Development of pollinated and unpollinated ovules in Ginkgo biloba: unravelling pollen's role in ovule tissue maturation.

In gymnosperms such as Ginkgo biloba, the pollen's arrival plays a key role in ovule development, before fertilization occurs. Accordingly, G. biloba female plants geographically isolated from male plants aborted all their ovules after the pollination drop emission, which is the event that allows the ovule to capture pollen grains. To decipher the mechanism induced by pollination required to avoid ovule senescence and then abortion, we compared the transcriptomic of pollinated and unpollinated ovules at three time points after the end of the emission of pollination drops. Transcriptomic and in situ expression analyses revealed that several key genes involved in programmed cell death such as senescence and apoptosis, DNA replication, and cell cycle were differentially expressed in unpollinated ovules compared to pollinated ones. Interestingly, we provided evidence that the pollen captured by the pollination drop affects auxin local accumulation and might cause the deregulation of key genes required for ovule's programmed cell death and activating both the cell cycle and DNA replication genes.

The genome of the Wollemi pine, a critically endangered "living fossil" unchanged since the Cretaceous, reveals extensive ancient transposon activity.

We present the genome of the living fossil, Wollemia nobilis, a southern hemisphere conifer morphologically unchanged since the Cretaceous. Presumed extinct until rediscovery in 1994, the Wollemi pine is critically endangered with less than 60 wild adults threatened by intensifying bushfires in the Blue Mountains of Australia. The 12 Gb genome is among the most contiguous large plant genomes assembled, with extremely low heterozygosity and unusual abundance of DNA transposons. Reduced representation and genome re-sequencing of individuals confirms a relictual population since the last major glacial/drying period in Australia, 120 ky BP. Small RNA and methylome sequencing reveal conservation of ancient silencing mechanisms despite the presence of thousands of active and abundant transposons, including some transferred horizontally to conifers from arthropods in the Jurassic. A retrotransposon burst 8-6 my BP coincided with population decline, possibly as an adaptation enhancing epigenetic diversity. Wollemia, like other conifers, is susceptible to Phytophthora, and a suite of defense genes, similar to those in loblolly pine, are targeted for silencing by sRNAs in leaves. The genome provides insight into the earliest seed plants, while enabling conservation efforts.

Identification of key regulatory genes involved in the sporophyte and gametophyte development in Ginkgo biloba ovules revealed by in situ expression analyses.

PREMISE: In Arabidopsis thaliana, the role of the most important key genes that regulate ovule development is widely known. In nonmodel species, and especially in gymnosperms, the ovule developmental processes are still quite obscure. In this study, we describe the putative roles of Ginkgo biloba orthologs of regulatory genes during ovule development. Specifically, we studied AGAMOUS (AG), AGAMOUS-like 6 (AGL6), AINTEGUMENTA (ANT), BELL1 (BEL1), Class III HD-Zip, and YABBY Ginkgo genes. METHODS: We analyzed their expression domains through in situ hybridizations on two stages of ovule development: the very early stage that corresponds to the ovule primordium, still within wintering buds, and the late stage at pollination time. RESULTS: GBM5 (Ginkgo ortholog of AG), GbMADS8 (ortholog of AGL6) and GbC3HDZ1-2-3 were expressed in both the stages of ovule development, while GbMADS1, GbAGL6-like genes (orthologs of AGL6), GbBEL1-2 and YABBY Ginkgo orthologs (GbiYAB1B and GbiYABC) seem mostly involved at pollination time. GbANTL1 was not expressed in the studied stages and was different from GbANTL2 and GbBEL1, which seem to be involved at both stages of ovule development. In Ginkgo, the investigated genes display patterns of expression only partially comparable to those of other studied seed plants. CONCLUSIONS: The expression of most of these regulatory genes in the female gametophyte region at pollination time leads to suggest a communication between the sporophytic maternal tissue and the developing female gametophyte, as demonstrated for well-studied model angiosperms.

Expression and Functional Analyses of Nymphaea caerulea MADS-Box Genes Contribute to Clarify the Complex Flower Patterning of Water Lilies.

Nymphaeaceae are early diverging angiosperms with large flowers characterized by showy petals and stamens not clearly whorled but presenting a gradual morphological transition from the outer elements to the inner stamens. Such flower structure makes these plant species relevant for studying flower evolution. MADS-domain transcription factors are crucial components of the molecular network that controls flower development. We therefore isolated and characterized MADS-box genes from the water lily Nymphaea caerulea. RNA-seq experiments on floral buds have been performed to obtain the transcript sequences of floral organ identity MADS-box genes. Maximum Likelihood phylogenetic analyses confirmed their belonging to specific MADS-box gene subfamilies. Their expression was quantified by RT-qPCR in all floral organs at two stages of development. Protein interactions among these transcription factors were investigated by yeast-two-hybrid assays. We found especially interesting the involvement of two different AGAMOUS-like genes (NycAG1 and NycAG2) in the water lily floral components. They were therefore functionally characterized by complementing Arabidopsis ag and shp1 shp2 mutants. The expression analysis of MADS-box genes across flower development in N. caerulea described a complex scenario made of numerous genes in numerous floral components. Their expression profiles in some cases were in line with what was expected from the ABC model of flower development and its extensions, while in other cases presented new and interesting gene expression patterns, as for instance the involvement of NycAGL6 and NycFL. Although sharing a high level of sequence similarity, the two AGAMOUS-like genes NycAG1 and NycAG2 could have undergone subfunctionalization or neofunctionalization, as only one of them could partially restore the euAG function in Arabidopsis ag-3 mutants. The hereby illustrated N. caerulea MADS-box gene expression pattern might mirror the morphological transition from the outer to the inner floral organs, and the presence of transition organs such as the petaloid stamens. This study is intended to broaden knowledge on the role and evolution of floral organ identity genes and the genetic mechanisms causing biodiversity in angiosperm flowers.

The role of pollination in controlling Ginkgo biloba ovule development.

Generally, in gymnosperms, pollination and fertilization events are temporally separated and the developmental processes leading the switch from ovule integument into seed coat are still unknown. The single ovule integument of Ginkgo biloba acquires the typical characteristics of the seed coat long before the fertilization event. In this study, we investigated whether pollination triggers the transformation of the ovule integument into the seed coat. Transcriptomics and metabolomics analyses performed on ovules just prior and after pollination lead to the identification of changes occurring in Ginkgo ovules during this specific time. A morphological atlas describing the developmental stages of ovule development is presented. The metabolic pathways involved in the lignin biosynthesis and in the production of fatty acids are activated upon pollination, suggesting that the ovule integument starts its differentiation into a seed coat before the fertilization. Omics analyses allowed an accurate description of the main changes that occur in Ginkgo ovules during the pollination time frame, suggesting the crucial role of the pollen arrival on the progression of ovule development.