Egg Cell Signaling by the Secreted Peptide ZmEAL1 Controls Antipodal Cell Fate

Abstract

Unlike in animals, female gametes of flowering plants are not the direct products of meiosis but develop from a functional megaspore after three rounds of free mitotic divisions. After nuclei migration and positioning, the eight-nucleate syncytium differentiates into the embryo sac, which contains two female gametes as well as accessory cells at the micropylar and chalazal pole, respectively. We report that an egg-cell-specific gene, ZmEAL1, is activated at the micropylar pole of the eight-nucleate syncytium. ZmEAL1 translation is restricted to the egg cell, resulting in the generation of peptide-containing vesicles directed toward its chalazal pole. RNAi knockdown studies show that ZmEAL1 is required for robust expression of the proliferation-regulatory gene IG1 at the chalazal pole of the embryo sac in antipodal cells. We further show that ZmEAL1 is required to prevent antipodal cells from adopting central cell fate. These findings show how egg cells orchestrate differentiation of the embryo sac.

Theoretical and experimental evidence indicates that there is no detectable auxin gradient in the angiosperm female gametophyte

“…Such an effect would be mediated by a hypothetical auxin-dependent NCAS, which is produced in sporophytic cells and controls cell specification in the FG. For instance, the peptide ZmEAL1 secreted from the egg cell has recently been shown to regulate cell fate within the maize FG in a non-cell-autonomous manner (Krohn et al, 2012). Similarly, peptides controlling cell fate in the FG could be secreted by surrounding nucellar cells.…”

Discovery of novel transcripts and gametophytic functions via RNA-seq analysis of maize gametophytic transcriptomes

“…This family is characterized by an EA1 box near the carboxyl terminus [59]. Three additional small peptide EAL genes have been described: ZMEAL1 (transcript maps upstream of and includes GRMZM2G576769) [64], ZMEAL2 (GRMZM2G157505) and GRMZM2G180950. BLAST querying for other small peptide ZmEA1 homologs in the B73 genome identified six additional genes ( Figure 6).…”

“…E, embryo sac expression; O, ovule without embryo sac expression; S, seedling expression; P, mature pollen expression. Rice genes from Krohn et al [64] are included for comparison. Posterior probability values are given at node positions.…”

Germline-Specific MATH-BTB Substrate Adaptor MAB1 Regulates Spindle Length and Nuclei Identity in Maize

“…However, the observations that MAB1-EGFP fluorescence cannot be detected during male and female meiosis and gametogenesis in maize and strongly accumulates in MG132-treated cells indicate that MAB1 is an unstable protein that is strongly regulated at the protein level itself and thus probably required only in minor amounts at short and defined stages during meiosis and gametogenesis. RNAi approaches in maize usually do not show mutant phenotype numbers above 40% in a heterozygous situation Srilunchang et al, 2010;Krohn et al, 2012), further supporting the hypothesis that the gene is strongly regulated and RNAi transcript amounts are sufficient to abolish gene activity.…”

“…Even though ACD is not as obvious during female gametogenesis, the polarity of the female gametophyte is established along its micropylar-chalazal axis and likely is already determined after meiosis during the first mitotic division of the functional megaspore (Webb and Gunning, 1990;Bajon et al, 1999). This polarity is clearly reflected by the successive establishment of two different gametic cells (egg and central cell), which are enclosed by highly specialized accessory cells at both poles (Huang and Russell, 1992;Pagnussat et al, 2009;Krohn et al, 2012). ACD and cell fate specification are closely related: (C) Schematic model of the CUL3-MATH-BTB-E3 ligase complex based on our data and the three-dimensional structure of human Hs SPOP and SCF Skp2 E3 complexes (Zheng et al, 2002;Zhuang et al, 2009).…”