Fleshy structures associated with the ovule/seed arose independently several times during gymnosperm evolution. Fleshy structures are linked to ovule/seed protection and dispersal, and are present in all the four lineages of extant gymnosperms. The ontogenetic origin of the fleshy structures could be different, and spans from the ovule funiculus in the Taxus baccata aril, the ovule integument in Ginkgo biloba, to modified bracts as in case of Ephedra species. This variability in ontogeny is reflected in the morphology and characteristics that these tissues display among the different species. This review aims to provide a complete overview of these ovule/seed-associated fleshy structures in living gymnosperms, reporting detailed descriptions for every genus. The evolution of these independently evolved structures is still unclear, and different hypotheses have been presented-protection for the seeds, protection to desiccation-each plausible but no one able to account for all their independent origins. Our purpose is to offer an extensive discussion on these fleshy structures, under different points of view (morphology, evolution, gene involvement), to stimulate further studies on their origin and evolution on both ecological and molecular levels.

Summary 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.

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.

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.

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