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Abstract Background Floral nectar is an important determinant of plant–pollinator interactions and an integral component of pollination syndromes, suggesting it is under pollinator-mediated selection. However, compared to floral display traits, we know little about the evolutionary ecology of nectar. Combining a literature review with a meta-analysis approach, we summarize the evidence for heritable variation in nectar traits and link this variation to pollinator response and plant fitness. We further review associations between nectar traits and floral signals and discuss them in the context of honest signalling and targets of selection. Scope Although nectar is strongly influenced by environmental factors, heritable variation in nectar production rate has been documented in several populations (mean h2 = 0.31). Almost nothing is known about heritability of other nectar traits, such as sugar and amino acid concentrations. Only a handful of studies have quantified selection on nectar traits, and few find statistically significant selection. Pollinator responses to nectar traits indicate they may drive selection, but studies tying pollinator preferences to plant fitness are lacking. So far, only one study conclusively identified pollinators as selective agents on a nectar trait, and the role of microbes, herbivores, nectar robbers and abiotic factors in nectar evolution is largely hypothetical. Finally, there is a trend for positive correlations among floral cues and nectar traits, indicating honest signalling of rewards. Conclusions Important progress can be made by studies that quantify current selection on nectar in natural populations, as well as experimental approaches that identify the target traits and selective agents involved. Signal–reward associations suggest that correlational selection may shape evolution of nectar traits, and studies exploring these more complex forms of natural selection are needed. Many questions about nectar evolution remain unanswered, making this a field ripe for future research.

Although nectar is known to be important, for example in plant–insect interactions, little has been known about the mechanism of its secretion; sucrose phosphate synthases are now reported to be essential for the synthesis of the sucrose component of nectar and the transporter protein SWEET9 is shown to mediate sucrose export into the extracellular space of the nectary. SWEET9 sucrose transporter key to nectar secretion Nectar is an important factor in interactions between plants and insects, mediating both pollination and defensive mutualisms. The function and composition of floral nectaries have been well characterized, but the mechanism of nectar secretion has remained elusive. In a study of three flowering plant species, Wolf Frommer and colleagues show that sucrose phosphate synthases are highly expressed in floral nectaries and are essential for the synthesis of the sucrose component of nectar. The transporter protein SWEET9 mediates sucrose export from the site of production, in the nectary parenchyma, to the extracellular space of the nectary. Angiosperms developed floral nectaries that reward pollinating insects 1 . Although nectar function and composition have been characterized, the mechanism of nectar secretion has remained unclear 2 . Here we identify SWEET9 as a nectary-specific sugar transporter in three eudicot species: Arabidopsis thaliana , Brassica rapa (extrastaminal nectaries) and Nicotiana attenuata (gynoecial nectaries). We show that SWEET9 is essential for nectar production and can function as an efflux transporter. We also show that sucrose phosphate synthase genes, encoding key enzymes for sucrose biosynthesis, are highly expressed in nectaries and that their expression is also essential for nectar secretion. Together these data are consistent with a model in which sucrose is synthesized in the nectary parenchyma and subsequently secreted into the extracellular space via SWEET9, where sucrose is hydrolysed by an apoplasmic invertase to produce a mixture of sucrose, glucose and fructose. The recruitment of SWEET9 for sucrose export may have been a key innovation, and could have coincided with the evolution of core eudicots and contributed to the evolution of nectar secretion to reward pollinators.

Historical assessment of nectar provision in the UK from the 1930s to 2007 shows an initial dramatic fall, but more recently nectar provision has increased; the diversity of nectar sources has fallen to the point that four species now produce half of the total UK nectar. UK nectar resources estimated There is widespread concern about recent declines in bees, butterflies and other insect pollinators. Declines in flowers have been suggested as a key cause, but the idea has not been fully tested until now. Mathilde Baude et al . provide a UK national-level assessment of a key resource on which pollinators depend — nectar. They determine the nectar value of most common British plants, and assess nectar production in 260 plant species, combining the data with historical vegetation surveys. The results show that total nectar resources declined in England and Wales between the 1930s and 1970s before stabilizing and then increased more recently, but the diversity of species providing the nectar kept declining for a further decade after that. By 2007, just four grassland plant species accounted for more than half of the national nectar provision. These trends mirror pollinator diversity, which declined in the mid-twentieth century but stabilized more recently. Small adjustments to the management cycle of improved grasslands, allowing white clover (the dominant resource species) to flower would increase nectar availability, although only a subset of pollinator species would benefit. There is considerable concern over declines in insect pollinator communities and potential impacts on the pollination of crops and wildflowers 1 , 2 , 3 , 4 . Among the multiple pressures facing pollinators 2 , 3 , 4 , decreasing floral resources due to habitat loss and degradation has been suggested as a key contributing factor 2 , 3 , 4 , 5 , 6 , 7 , 8 . However, a lack of quantitative data has hampered testing for historical changes in floral resources. Here we show that overall floral rewards can be estimated at a national scale by combining vegetation surveys and direct nectar measurements. We find evidence for substantial losses in nectar resources in England and Wales between the 1930s and 1970s; however, total nectar provision in Great Britain as a whole had stabilized by 1978, and increased from 1998 to 2007. These findings concur with trends in pollinator diversity, which declined in the mid-twentieth century 9 but stabilized more recently 10 . The diversity of nectar sources declined from 1978 to 1990 and thereafter in some habitats, with four plant species accounting for over 50% of national nectar provision in 2007. Calcareous grassland, broadleaved woodland and neutral grassland are the habitats that produce the greatest amount of nectar per unit area from the most diverse sources, whereas arable land is the poorest with respect to amount of nectar per unit area and diversity of nectar sources. Although agri-environment schemes add resources to arable landscapes, their national contribution is low. Owing to their large area, improved grasslands could add substantially to national nectar provision if they were managed to increase floral resource provision. This national-scale assessment of floral resource provision affords new insights into the links between plant and pollinator declines, and offers considerable opportunities for conservation.

Flowers can be highly variable in nectar volume and chemical composition, even within the same plant, but the causes of this variation are not fully understood. One potential cause is nectar-colonizing bacteria and yeasts, but experimental tests isolating their effects on wildflowers are largely lacking. This study examines the effects of dominant species of yeasts and bacteria on the hummingbird-pollinated shrub, Mimulus aurantiacus, in California. Wildflowers were inoculated with field-relevant titres of either the yeast Metschnikowia reukaufii or the bacterium Neokomagataea sp. (formerly Gluconobacter sp.), both isolated from M. aurantiacus nectar. Newly opened flowers were bagged, inoculated, harvested after 3 d and analysed for microbial abundance, nectar volume, and sugar and amino acid concentration and composition. Yeast inoculation reduced amino acid concentration and altered amino acid composition, but had no significant effect on nectar volume or sugar composition. In contrast, bacterial inoculation increased amino acid concentration, enhanced the proportion of nectar sugars comprised by monosaccharides, and reduced nectar volume. The results presented suggest that microbial inhabitants of floral nectar can make nectar characteristics variable among flowers through divergent effects of yeasts and bacteria on nectar chemistry and availability, probably modifying plant-pollinator interactions.

The evolution of novel features, such as eyes or wings, that allow organisms to exploit their environment in new ways can lead to increased diversification rates. Therefore, understanding the genetic and developmental mechanisms involved in the origin of these key innovations has long been of interest to evolutionary biologists. In flowering plants, floral nectar spurs are a prime example of a key innovation, with the independent evolution of spurs associated with increased diversification rates in multiple angiosperm lineages due to their ability to promote reproductive isolation via pollinator specialization. As none of the traditional plant model taxa have nectar spurs, little is known about the genetic and developmental basis of this trait. Nectar spurs are a defining feature of the columbine genus Aquilegia (Ranunculaceae), a lineage that has experienced a relatively recent and rapid radiation. We use a combination of genetic mapping, gene expression analyses, and functional assays to identify a gene crucial for nectar spur development, POPOVICH (POP), which encodes a C2H2 zinc-finger transcription factor. POP plays a central role in regulating cell proliferation in the Aquilegia petal during the early phase (phase I) of spur development and also appears to be necessary for the subsequent development of nectaries. The identification of POP opens up numerous avenues for continued scientific exploration, including further elucidating of the genetic pathway of which it is a part, determining its role in the initial evolution of the Aquilegia nectar spur, and examining its potential role in the subsequent evolution of diverse spur morphologies across the genus.

Establishing how plants contribute food and refuge to insects can be challenging for small species that are difficult to observe in their natural habitat, such as disease vectoring mosquitoes. Currently indirect methods of plant-host identification rely on DNA sequencing of ingested plant material but are often unsuccessful for small insects that feed primarily on plant sugars or have little contact with plant cells. Here we developed an innovative approach to determine species-specific phytophagy by detecting taxon-specific plant secondary metabolites (PSMs) in nectar. Two mosquito species were exposed to three PSMs, each present in the nectar of a known plant host, firstly from dosed sucrose solutions and secondly from flowers. Both experiments yielded high rates of PSM detection in mosquitoes using liquid chromatography-mass spectrometry (LC-MS). PSMs were consistently detected in mosquitoes up to 8 h post-ingestion. In experiments consisting of two or three plant species, multiple PSMs from different host plants could be detected. These positive results demonstrate that PSMs could be useful indicators of insect plant-hosts selection in the wild. With expanded knowledge of nectar-based PSMs across a landscape, improved knowledge of plant-host relationships could be achieved where direct observations in their natural habitat are lacking. Increasing understanding of vector insect ecology will have an important role in tackling vector-borne disease.

Background Floral nectar typically functions as a pollinator reward in mutualistic flower-pollinator interactions, with this mutualism sometimes strengthened when plants provide the pollinators with brood sites and larval food as rewards. The functional and molecular mechanisms underpinning such rewards remain unclear. Results We present strong circumstantial evidence supporting a fungus-mediated plant-pollinator mutualism in a beetle-pollinated early-divergent angiosperm, Monoon laui (Annonaceae), which has flowers that produce exudates on both the stigmas and inner petals, with fungi that develop on the inner petals subsequently consumed by insect larvae. The identities of the pollinators and larvae, as well as the fungal communities borne on the pollinators and petals, indicate that the pollinators disperse fungi while ovipositing on the petals. The nutritional value of the two exudates reveals that the stigmatic exudate is sugar-rich, whereas the inner petal exudate has a greater amino acid content. Transcriptomic and proteomic comparisons between the two organs and their exudates corroborate the nutritional profiles, with a stronger immune response on stigmas. Conclusions Both stigmatic exudate and petal nectar of Monoon laui function as a pollinator reward, while petals with their nectar are moreover critical in the fungus-mediated plant-pollinator mutualism, as they are likely to be closely adapted to the requirements of the pollinators by providing them with brood sites and larval food, thereby increasing their population size during the flowering season and promoting pollination success.

The floral nectar of angiosperms harbors a variety of microorganisms that depend predominantly on animal visitors for their dispersal. Although some members of the genus Acinetobacter and all currently known species of Rosenbergiella are thought to be adapted to thrive in nectar, there is limited information about the response of these bacteria to variation in the chemical characteristics of floral nectar. We investigated the growth performance of a diverse collection of Acinetobacter (n = 43) and Rosenbergiella (n = 45) isolates obtained from floral nectar and the digestive tract of flower-visiting bees in a set of 12 artificial nectars differing in sugar content (15% w/v or 50% w/v), nitrogen content (3.48/1.67 ppm or 348/167 ppm of total nitrogen/amino nitrogen), and sugar composition (only sucrose, 1/3 sucrose + 1/3 glucose + 1/3 fructose, or 1/2 glucose + 1/2 fructose). Growth was only observed in four of the 12 artificial nectars. Those containing elevated sugar concentration (50% w/v) and low nitrogen content (3.48/1.67 ppm) were limiting for bacterial growth. Furthermore, phylogenetic analyses revealed that the ability of the bacteria to grow in different types of nectar is highly conserved between closely related isolates and genotypes, but this conservatism rapidly vanishes deeper in phylogeny. Overall, these results demonstrate that the ability of Acinetobacter spp. and Rosenbergiella spp. to grow in floral nectar largely depends on nectar chemistry and bacterial phylogeny.

Floral nectar contains mainly sugars but also amino acids, organic acids, inorganic ions and secondary compounds to attract pollinators. The genus Nicotiana exhibits great diversity among species in floral morphology, flowering time, nectar compositions, and predominant pollinators. We studied nectar samples of 20 Nicotiana species, composed equally of day- and night-flowering plants and attracting different groups of pollinators (e.g. hummingbirds, moths or bats) to investigate whether sugars, amino acids, organic acids and inorganic ions are influenced by pollinator preferences. Glucose, fructose and sucrose were the only sugars found in the nectar of all examined species. Sugar concentration of the nectar of day-flowering species was 20% higher and amino acid concentration was 2-3-fold higher compared to the nectar of night-flowering species. The sucrose-to-hexose ratio was significantly higher in night-flowering species and the relative share of sucrose based on the total sugar correlated with the flower tube length in the nocturnal species. Flowers of different tobacco species contained varying volumes of nectar which led to about 150-fold higher amounts of total sugar per flower in bat- or sunbird-pollinated species than in bee-pollinated or autogamous species. This difference was even higher for total amino acids per flower (up to 1000-fold). As a consequence, some Nicotiana species invest large amounts of organic nitrogen for certain pollinators. Higher concentrations of inorganic ions, predominantly anions, were found in nectar of night-flowering species. Therefore, higher anion concentrations were also associated with pollinator types active at night. Malate, the main organic acid, was present in all nectar samples but the concentration was not correlated with pollinator type. In conclusion, statistical analyses revealed that pollinator types have a stronger effect on nectar composition than phylogenetic relations. In this context, nectar sugars and amino acids are more strongly correlated with the preferences of predominant pollinators than organic acids and inorganic ions.

Background and AimsUnderstanding the evolutionary patterns of ecologically relevant traits is a central goal in plant biology. However, for most important traits, we lack the comprehensive understanding of their taxonomic distribution needed to evaluate their evolutionary mode and tempo across the tree of life. Here we evaluate the broad phylogenetic patterns of a common plant-defence trait found across vascular plants: extrafloral nectaries (EFNs), plant glands that secrete nectar and are located outside the flower. EFNs typically defend plants indirectly by attracting invertebrate predators who reduce herbivory.MethodsRecords of EFNs published over the last 135 years were compiled. After accounting for changes in taxonomy, phylogenetic comparative methods were used to evaluate patterns of EFN evolution, using a phylogeny of over 55 000 species of vascular plants. Using comparisons of parametric and non-parametric models, the true number of species with EFNs likely to exist beyond the current list was estimated.Key ResultsTo date, EFNs have been reported in 3941 species representing 745 genera in 108 families, about 1–2 % of vascular plant species and approx. 21 % of families. They are found in 33 of 65 angiosperm orders. Foliar nectaries are known in four of 36 fern families. Extrafloral nectaries are unknown in early angiosperms, magnoliids and gymnosperms. They occur throughout monocotyledons, yet most EFNs are found within eudicots, with the bulk of species with EFNs being rosids. Phylogenetic analyses strongly support the repeated gain and loss of EFNs across plant clades, especially in more derived dicot families, and suggest that EFNs are found in a minimum of 457 independent lineages. However, model selection methods estimate that the number of unreported cases of EFNs may be as high as the number of species already reported.ConclusionsEFNs are widespread and evolutionarily labile traits that have repeatedly evolved a remarkable number of times in vascular plants. Our current understanding of the phylogenetic patterns of EFNs makes them powerful candidates for future work exploring the drivers of their evolutionary origins, shifts, and losses.