Nectar is a major flower attractant and reward for insects for pollination. Liriodendron, a genus of the Magnoliaceae family, has only two relict species, L. chinense and L. tulipifera, that are considered "basal angiosperms" according to plant evolutionary history. The flowers of Liriodendron plants are insect pollinated and secrete nectar to attract pollinators. To date, the morphology and anatomy of the nectary, the mechanism of nectar secretion and the molecular mechanism involved in

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... development in Liriodendron remain poorly understood. Methods In this study, we examined the nectary surface cells and the change in starch in L. tulipifera by using scanning electron microscopy and periodic acid-Schiff techniques to select definitive samples for next research. Transcriptome sequencing was performed on the top and middle parts of the immature nectary and the middle parts of the mature nectary and the postsecreted nectary in L. tulipifera. We evaluated the expression profiles of 22 DEGs that were closely related to nectary development and nectar secretion for realtime quantitative PCR analysis. Results The L. tulipifera nectary is a starch-storing nectary and is located in the top and middle parts of L. tulipifera petals. After analyzing the RNA-seq data, we obtained 115.26 Gb clean data in 12 libraries and mapped the results to the L. chinense reference genome with 71.02%-79.77% efficiency. In total, 26,955 DEGs were identified by analyzing six different pairwise comparisons. The flavonoid biosynthesis, phenylpropanoid biosynthesis, anthocyanin biosynthesis and starch and sucrose metabolism pathways were enriched and related to nectar secretion and pigment change. We identified 56 transcription factor 3 families, and members of the TCP, Trihelix, C2H2, ERF, and MADS families changed dynamically during nectary development. Moreover, to further verify the accuracy of the RNA-seq results, we validated the expression profiles of 22 candidate genes. Conclusions We evaluated the nectary development and secretion process comprehensively and identified many related candidate genes in L. tulipifera. These findings suggest that the nectary may play important roles in flavonoid synthesis and petal color presentation. Background The original attractant for insects to insect-pollinated plants was pollens; however, nectar, an inexpensive foodstuff, was generated as an alternative to pollen, and then, the nectary was formed [1] . Owing to its diversity in position and structure, the nectary is classified into different categories [2] . According to their position, nectaries can be classified into two types: floral and extrafloral nectaries. Pteridium aquilinum is an ancient extrafloral plant with nectaries on its fronds [3] [4] [5] . According to their structure, nectaries can be divided into two groups: structural and nonstructural nectaries [6, 7] . The structure of the most angiosperms nectary contains three major components: epidermis, specialized parenchyma and vascular bundle [8, 9] . During nectary development and nectar secretion in most plants, starch grains accumulate and then degrade. Therefore, according to whether starch undergoes regular dynamic changes, the nectary can also be divided into two groupings: starch-storing nectary and non-starch-storing nectary [10] . Thus far, the anatomy and taxonomy of nectaries have been comprehensively studied, and substantial achievements have been obtained. However, research on the biochemistry and molecular biology of the nectary is delayed. Currently, research is focused more on the molecular mechanism of the floral nectary rather than the anatomy and taxonomy of the floral nectary. Several studies on model