Metabolic and transcriptomic analyses of nectaries of A. comosus and N. tabacum
Background: Nectar is offered by numerous flowering plants to attract pollinators. To date, the production and secretion of nectar have been analyzed mainly in eudicots, particularly rosids such as Arabidopsis. However, due to the enormous diversity of flowering plants, further research on other plant species, especially monocots, is needed. Ananas comosus (monocot) is an economically important species and ideal for such analyses because it produce easily accessible nectar in sufficient quantities. In addition, the analyses were also carried out with Nicotiana tabacum (dicot, asterids) for comparison. Results: We performed transcriptome sequencing (RNA-Seq) analyses of the nectaries of Ananas comosus and Nicotiana tabacum, to test whether mechanisms described for nectar production and secretion in Arabidopsis are also present in these plant species. The focus of these analyses is on carbohydrate metabolism and transport (e.g., sucrose-phosphate synthases, invertases, sucrose synthases, SWEETs and further sugar transporters). In addition, the metabolites were analyzed in the nectar, nectaries and leaves of both plant species to address the question whether concentration gradients for different metabolites exist between the nectaries and nectar.  The nectar of N. tabacum contains large amounts of glucose, fructose and sucrose, and the sucrose concentration in the nectar appears to be similar to the sucrose concentration in the nectaries. Nectar production and secretion in this species closely resembles corresponding processes in some other dicots, including sucrose synthesis in nectaries and sucrose secretion by SWEET9.  The nectar of A. comosus also contains large amounts of glucose, fructose and sucrose and in this species the sucrose concentration in the nectar appears to be higher than the sucrose concentration in the nectaries. Furthermore, orthologs of SWEET9 appear to be generally absent in A. comosus and other monocots. Therefore, sucrose export by SWEETs from the nectaries into the nectar can be excluded, rather, other mechanisms, such as active sugar export or exocytosis, are more likely.  Conclusion: The mechanisms of nectar production and secretion in N. tabacum appear to be largely similar to those in other dicots, whereas in the monocotyledonous species A. comosus, different synthesis and transport processes are involved.