New paradigms in plant embryogenesis: maternal control comes in different flavors

Classical maternal-effect genes in animals, including several Polycomb-group genes, are thought to produce their effects through expression of these genes in diploid maternal cells and by deposition of their stable products, usually mRNA, in the egg-cell cytoplasm. MEDEA mRNA cannot be detected in flower buds during early development of the female gametophyte, but is detectable by RT-PCR in mature gynoecia before and after fertilization, and throughout embryogenesis. Therefore, MEDEA mRNA is present, and its gametophytic maternal effect is exerted, at a time when most, if not all, female gametophytic cells are absent. To explain these observations, Grossniklaus and colleagues suggest two alternative, but not mutually exclusive, interpretations. It is possible that an unusually stable form of MEDEA mRNA is deposited in the female gametophyte, which remains detectable until very late stages of embryogenesis. To explain the genetics of inheritance of the medea alleles, MEDEA expression must not occur in the diploid (sporophytic) cells of the ovule. Thus the site of expression could be in the late female-gametophytic cells. Alternatively, MEDEA may be an imprinted gene whose maternally inherited copy, but not the paternal copy, is active in the mature female gametophyte and in the embryo throughout the latter's development. In a plant heterozygous for the medea mutation 50% of the female gametophytes inherit the mutant allele. The inactive (possibly due to imprinting) wild-type copy cannot rescue the loss of function of the mutant allele when the former is inherited through the pollen. While it is possible that the imprinted allele is inactive, there is no formal reason why the opposite may not be true: maternal imprinting may be necessary for activation of this gene. It is also formally possible that MEDEA mRNA is transcribed exclusively before fertilization such that RT-PCR signals from late stages are from the stable mRNA that remains in the embryo sac throughout embryogenesis. Therefore, more direct evidence for imprinting of MEDEA must await results of in situ hybridization analysis, and further molecular and genetic characterization of the gene and its product.An alternative interpretation addressed by Grossniklaus and his associates is that activity of wild-type MEDEA may be needed in the endosperm indirectly to influence embryo development (Fig. 2Fig. 2). If the gene product is required in a dose dependent manner, the single wild-type copy of the gene (inherited through the pollen) in a triploid endosperm may be insufficient to support embryogenesis. To test this hypothesis, the authors pollinated flowers of a diploid plant heterozygous for medea with pollen from a tetraploid line. Two wild-type paternal alleles of MEDEA in a tetraploid endosperm did not rescue the lethality of a heterozygous embryo, presumably containing one mutant (maternal) and two wild-type (paternal) alleles, within a mutant gametophyte. Therefore, two wild-type copies of MEDEA in the endosperm are not sufficient for normal embryogenesis, which argues against an endosperm effect. A further interpretation that these experiments fail to resolve, however, is whether the ratio of the wild-type MEDEA gene copies to the chromosome ploidy in the endosperm cells is important for embryo rescue. For example, in endosperm cells a ratio of chromosome ploidy to the number of copies of the wild-type MEDEA gene of 1.7 or less may be required for normal embryogenesis (Table 1Table 1). Some genes involved in animal sex determination show a similar dependence. Furthermore, a chromatin-remodeling protein could be expected to act in this fashion because it may bind to cis-acting sites on the chromosome, and therefore could potentially be titrated by an excess of chromatin binding sites.Table 1Effects of ploidy level and MEDEA genotype on embryoMEDEA genotypeGenotypeEggPollenEmbryoEndospermEmbryo phenotypePloidy to MEDEA ratio in endosperm+−+/−++/−Normal1.5−+−/+− −/+Abnormal3.0−++−/++− −/++Abnormal2.0− −+− −/+− − − −/+Abnormal5.0− ++− +/+− −++/+Normal1.7Why should MEDEA's influence, that of restricting cell division, be potentially `silenced' through the male germ-line by imprinting? The answer may lie in the endosperm. Two copies of the active gene in endosperm cells may slow down endosperm mitotic division relative to that of the single active copy in the embryo cells. This gives the embryo a 2:1 advantage compared to the 3:2 advantage in the absence of gene silencing by paternal imprinting. In an ancestral plant with no endosperm this 50% numerical advantage may have been important in relegating one of the two competing embryos (a consequence of double fertilization of two sister eggs in these plants) to the subordinate endosperm fate.There are at least three other Arabidopsis genes that also exhibit gametophytic maternal effects: FERTILIZATION INDEPENDENT SEED (FIS1, FIS2, FIS3) and FERTILIZATION INDEPENDENT ENDOSPERM (FIE, probably identical to FIS1)[6xA mutation that allows endosperm development without fertilization. Ohad, N. et al. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 5319–5324Crossref | PubMed | Scopus (252)See all References, 7xFertilization-independent seed development in Arabidopsis thaliana. Chaudhury, A.M. et al. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 4223–4228Crossref | PubMed | Scopus (302)See all References]. Mutations in these genes trigger seed development without fertilization, with accompanying incomplete embryo and endosperm development. This important gene group appears to control an early step in sexual seed development. It seems unlikely that MEDEA belongs to the same functional class because parental imprinting is unfavorable to asexual reproduction[3xGenomic imprinting in endosperm: its effect on seed development in crosses between species, and between different ploidies of the same species, and its implications for the evolution of apomixis. Haig, D. and Westoby, M. Philos. Trans. R. Soc. London Ser. B. 1991; 333: 1–13CrossrefSee all References[3]. Hence, a gene that negatively regulates apomixis, perhaps one belonging to the FIS/FIE class or one as yet undiscovered, may turn out to promote imprinting; its mutant allele may, therefore, suppress medea's lethal gametophytic effect on the embryo.The search for maternal-effect genes in flowering plants has just begun. Sporophytic and gametophytic maternal effects discovered earlier[4xMaternal effects of the short integument mutation on embryo development in Arabidopsis. Ray, S., Golden, T., and Ray, A. Dev. Biol. 1996; 180: 365–369Crossref | PubMed | Scopus (73)See all References, 5xDown regulation of ovule-specific MADS box genes from petunia results in maternally controlled defects in seed development. Colombo, L. et al. Plant Cell. 1997; 9: 703–715PubMedSee all References, 6xA mutation that allows endosperm development without fertilization. Ohad, N. et al. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 5319–5324Crossref | PubMed | Scopus (252)See all References, 7xFertilization-independent seed development in Arabidopsis thaliana. Chaudhury, A.M. et al. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 4223–4228Crossref | PubMed | Scopus (302)See all References]highlight the importance of cellular cross talk among the three developmental partners: sporophyte, endosperm and embryo[14xThree's company: regulatory cross-talk during seed development. Ray, A. Plant Cell. 1997; 9: 665–667See all References[14]. For example, genetic evidence suggests that expression of SHORT INTEGUMENTS1 occurs in the sporophyte but that its product, or its downstream effector, is needed in the embryo for normal morphogenesis[4xMaternal effects of the short integument mutation on embryo development in Arabidopsis. Ray, S., Golden, T., and Ray, A. Dev. Biol. 1996; 180: 365–369Crossref | PubMed | Scopus (73)See all References[4]. In this respect, the short integuments1 mutation closely resembles the classical maternal-effect mutations in animals although the precise cellular mechanism involved may be quite different. In petunia, expression of two putative MADS box transcription-factor genes, FBP7 and FBP11, is needed in the sporophyte for normal endosperm development[5xDown regulation of ovule-specific MADS box genes from petunia results in maternally controlled defects in seed development. Colombo, L. et al. Plant Cell. 1997; 9: 703–715PubMedSee all References[5]. Now MEDEA points at yet another twist in this fascinating story: the potential role of germ-line specific imprinting. Coordinate evolution of placental mammals and endosperm-containing angiosperms may reflect the rise of parental imprinting as an important evolutionary influence on sexuality.