The appearance of seeds represents an enormous step forward for the evolutionary success of plants. In seeds the embryo is enclosed by a coat that has protective functions and can also influence the process of germination. Seeds can remain viable for a long time after their release from the “mother” plant therefore they have the time to be dispersed far away and to colonize also distant territories, something that vegetating plants cannot do because of their sessility.
Seed producing plants named Spermatophytes, comprise both Angiosperms and Gymnosperms. Angiosperm ovules are contained inside ovaries and develop into seeds following fertilization, while the ovaries are transformed into fruits. In Gymnosperms there are no ovaries and only their seed coat generally protects embryos, although there are cases where a hard cone may enclose and protect the seeds prior to their shedding. In some Gymnosperms species the seed coat develops into a fleshy fruit-like structure that is involved in the process of seed dispersal. It thus appears that in both Gymnosperms and Angiosperms the seed coat plays a role of paramount importance to secure high probabilities of success to the newly formed progeny of plants. Notwithstanding its importance, almost nothing is known regarding the molecular aspects of seed coat formation in Gymnosperms while there is some knowledge for a few Angiosperms. We propose to study the early steps of seed coat differentiation from an evolutionary point of view with the aim to elucidate the molecular mechanisms involved in the transformation of the ovule integument/s into a seed coat. To do so we propose to study this process in an Angiosperm (Arabidopsis thaliana) and in a Gymnosperm (Ginkgo biloba). Ginkgo was selected because its genome was recently sequenced but also because it has a feature that makes it of particular interest: the outermost layer of its integument becomes fleshy and ripens like a fruit, thus having the characteristics of both a seed coat and a fleshy “fruit”. Arabidopsis was chosen because its genome is also known and it is a model plant for which the molecular development of its reproductive structures is far better studied compared to other Angiosperms. In spite of the anatomical differences and taxonomic distance between the two species, a unifying characteristic is that in both cases MADS-box genes, important regulators of development, belonging to the same class seem to be involved in seed coat differentiation.
The seed coat starts to differentiate upon fertilization in Arabidopsis and upon pollination in Ginkgo. In both cases seed coat development should be repressed until a signal is perceived in the ovule integument tissues that triggers the start of the developmental process that leads to seed coat formation. Understanding these processes at the molecular level forms the core of this project proposal and will be studied in both species using various approaches. In particular, transcriptome changes will be studied in order to understand which genes are differential expressed during the early phases of seed coat formation, and to this purpose particular attention will be paid to regulatory genes and especially to MADS-domain transcription factors encoding genes. Upon identification of interesting genes that are differentially expressed during the early steps of transformation of ovule integuments into seed coat, their detailed expression will be studied by using real time PCR and in situ hybridization analyses. Moreover, since it is becoming increasingly evident that during development of plant reproductive structures the expression of genes may be regulated through epigenetic mechanisms, also the state of epigenetic marks on specific regulatory genes and at a genome wide scale will be investigated during the early steps of seed coat formation in both species. In this respect it is important to consider that PRC proteins are rated amon