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Post-secretory synthesis of a natural analog of iron-gall ink in the black nectar of Melianthus spp.

ABSTRACT: The black nectar of Melianthus flowers is thought to serve as a visual attractant to pollinators, but the chemical identity and synthesis of the black pigment are unknown. Here we report that the black nectar contains a natural analog of iron-gall ink, which humans have used since medieval times. Specifically, dark black nectar at anthesis contains high levels of ellagic acid and iron; synthetic solutions of ellagic acid and iron(III) recapitulate the black color of the nectar. Conversely, lightly colored nectars before and after anthesis contain significantly lower levels of ellagic acid and iron, but higher levels of gallic acid. We then explored the possibility of post-secretory synthesis of ellagic acid from gallic acid. Indeed, Melianthus nectar contains a peroxidase that oxidizes gallic acid to form ellagic acid. Reactions containing the nectar peroxidase, gallic acid, hydrogen peroxide, and iron can fully recreate the black color of the nectar. Visual modeling indicates that the black color is both visible and conspicuous to birds within the context of the flower. In summary, the black nectar of Melianthus is derived from an ellagic acid-Fe complex analogous to iron-gall ink and is likely involved in the attraction of passerine bird pollinators.

ORGANISM(S): Melianthus elongatus

PROVIDER: GSE221730 | GEO | 2023/01/01

REPOSITORIES: GEO

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Project description:Floral nectar is a rich secretion produced by nectaries offered as an attractive reward to pollinators. Floral nectaries secrete an aqueous solution largely composed of sugars and amino acids as well as an array of proteins called nectarins. These provide one of the main mechanisms of floral defense in Nicotiana nectar. Due to its composition, floral nectar provides an ideal source of nutrients for specialized microorganisms that can change the nectar composition. Understanding of floral defenses and the role of nectar proteins is essential to predict the impacts in microbial growth, nectar composition, and relationships between plants and pollinators. In order to deepen knowledge about Nicotiana spp. nectar proteins, we used LC-MS/MS-based comparative proteomic analysis to identify 22 proteins from Petunia hybrida, 35 proteins from Datura stramonium, and 144 proteins from 23 different species of Nicotiana. GO enrichment analysis and secretory signal peptide prediction demonstrated that defense/stress was the largest group of proteins in nectars with differential abundance and distribution throughout genus Nicotiana. The Nicotiana spp. proteome consisted of 105 exclusive proteins such as lipid transfer proteins (LTPs), Nectar Redox Cycle proteins, proteases inhibitors (PI), and PR-proteins. Analysis by taxonomic sections demonstrated that LTPs were the most abundant group of proteins in the nectar of Undulatae and Noctiflora sections while nectarins were more abundant proteins in Rusticae, Suaveolens, Polydicliae, and Alata sections. Peroxidases (Pox) and chitinases (Chit) were exclusive to P. hybrida nectar, while D. stramonium proteome had only seven unique proteins. Significant differences were identified between the proteome of taxonomic sections providing relevant insights into the group of proteins related to defense/stress associated with Nectar Redox Cycle, antimicrobial proteins and signaling pathways. The activity of floral nectar proteins is suggested impact the microbial growth. The knowledge about these proteomes provides significant insights into the diversity of proteins secreted in the nectars and the array of mechanisms used by Nicotiana spp. in its floral defense.

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Post-secretory synthesis of a natural analog of iron-gall ink in the black nectar of Melianthus spp.

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