Transcriptomic and microstructural analyses in Liriodendron tulipifera Linn. reveal candidate genes involved in nectary development and nectar secretion
Abstract Background 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 nectary 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 real-time 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 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.