We are interested in the intra- and intercellular mechanisms underlying the specification of the distinct cell types. To identify factors necessary for the regulation of egg cell fate, we have screened for mutants with deviating egg cell marker gene expression. The first class of mutants identified includes the lachesis (lis), clotho (clo) and atropos (ato) mutants. All three mutants are characterized by the formation of supernumerary egg cells (Fig. 1), which differentiate at the expense of the adjacent synergids and central cell. Additionally, antipodals of mutant gametophytes can adopt a central cell fate. Our data show that all cells in the female gametophyte can differentiate into gametes, and that this gametic competence is repressed by LIS, CLO and ATO (Groß-Hardt et al., 2007, Moll et al., 2008).
GUS expression in wildtype (A) is restricted to the egg cell. By contrast, in lis, clo and ato gametophytes, GUS expression is also detected in synergids and central cell (B-D) (Moll et al., 2008).
Promoter-reporter analysis suggests that CLO and ATO are expressed in all female gametophytic cells. By contrast, in mature female gametophytes pLIS::NLS_GUS is only detected in the egg and the central cell. Making use of an RNAi approach, we could additionally show that egg-cell specific downregulation of LIS affects the development of synergids, central cell and antipodals. Our results thus suggest that the egg cell acts as a signaling center for the differentiation of female gametophytic cells (Völz et al., 2012). The second class of mutants shows no, or reduced, egg cell marker expression, whereas expression of an antipodal cell marker is unchanged (Fig. 2).
A Ovule showing GUS expression in the egg (arrow) and antipodal cell (star). B Ovule with GUS expression in antipodals only (Kägi and Groß-Hardt, 2010).