New perspectives in nectar evolution and ecology: simple alimentary reward or a complex multiorganism interaction?

Abstract

Floral and extra-floral nectars are secretions elaborated by specific organs (nectaries) that can be associated with plant reproductive structures (the so-called floral nectaries found only in angiosperms) or vegetative parts (extrafloral nectaries). These secretions are common in terrestrial vascular plants, especially angiosperms. Although gymnosperms do not seem to have true nectar, their ovular secretions may share evolutionary links with angiosperm nectar. Nectar is generally involved in interactions with animals and by virtue of its sugar and amino acid content, it has been considered a reward offered by plants to animals in exchange for benefits, mainly pollination and indirect defense against herbivores. These relationships are often cited as examples of classical mutualistic interactions. Nonetheless, recent studies dealing with compounds less abundant than sugars and amino acids challenge this view and suggest that nectar is much more complex than simply a reward in the form of food. Nectar proteins (nectarins) and nectar secondary compounds have no primary nutritious function but are involved in plant-animal relationships in other ways. Nectarins protect against proliferation of microorganisms and infection of plant tissues by pathogens. Nectar secondary compounds can be involved in modulating the behavior of nectar feeders, maximizing benefits for the plant. Nectar-dwelling microorganisms (mainly yeasts) were recently revealed to be a third partner in the scenario of plant-animal interactions mediated by nectar. There is evidence that yeast has a remarkable impact on nectar feeder behavior, although the effects on plant fitness have not yet been clearly assessed. Keywords nectar; plant-animal interactions; indirect defense; pollination; nectar secondary compounds; nectar proteins; microorganisms The evolution and diversity of nectaries and nectarAccording to the classical theory of plant-animal coevolution, nectar -a sugary plant secretion -is considered a trait that evolved in two types of mutualistic interactions involving plants and animals: indirect defense against herbivores and pollen dispersal.Plants may defend against herbivores directly, through anatomical and/or chemical cues, or indirectly by attracting pugnacious ants that prey on herbivores or deter them from feeding on the plant. Indirect defense against herbivores is mediated by so-called extra-floral nectaries, i.e., nectaries generally located in vegetative parts of the plant (Fig. 1a) and never involved in pollination. On the other hand, pollen dispersal is favored by nectar close to the reproductive organs of angiosperm flowers, in so-called floral nectaries (Fig. 1b).

Yeast–Bacterium Interactions: The Next Frontier in Nectar Research

“…The high sugar concentration of floral nectar exerts osmotic pressure on microbes and represents a filter for microbial life [46,76]. However, high sugar concentration can encourage growth of a wide range of osmotolerant microorganisms including plant pathogens [77,78]. Consequently, it has been hypothesized that some plants may resist microbial colonization of nectar by producing high levels of hydrogen peroxide and other reactive oxygen species, toxic secondary metabolites from diverse chemical families (e.g.…”

“…[62, [78][79][80][81][82]. These chemicals are geographically and phylogenetically widespread across the plant kingdom, although species may vary in defense mechanisms [62,79].…”

Floral Metabolism of Sugars and Amino Acids: Implications for Pollinators’ Preferences and Seed and Fruit Set

“…In the last decade, research on nectar had seen a renaissance brought about by targeted nectary transcriptomics and metabolomics (Kram et al, 2009;Noutsos et al, 2015) and the discovery of the mechanisms regulating nectar secretion, such as transporters (Lin et al, 2014), hormones (Wiesen et al, 2016), and transcription factors (Liu et al, 2009). These recent discoveries on nectar biology have been compiled in exhaustive reviews (Nepi, 2017;Roy et al, 2017). Hence, we exclusively focus discussion here on the chemical modifications of , and Oligo-Peptide Transporters (OPT) mediate the uptake of neutral (gray circles) and acidic (gray circles with a minus sign) amino acids, neutral and basic (gray circles with a plus sign) amino acids, and tetrapeptides and pentapeptides (purple rectangles), respectively.…”

Floral nectary and osmophore of Epipactis helleborine (L.) Crantz (Orchidaceae)

“…Nectar, an aqueous solution with sugars, is regarded as a trait that appeared in plant–animal coevolution facilitated pollen dispersal and indirect defence against herbivores. The pollen dispersal is promoted by floral nectaries, where nectar is produced close to the reproductive organs, whereas the defence against herbivores by extra-floral nectaries, where nectar is exuded on the vegetative parts and is not involved in pollination (Nepi 2017 ). In orchid flowers, the nectaries can be formed as shallow, labellar nectaries, nectar spurs growing out from the lip base or the fused sepals (van der Pijl and Dodson 1969 ), labellar callus (Davies et al 2005 ) or as cuniculus embedded in ovary (Dressler 1990 ).…”