on either side of the egg cell, proved to be the origin of the signal, as identified by a laser ablation experiment in the semi-in vitro Torenia system (Higashiyama et al, 2001), and later in the myb98 mutant of Arabidopsis (Kasahara et al, 2005). This attractant(s) derived from the synergid cell is thought to be the true attractant.…”