Abstract
The central cell of the female gametophyte plays a role in pollen tube guidance and in regulating the initiation of endosperm development. Following fertilization, the central cell gives rise to the seed's endosperm, which nourishes the developing embryo within the seed. The molecular mechanisms controlling specification and differentiation of the central cell are poorly understood. We identified AGL61 in a screen for transcription factor genes expressed in the female gametophyte. AGL61 encodes a Type I MADS domain protein, which likely functions as a transcription factor. Consistent with this, an AGL61-green fluorescent protein fusion protein is localized to the nucleus. In the context of the ovule and seed, AGL61 is expressed exclusively in the central cell and early endosperm. agl61 female gametophytes are affected in the central cell specifically. The morphological defects include an overall reduction in size of the central cell and a reduced or absent central cell vacuole. When fertilized with wild-type pollen, agl61 central cells fail to give rise to endosperm. In addition, synergid- and antipodal-expressed genes are ectopically expressed in agl61 central cells. The expression pattern and mutant phenotype of AGL61 are similar to those of AGL80, suggesting that AGL61 may function as a heterodimer with AGL80 within the central cell; consistent with this, AGL61 and AGL80 interact in yeast two-hybrid assays. Together, these data suggest that AGL61 functions as a transcription factor and controls the expression of downstream genes during central cell development.
Highlights
The central cell of the female gametophyte plays a role in pollen tube guidance and in regulating the initiation of endosperm development
These results suggest that an AGL61-AGL80 heterodimer functions in the central cell to control the expression of downstream genes that are critical for central cell and endosperm development
We harvested ovaries from male sterility1 and determinant infertile1, extracted RNA, and used realtime reverse transcription (RT)-PCR to assay the expression of genes within this gene family. ms1 ovules are normal and dif1 ovules lack female gametophytes (Steffen et al, 2007); genes exhibiting reduced expression in dif1 ovaries relative to ms1 ovaries are likely to be expressed in the female gametophyte
Summary
The central cell of the female gametophyte plays a role in pollen tube guidance and in regulating the initiation of endosperm development. The expression pattern and mutant phenotype of AGL61 are similar to those of AGL80, suggesting that AGL61 may function as a heterodimer with AGL80 within the central cell; consistent with this, AGL61 and AGL80 interact in yeast two-hybrid assays. The central cell expresses a set of genes that represses endosperm development in the absence of fertilization. The FIS proteins are related to the Polycombgroup (PcG) proteins involved in heritable silencing of homeotic gene expression in Drosophila and mammals These observations have led to a model in which the FIS proteins form a complex that represses genes involved in endosperm development within the central cell (for review, see Curtis and Grossniklaus, 2008). Most species including Arabidopsis and cereals undergo the Polygonum pattern of megagametogenesis
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