Damian R. Page

Identification of new members of Fertilisation independent Seed Polycomb Group pathway involved in the control of seed development in Arabidopsis thaliana

In higher plants, double fertilisation initiates seed development. One sperm cell fuses with the egg cell and gives rise to the embryo, the second sperm cell fuses with the central cell and gives rise to the endosperm. The endosperm develops as a syncytium with the gradual organisation of domains along an anteroposterior axis defined by the position of the embryo at the anterior pole and by the attachment to the placenta at the posterior pole. We report that ontogenesis of the posterior pole in Arabidopsis thaliana involves oriented migration of nuclei in the syncytium. We show that this migration is impaired in mutants of the three founding members of the FERTILIZATION INDEPENDENT SEED (FIS) class, MEDEA (MEA), FIS2 and FERTILIZATION INDEPENDENT ENDOSPERM (FIE). A screen based on a green fluorescent protein (GFP) reporter line allowed us to identify two new loci in the FIS pathway, medicis and borgia. We have cloned the MEDICIS gene and show that it encodes the Arabidopsis homologue of the yeast WD40 domain protein MULTICOPY SUPRESSOR OF IRA (MSI1). The mutations at the new fis loci cause the same cellular defects in endosperm development as other fis mutations, including parthenogenetic development, absence of cellularisation, ectopic development of posterior structures and overexpression of the GFP marker.

The art and design of genetic screens: Arabidopsis thaliana

Molecular genetic studies rely on well-characterized organisms that can be easily manipulated. Arabidopsis thaliana — the model system of choice for plant biologists — allows efficient analysis of plant function, combining classical genetics with molecular biology. Although the complete sequence of the Arabidopsis genome allows the rapid discovery of the molecular basis of a characterized mutant, functional characterization of the Arabidopsis genome depends on well-designed forward genetic screens, which remain a powerful strategy to identify genes that are involved in many aspects of the plant life cycle.

Positive darwinian selection at the imprinted MEDEA locus in plants

In mammals and seed plants, a subset of genes is regulated by genomic imprinting where an allele’s activity depends on its parental origin. The parental conflict theory suggests that genomic imprinting evolved after the emergence of an embryo-nourishing tissue (placenta and endosperm), resulting in an intragenomic parental conflict over the allocation of nutrients from mother to offspring. It was predicted that imprinted genes, which arose through antagonistic co-evolution driven by a parental conflict, should be subject to positive darwinian selection. Here we show that the imprinted plant gene MEDEA (MEA), which is essential for seed development, originated during a whole-genome duplication 35 to 85 million years ago. After duplication, MEA underwent positive darwinian selection consistent with neo-functionalization and the parental conflict theory. MEA continues to evolve rapidly in the out-crossing species Arabidopsis lyrata but not in the self-fertilizing species Arabidopsis thaliana, where parental conflicts are reduced. The paralogue of MEA, SWINGER (SWN; also called EZA1), is not imprinted and evolved under strong purifying selection because it probably retained the ancestral function of the common precursor gene. The evolution of MEA suggests a late origin of genomic imprinting within the Brassicaceae, whereas imprinting is thought to have originated early within the mammalian lineage.

Genomic imprinting and seed development: endosperm formation with and without sex.

During seed development, coordinated developmental programs lead to the formation of the embryo, endosperm and seed coat. The maternal effects of the genes affected in the fertilisation-independent seed class of mutants play an important role in seed development. The plant Polycomb proteins MEDEA and FERTILIZATION-INDEPENDENT ENDOSPERM physically interact and form a complex, in a manner similar to that of their counterparts in animals. Maternal-effect phenotypes can result from regulation by genomic imprinting, a phenomenon of critical importance for both sexual and apomictic seed development.

Identification of a DNA methylation-independent imprinting control region at the Arabidopsis MEDEA locus

Genomic imprinting is exclusive to mammals and seed plants and refers to parent-of-origin-dependent, differential transcription. As previously shown in mammals, studies in Arabidopsis have implicated DNA methylation as an important hallmark of imprinting. The current model suggests that maternally expressed imprinted genes, such as MEDEA (MEA), are activated by the DNA glycosylase DEMETER (DME), which removes DNA methylation established by the DNA methyltransferase MET1. We report the systematic functional dissection of the MEA cis-regulatory region, resulting in the identification of a 200-bp fragment that is necessary and sufficient to mediate MEA activation and imprinted expression, thus containing the imprinting control region (ICR). Notably, imprinted MEA expression mediated by this ICR is independent of DME and MET1, consistent with the lack of any significant DNA methylation in this region. This is the first example of an ICR without differential DNA methylation, suggesting that factors other than DME and MET1 are required for imprinting at the MEA locus.

Engineering of Apomixis in Crop Plants: What Can We Learn from Sexual Model Systems?

The development of apomixis technology in crop plants is a desirable goal. Apomixis is the asexual reproduction through seeds, which occurs in over 400 flowering plants (Nogler, 1984). The introduction of clonal reproduction to crop plants will allow the indefinite propagation of any desirable genotype (including that of heterozygous F1 hybrids) and will completely transform current breeding and seed production strategies. Developmental aspects of apomixis (Koltunow, 1993; Grossniklaus, 2001; Spillane et al., 2001), its genetic control (Savidan, 2000; Grossniklaus et al., 2001a; Grimanelli et al., 2001), and its potential use in agriculture (Koltunow et al., 1995; Hanna et al., 1998; Jefferson and Bicknell, 1996; Thoenissen, 2001) have been extensively reviewed. Here, we provide a short summary of developmental and genetic aspects and report on our program using sexual model systems to identify genes and promoters relevant to the engineering of apomixis.