Analysis of communication pathways during seed development in Arabidopsis thaliana

Seed development in flowering plants requires coordination between the two genetically different fertilization products, the embryo and the endosperm and the surrounding maternal tissues, the integuments. However, little is known about the coordination of endosperm and embryo growth. In Arabidopsis, mutations in the cell cycle regulator CYCLIN DEPENDENT KINASE A;1 (CKDA;1) result in pollen that only successfully fertilizes the egg cell and seeds generated from crosses with cdka;1 pollen develop endosperm with solely maternal contribution. Here, fertilization by the cdka;1 mutant pollen was used to dissect early seed development. Crosses of 14 Arabidopsis accessions pollinated with cdka;1 mutant pollen revealed a large natural genetic variation with regard to the development of endosperm without paternal contribution. This work revealed a surprisingly large degree of autonomy in embryo growth, but also showed the embryo’s growth restrictions with regard to endosperm size. By using a recombinant inbred line population between the two Arabidopsis accessions Bayreuth-0 and Shahdara four QTLs were discovered, two main and four complex loci that influence the development of unfertilized endosperm. The genes of two DNA N-glycosylases ROS1 and DME, which catalyze the demethylation of symmetrical cytosine methylation, lay inside the two intervals of one of the two complex QTLs. A functional analysis revealed a new aspect of ROS1 and DME in restricting the proliferation of unfertilized endosperm. Moreover, ros1-dme double mutants could rescue the observed seed abortion upon cdka;1 pollination dependent on the activity of the methyltransferase MET1 during the sporophytic phase. Surprisingly, the rescue was independent of dme co-transmission, indicating an effect of ros1 together with dme in trans. The inheritance pattern of the mutant phenotypes revealed a paramutation-like phenomenon and the detection of almost 100% relative methylation levels on PHE1 and AGL36 sequence loci in ros1-dme double compared to the single mutants and Col-0 wild-type plants suggested that hypermethylation caused the mutant phenotype. The observed hypermethylation is likely to be established by in trans interactions between homologous DNA sequences on different chromosomes in a dominant paternal manner. These findings confirm previous results that endosperm formation is, beside other factors, triggered by the alteration of methylation levels prior to fertilization. Furthermore, DNA methylation patterns can probably be transferred via the paternal gametes, influencing not only the epigenetic status of the sporophyte, but also of the following gametophyte and affecting seed development after subsequent fertilization

Male fertility in Arabidopsis requires active DNA demethylation of genes that control pollen tube function.

Active DNA demethylation is required for sexual reproduction in plants but the molecular determinants underlying this epigenetic control are not known. Here, we show in Arabidopsis thaliana that the DNA glycosylases DEMETER (DME) and REPRESSOR OF SILENCING 1 (ROS1) act semi-redundantly in the vegetative cell of pollen to demethylate DNA and ensure proper pollen tube progression. Moreover, we identify six pollen-specific genes with increased DNA methylation as well as reduced expression in dme and dme;ros1. We further show that for four of these genes, reinstalling their expression individually in mutant pollen is sufficient to improve male fertility. Our findings demonstrate an essential role of active DNA demethylation in regulating genes involved in pollen function

Genome-Wide Transcript Profiling of Endosperm without Paternal Contribution Identifies Parent-of-Origin–Dependent Regulation of AGAMOUS-LIKE36

Seed development in angiosperms is dependent on the interplay among different transcriptional programs operating in the embryo, the endosperm, and the maternally-derived seed coat. In angiosperms, the embryo and the endosperm are products of double fertilization during which the two pollen sperm cells fuse with the egg cell and the central cell of the female gametophyte. In Arabidopsis, analyses of mutants in the cell-cycle regulator CYCLIN DEPENDENT KINASE A;1 (CKDA;1) have revealed the importance of a paternal genome for the effective development of the endosperm and ultimately the seed. Here we have exploited cdka;1 fertilization as a novel tool for the identification of seed regulators and factors involved in parent-of-origin–specific regulation during seed development. We have generated genome-wide transcription profiles of cdka;1 fertilized seeds and identified approximately 600 genes that are downregulated in the absence of a paternal genome. Among those, AGAMOUS-LIKE (AGL) genes encoding Type-I MADS-box transcription factors were significantly overrepresented. Here, AGL36 was chosen for an in-depth study and shown to be imprinted. We demonstrate that AGL36 parent-of-origin–dependent expression is controlled by the activity of METHYLTRANSFERASE1 (MET1) maintenance DNA methyltransferase and DEMETER (DME) DNA glycosylase. Interestingly, our data also show that the active maternal allele of AGL36 is regulated throughout endosperm development by components of the FIS Polycomb Repressive Complex 2 (PRC2), revealing a new type of dual epigenetic regulation in seeds

Reproductive cross-talk: seed development in flowering plants

Flowering plants have evolved to be a predominant life form on earth. A common principle of flowering plants and probably one of the main reasons for their evolutionary success is the rapid development of an embryo next to a supporting tissue called the endosperm. The embryo and the endosperm are protected by surrounding maternal tissues, the integuments, and the trinity of integuments, embryo and endosperm comprise the plant seed. For proper seed development, these three structures have to develop in a highly controlled and co-ordinated manner, representing a paradigm for cell-cell communication during development. Communication pathways between the endosperm and the seed coat are now beginning to be unravelled. Moreover, recently isolated mutants affecting plant reproduction have allowed a genetic dissection of seed development, and revealed that the embryo plays a previously unrecognized yet important role in co-ordinating seed development

Natural Variation in the Degree of Autonomous Endosperm Formation Reveals Independence and Constraints of Embryo Growth During Seed Development in Arabidopsis thaliana

Seed development in flowering plants is a paradigm for the coordination of different tissues during organ growth. It requires a tight interplay between the two typically sexually produced structures: the embryo, developing from the fertilized egg cell, and the endosperm, originating from a fertilized central cell, along with the surrounding maternal tissues. Little is known about the presumptive signal transduction pathways administering and coordinating these different tissues during seed growth and development. Recently, a new signal has been identified emanating from the fertilization of the egg cell that triggers central cell proliferation without prior fertilization. Here, we demonstrate that there exists a large natural genetic variation with respect to the outcome of this signaling process in the model plant Arabidopsis thaliana. By using a recombinant inbred line population between the two Arabidopsis accessions Bayreuth-0 and Shahdara, we have identified two genetic components that influence the development of unfertilized endosperm. Exploiting this natural variation, we could further dissect the interdependence of embryo and endosperm growth during early seed development. Our data show an unexpectedly large degree of independence in embryo growth, but also reveal the embryo's developmental restrictions with respect to endosperm size. This work provides a genetic framework for dissection of the interplay between embryo and endosperm during seed growth in plants