Final Report Summary - SEEDS OF LIFE (Seeds of life - analysis of seed growth and development) Flowering plants (Angiosperms) have evolved as one major life form on earth. One common principle of flowering plants and probably one of the main reasons for their evolutionary success is the rapid development of an embryo along with a nourishing tissue, called the endosperm. Embryo and endosperm are surrounded by maternal tissues and build the plant seed. Seeds are a fascinating biological structure and their formation is an essential step in the plant life cycle. Seeds also represent the major food source for animals and humans and for this reason, are an indispensable unit in the ecosystem on earth. The general aim of this project was to understand molecular mechanisms and principles of seed growth control. As model species the eudicot Arabidopsis thaliana and the monocotyledonous species Brachypopdium distachyon were used. Detailed histological studies allowed us to define the morphogenetic framework of seed development in Brachypodium. In addition, first mutagenesis screens were conducted and mutants with novel phenotypes were obtained. Subsequent molecular identification is greatly facilitated through the availability of the entire genome sequence of Brachypodium to which we have also contributed by annotating especially cell-cycle and chromatin-regulator genes. Particular focus was on the understanding of the cell-proliferation machinery such as cyclin-dependent kinases (CDKs) and cyclins as the major driving forces of seed growth. A genetic framework of cell proliferation activity was established and currently, results obtained in Arabidopsis are compared with situation in Brachypodium, for instance by overexpressing cell-cycle genes in Brachypodium. Our functional studies of seed-growth regulators show that for the free-nuclear divisions of the endosperm high levels of mitotic CDK-cyclin complexes are not only necessary but also sufficient to trigger proliferation. Moreover, we have identified type I MADS box transcription factors as important regulators of plant reproduction. A possible direct target of these MADS box transcription factors is the cell proliferation machinery. In turn, we found that downstream of CDK-cyclin complexes are not only cell-cycle targets but also many other proteins regulating cell-physiological processes such as energy metabolism. Thus, a regulatory cascade, from transcription factors to cell developmental and physiological processes is emerging. Many of the identified MADS box factors involved in seed development appear to be expressed in a parent-of-origin dependent manner, i.e. they are imprinted. One example is the recently characterized gene AGL36. Current focus is therefore on the epigenetic regulation of gene expression during plant reproduction. We have found that tight control of DNA methylation is important for the repression of a premature onset of seed development, i.e. endosperm proliferation without fertilization. In addition, we could show that the control of histone modification is key for setting the stage of the seed developmental phase. Ongoing research aims at an understanding of the kinetics of epigenetic changes during seed formation and maturation. We have successfully used natural variation to identify parameters of seed development in Arabidopsis. In particular we have identified several novel QTLs that influence seed size and mass. Interestingly, we find not only a high level of genetic variation in genes regulating seed development in natural population but also epigenetic variation indicating that the different pathways that govern seed development and growth are tightly interconnected at different regulatory levels.
