Convergent evolution of a novel blood-red nectar pigment in vertebrate-pollinated flowers

Abstract

Nesocodon mauritianus (Campanulaceae) produces a blood-red nectar that has been proposed to serve as a visual attractant for pollinator visitation. Here we show that the red color of the nectar is derived from a novel alkaloid termed nesocodin. The first nectar produced is acidic and pale yellow in color, but slowly becomes alkaline before taking on its characteristic red color. Three enzymes secreted into the nectar are either necessary or sufficient for pigment production, including (1) a carbonic anhydrase that creates an alkaline environment, (2) an aryl alcohol oxidase that generates sinapaldehyde, a pigment precursor, and (3) a ferritin-like catalase that protects nesocodin from degradation by hydrogen peroxide. Our findings demonstrate how these three enzymatic activities allow for the condensation of sinapaldehyde and proline to form a novel pigment with a stable imine bond, which in turn is attractive to Phelsuma geckos, the presumed pollinators of Nesocodon. We also identify nesocodin in the red nectar of the distantly related Jaltomata herrerae and provide evidence for convergent evolution of this trait. While the overall enzymatic activities required for red pigment formation in both Nesocodon and J. herrerae nectars are identical, the associated genes encoding the enzymes are not orthologous and, in the case of the aryl alcohol oxidase, even belong to different protein families. This work cumulatively identifies a novel, convergently evolved trait in two vertebrate-pollinated species, suggesting the red pigment is selectively favored and that only a limited number of compounds are likely to underlie this adaptation.

Sweet solutions: nectar chemistry and quality

“…These pharmacological manipulations of pollinator behaviour may benefit the plant through enhanced pollen transfer but are unlikely to benefit pollinators. In a very different example of pollinator attraction through secondary metabolites, coloured nectar tends to be associated with vertebrate pollinators, often on islands [114], and it has now been shown that the blood-red nectar of flowers attractive to geckos is owing to an alkaloid pigment, nesocodin [115].…”

A phosphatase gene is linked to nectar dihydroxyacetone accumulation in mānuka (Leptospermum scoparium)

“…However, although progress has been made on identifying the genetic basis for variation of nectar volume (Galliot et al, 2006;Wessinger et al, 2014;Barstow et al, 2022) or sucrose as a percentage of total sugars (Prasifka et al, 2018), such knowledge is limited for the other nectar components. Enzymes and/or candidate genes (Roy et al, 2022;Magner et al, 2023) and a QTL (Kostyun The accumulation of DHA in nectar of m anuka and some other species of Leptospermum results in honey, which is highly valued both as a food and as a medicinal product. The economic importance of m anuka honey has resulted in it being not only one of the only woody perennial species for which nectary biology and nectar traits have been studied in detail but also one of the few species with green, photosynthetic nectaries to have been characterised.…”

On the horizon for nectar‐related research

“…For example, the integration of transcriptomic, biochemical, and metabolomic analyses led to the discovery of pathways that produce a novel blood‐red pigment in Nesocodon mauritianus nectar. This red nectar is a crucial visual cue for geckos, the likely primary pollinators of N. mauritianus (Roy et al, 2021 [Preprint]). Specialized nectar metabolites such as nicotine, caffeine, and gelsemine have been shown to play crucial roles in modifying pollinator visitation (reviewed by Stevenson et al, 2017), while the nectar microbiome also affects pollinator attraction (e.g., Colda et al, 2021).…”

“…A number of research avenues that are on the horizon will provide a framework for identifying the cellular and physiological dynamics of nectar production. A key direction will be visualizing sugar and metabolite processing and transport during active nectar production and secretion using techniques such as microscopic imaging coupled with cell permeable dyes and molecular probes (e.g., Solhaug et al, 2021) (Figure 1L). Along similar lines, experiments utilizing carbon/nitrogen isotopes will shed light on how amino acids and sugars are trafficked into nectar, whether they are synthesized in nectaries de novo, derived from vasculature sources, or a mixture of both (e.g., Solhaug et al, 2021).…”

“…A key direction will be visualizing sugar and metabolite processing and transport during active nectar production and secretion using techniques such as microscopic imaging coupled with cell permeable dyes and molecular probes (e.g., Solhaug et al, 2021) (Figure 1L). Along similar lines, experiments utilizing carbon/nitrogen isotopes will shed light on how amino acids and sugars are trafficked into nectar, whether they are synthesized in nectaries de novo, derived from vasculature sources, or a mixture of both (e.g., Solhaug et al, 2021). Plant hormones such as auxin, gibberellins, and jasmonic acid play critical roles in nectar production (reviewed by Roy et al, 2017).…”