Attract Insect Pollinators Research Articles

Three's a Crowd: Trade-Offs between Attracting Pollinators and Ant Bodyguards with Nectar Rewards in Turnera.

Many plants attract insect pollinators with floral nectar (FN) and ant "bodyguards" with extrafloral nectar (EFN). If nectar production is costly or physiologically linked across glands, investment in one mutualism may trade off with investment in the other. We confirmed that changes in FN and EFN availability alter pollination and ant defense mutualisms in a field population of Turnera ulmifolia. Plants with additional FN tended to produce more seeds, while plants with reduced EFN production experienced less florivory. We then mimicked the consumptive effects of mutualists by removing FN or EFN daily for 50 days in a full factorial design using three Turnera species (T. joelii, T. subulata, and T. ulmifolia) in a glasshouse experiment. For T. ulmifolia and T. subulata, but not T. joelii, removing either nectar reduced production of the other, showing for the first time that EFN and FN production can trade off. In T. subulata, increased investment in FN decreased seed set, suggesting that nectar production can have direct fitness costs. Through the linked expression of EFN and FN, floral visitors may negatively affect biotic defense, and extrafloral nectary visitors may negatively affect pollination.

Chemical and morphological filters in a specialized floral mimicry system.

Many plant species attract insect pollinators through chemical mimicry of their oviposition sites, often detaining them in a trap chamber that ensures pollen transfer. These plant mimics are considered to be unspecialized at the pollinator species level, yet field observations of a mycoheterotrophic rainforest orchid (Gastrodia similis), which emits an odour reminiscent of rotting fruit, indicate that it is pollinated by a single drosophilid fly species (Scaptodrosophila bangi). We investigated the roles of floral volatiles and the dimensions of the trap chamber in enforcing this specialization, using gas chromatography-mass spectrometry analyses, bioassays and scanning electron microscopy. We showed that G.similis flowers predominantly emit three fatty-acid esters (ethyl acetate, ethyl isobutyrate and methyl isobutyrate) that were shown in experiments to attract only Scaptodrosophila flies. We additionally showed that the trap chamber, which flies enter into via a touch-sensitive 'trapdoor', closely matches the body size of the pollinator species S.bangi and plays a key role in pollen transfer. Our study demonstrates that specialization in oviposition site mimicry is due primarily to volatile chemistry and is reflected in the dimensions of the trapping apparatus. It also indicates that mycoheterotrophic plants can be specialized both on mycorrhizal fungi and insect pollinators.

Fire and Canopy Removal Effects on Demography and Reproduction in Turkeybeard (Xerophyllum asphodeloides), a Fire-Dependent Temperate Forest Herb

We examined the effects of fire and canopy disturbance on turkeybeard (Xerophyllum asphodeloides [L.] Nutt.), a rare temperate forest herb. We used long-term data and a fire and canopy alteration experiment with controls to address our hypothesis that turkeybeard is fire-adapted. Analyses of a primary study population demonstrated that turkeybeard is a long-lived, infrequently flowering perennial with high survival and rapid postfire resprouting ability. Experimental responses showed that population-level flowering and inflorescence production increased 60–280% across treatments compared to controls in two posttreatment seasons. Seed production/inflorescence was significantly higher in treated plants. Fire and canopy removal treatments had a positive and additive effect on seed set compared to controls. Additional population surveys and pollination experiments showed that turkeybeard exhibited low flowering levels in undisturbed forest and required outcrossing for good seed set. Burning released plants from these limiting factors by inducing mass-flowering and altering the forest habitat to attract insect pollinators. Our research demonstrates that turkeybeard is one of the few known definitively fire-adapted montane forest understory herbs in the eastern United States, such that fire management including prescribed burning is recommended for its conservation and management. It also adds new impetus for studying fire’s ecological role in these lands.

Plants and the human brain

We're all familiar with the idea that plant-derived chemicals can have an impact on the functioning of the human brain. Most of us reach for a cup of coffee or tea in the morning, many of us occasionally eat some chocolate, some smoke a cigarette or take an herbal supplement, and some people use illicit drugs. We know a great deal about the mechanisms by which the psychoactive components of these various products have their effects on human brain function, but the question of why they have these effects has been almost totally ignored. This book sets out to describe not only how, but more importantly why, plant- and fungus-derived chemicals have their effects on the human brain. The answer to this last question resides, in part, with the terrestrial world's two dominant life forms, the plants and the insects, and the many ecological roles the 'secondary metabolite' plant chemicals are trying to play; for instance, defending the plant against insect herbivores whilst attracting insect pollinators. The answer also resides in the intersecting genetic heritage of mammals, plants, and insects and the surprising biological similarities between the three taxa. In particular it revolves around the close correspondence between the brains of insects and humans, and the intercellular signaling pathways shared by plants and humans. Plants and the Human Brain describes and discusses both how and why phytochemicals affect brain function with respect to the three main groups of secondary metabolites: the alkaloids, which provide us with a host of poisons, a handful of hallucinogens, and most drugs of abuse (e.g. morphine, cocaine and nicotine); the phenolics, which constitute a significant and beneficial part of our natural diet; and the terpenes, a group of multifunctional compounds which provide us with the active components of cannabis and a multitude of herbal extracts.

A regulatory network for coordinated flower maturation.

For self-pollinating plants to reproduce, male and female organ development must be coordinated as flowers mature. The Arabidopsis transcription factors AUXIN RESPONSE FACTOR 6 (ARF6) and ARF8 regulate this complex process by promoting petal expansion, stamen filament elongation, anther dehiscence, and gynoecium maturation, thereby ensuring that pollen released from the anthers is deposited on the stigma of a receptive gynoecium. ARF6 and ARF8 induce jasmonate production, which in turn triggers expression of MYB21 and MYB24, encoding R2R3 MYB transcription factors that promote petal and stamen growth. To understand the dynamics of this flower maturation regulatory network, we have characterized morphological, chemical, and global gene expression phenotypes of arf, myb, and jasmonate pathway mutant flowers. We found that MYB21 and MYB24 promoted not only petal and stamen development but also gynoecium growth. As well as regulating reproductive competence, both the ARF and MYB factors promoted nectary development or function and volatile sesquiterpene production, which may attract insect pollinators and/or repel pathogens. Mutants lacking jasmonate synthesis or response had decreased MYB21 expression and stamen and petal growth at the stage when flowers normally open, but had increased MYB21 expression in petals of older flowers, resulting in renewed and persistent petal expansion at later stages. Both auxin response and jasmonate synthesis promoted positive feedbacks that may ensure rapid petal and stamen growth as flowers open. MYB21 also fed back negatively on expression of jasmonate biosynthesis pathway genes to decrease flower jasmonate level, which correlated with termination of growth after flowers have opened. These dynamic feedbacks may promote timely, coordinated, and transient growth of flower organs.

Beyond pollinator attraction: extra-floral displays deter herbivores in a Mediterranean annual plant

The evolution of several floral traits is thought to be driven by multiple selective agents, including pollinators and herbivores. Similar combinations of selection pressures may have shaped extra-floral traits. The conspicuous purple tufts of leaves (“flags”), which often terminate vertical inflorescences in the Mediterranean annual Salvia viridis, were shown to attract insect pollinators to the flowering patch. Here we test whether they also function as anti-herbivore signals. We determined the aposematic potential of S. viridis flags on three levels: concentrations of anthocyanins, suggested to function as aposematic visual signals, in leaves and flags; spectrometry to estimate whether the color-vision system of two common Mediterranean generalist herbivores (locusts and goats) can discriminate flags from leaves; and choice experiments to determine food preferences of the same herbivores. Anthocyanin concentrations in flags were >10-fold higher than in leaves. Flags exhibited peak reflectance at 450 and 700 nm wavelengths, while leaves reflected maximally at 550 nm. According to the Vorobyev-Osorio color vision model, these differences in color reflection are likely to allow visual discrimination by herbivores. Goats preferred feeding on clipped inflorescences over control inflorescences. Locusts preferred leaves over flags. To test whether this was due to deterrence from the flags’ coloration, we also offered them choice between leaves and a rare, white morph, of the flags. The locusts chose both equally immediately after presentation, but leaves attracted more individuals after 5 min of feeding. The locusts also preferred green cabbage over anthocyanin-rich red cabbage. These results support the function of colorful extra-floral displays as warning signals.

Nanoridges that characterize the surface morphology of flowers require the synthesis of cutin polyester.

Distinctive nanoridges on the surface of flowers have puzzled plant biologists ever since their discovery over 75 years ago. Although postulated to help attract insect pollinators, the function, chemical nature, and ontogeny of these surface nanostructures remain uncertain. Studies have been hampered by the fact that no ridgeless mutants have been identified. Here, we describe two mutants lacking nanoridges and define the biosynthetic pathway for 10,16-dihydroxypalmitate, a major cutin monomer in nature. Using gene expression profiling, two candidates for the formation of floral cutin were identified in the model plant Arabidopsis thaliana: the glycerol-3-phosphate acyltransferase 6 (GPAT6) and a member of a cytochrome P450 family with unknown biological function (CYP77A6). Plants carrying null mutations in either gene produced petals with no nanoridges and no cuticle could be observed by either scanning or transmission electron microscopy. A strong reduction in cutin content was found in flowers of both mutants. In planta overexpression suggested GPAT6 preferentially uses palmitate derivatives in cutin synthesis. Comparison of cutin monomer profiles in knockouts for CYP77A6 and the fatty acid omega-hydroxylase CYP86A4 provided genetic evidence that CYP77A6 is an in-chain hydroxylase acting subsequently to CYP86A4 in the synthesis of 10,16-dihydroxypalmitate. Biochemical activity of CYP77A6 was demonstrated by production of dihydroxypalmitates from 16-hydroxypalmitate, using CYP77A6-expressing yeast microsomes. These results define the biosynthetic pathway for an abundant and widespread monomer of the cutin polyester, show that the morphology of floral surfaces depends on the synthesis of cutin, and identify target genes to investigate the function of nanoridges in flower biology.

Development of a complex floral trait: The pollinator‐attracting petal spots of the beetle daisy, Gorteria diffusa (Asteraceae)

Angiosperms possess a variety of complex floral traits that attract animal pollinators. Dark petal spots have evolved independently many times across the angiosperm phylogeny and have been shown to attract insect pollinators from several lineages. Here we present new data on the ontogeny and morphological complexity of the elaborate insect-mimicking petal spots of the South African daisy species, Gorteria diffusa (Asteraceae), commonly known as the beetle daisy, although it is fly-pollinated. Using light and scanning electron microscopy and histology, we identified three distinct specialized cell types of the petal epidermis that compose the petal spot. Sophisticated patterning of pigments, cuticular elaborations, and multicellular papillate trichomes make the G. diffusa petal spot a uniquely complex three-dimensional floral ornament. Examination of young inflorescence meristems revealed that G. diffusa ray florets develop (and probably also initiate) basipetally, in the opposite direction to the disc florets-a developmental phenomenon that has been found in some other daisies, but which contradicts conventional theories of daisy inflorescence architecture. Using these ontogenetic and morphological data, we have identified the mechanism by which G. diffusa patterns its insect-mimicking petal spots, and we propose a testable model for the genetic regulation of petal spot identity.

Learning of colored targets with vertical and horizontal components by bumblebees (<i>Bombus terrestris</i> L.)

Colorful tufts of bracts, which attract insect pollinators, terminate the vertical inflorescences of several plant species. These flag-like bracts contrast in color with the leaves below them, and are oriented perpendicular to the flowers on the inflorescence. We studied how color contrast and perpendicular orientation affect the feeding choices of bumblebees in laboratory experiments. We trained bees to feeders with perpendicular two-color displays, and subsequently recorded their choices among feeders that displayed only one of these cues. The bees preferred perpendicular displays that resembled the training model in the color of the horizontal component. We then evaluated the effects of the horizontal vs. vertical display component on foraging choices. After training bees to two-color, perpendicular displays, we allowed them to choose between displays with either the same horizontal or the same vertical component as the training model. Foragers mostly oriented to the horizontal displays to which they h...

An ortholog of MIXTA‐like2 controls epidermal cell shape in flowers of Thalictrum

Here, we investigated the genetic underpinnings of pollination-related floral phenotypes in Thalictrum, a ranunculid with apetalous flowers. The variable presence of petaloid features in other floral organs correlates with distinct adaptations to insect vs. wind pollination. Conical cells are present in sepals or stamens of insect-pollinated species, and in stigmas. We characterized a Thalictrum ortholog of the Antirrhinum majus transcription factor MIXTA-like2, responsible for conical cells, from three species with distinct floral morphologies, representing two pollination syndromes. Genes were cloned by PCR and analysed phylogenetically. Expression analyses were conducted by quantitative PCR and in situ hybridization, followed by functional studies in transgenic tobacco. The cloned genes encode R2R3 MYB proteins closely related to Antirrhinum AmMYBML2 and Petunia hybrida PhMYB1. Spatial expression by in situ hybridization overlaps areas of conical cells. Overexpression in tobacco induces cell outgrowths in carpel epidermis and significantly increases the height of petal conical cells. We have described the first orthologs of AmMIXTA-like2 outside the core eudicots, likely ancestral to the MIXTA/MIXTA-like1 duplication. The conserved role in epidermal cell elongation results in conical cells, micromorphological markers for petaloidy. This adaptation to attract insect pollinators was apparently lost after the evolution of wind pollination in Thalictrum.

A Rudimentary Labellar Speculum in Cymbidium lowianum (Rchb.f.) Rchb.f. and Cymbidium devonianum Paxton (Orchidaceae)

The labellar 'hairs' of some Cymbidium spp. are said to be thin-walled and to contain 'plasma', oil and sugars and it has long been speculated that they may function as food-hairs. However, the present authors' preliminary studies showed that certain atypical papillae may have a different role and, by reflecting light, function as a speculum. The purpose of the paper is to test this hypothesis. Light microscopy, scanning electron microscopy, transmission electron microscopy, histochemistry and ultraviolet photography were used to investigate the structure, food content and light-reflecting properties of these papillae. The labellum of Cymbidium lowianum (Rchb.f.) Rchb.f. is densely clothed with obconical to conical papillae with wide bases and pointed tips. However, on either side of the median axis of the lip occur silvery patches comprising papillae with truncated tips and it is thought that these reflect light and thereby attract insect pollinators. Similar patches are also found in Cymbidium devonianum Paxton, and in both species, they are set against a reddish background, which, since bees cannot perceive this colour, probably appears dark to the insect thus enhancing the visual impact of the light-reflecting patches. In Cymbidium tigrinum Parish ex Hook. and Cymbidium mastersii Griff. ex Lindl., however, the labellum is mainly white and no light-reflecting patches were observed. Instead, unlike C. lowianum and C. devonianum, these species are highly fragrant and the attraction of insects probably depends to a greater extent on olfactory cues. In C. lowianum both types of papillae contain protein, starch and lipid bodies but only protein is seemingly present at elevated concentrations. However, lipoidal material also occurs upon the surface of the labellum and it is possible that this may be gathered by insects as reported for C. iridifolium A. Cunn (syn. C. madidum Lindl.). The labellar papillae of C. lowianum, thus, have the potential to function as food-hairs, although direct evidence for this is lacking.

Mimicry in plant-parasitic fungi

Mimicry is the close resemblance of one living organism (the mimic) to another (the model), leading to misidentification by a third organism (the operator). Similar to other organism groups, certain species of plant-parasitic fungi are known to engage in mimetic relationships, thereby increasing their fitness. In some cases, fungal infection can lead to the formation of flower mimics (pseudo flowers) that attract insect pollinators via visual and/or olfactory cues; these insects then either transmit fungal gametes to accomplish outcrossing (e.g. in some heterothallic rust fungi belonging to the genera Puccinia and Uromyces) or vector infectious spores to healthy plants, thereby spreading disease (e.g. in the anther smut fungus Microbotryum violaceum and the mummy berry pathogen Monilinia vaccinii-corymbosi). In what is termed aggressive mimicry, some specialized plant-parasitic fungi are able to mimic host structures or host molecules to gain access to resources. An example is M. vaccinii-corymbosi, whose conidia and germ tubes, respectively, mimic host pollen grains and pollen tubes anatomically and physiologically, allowing the pathogen to gain entry into the host's ovary via stigma and style. We review these and other examples of mimicry by plant-parasitic fungi and some of the mechanisms, signals, and evolutionary implications.

The structural and developmental characteristics of floral nectaries of Litchi chinensis and their biological significance

Floral nectaries attract insect pollinators through secreted nectar. The structure and the size of floral nectaries are determining factors of the quantity and concentration of the nectar produced, which may consequently influence the patterns of insect visitation. In this study, we examined the structural and developmental characteristics of the floral nectaries of Litchi chinensis of “Nuomici” and “Feizixiao”. In two variety of L. chinensis, the floral nectaries of both functional male and female flowers are located between the cup-shaped calyx and the ovary. These floral nectaries are typical structural nectaries, formed as protrusions on the surface of flower receptacles, and at the mature stage, they are composed of epidermis, nectariferous tissue, and vascular tissue containing only phloem elements. The epidermis of the floral nectaries is covered with numerous epidermal hairs, with sporadic stomata distributed on the top surface of the nectaries. Before the nectaries start to secret nectar, the nectariferous tissues are filled with numerous starch grains, which display regular changes before, during, and after nectar secretion. The nectariferous tissue contains two cell types of different morphological characteristics. These cell types presumably have different functions, either for nectar synthesis, or for protecting nectaries from damage by phenolic compounds. The primordia of these nectaries initiate during floral organ differentiation, and its development coincides with the formation of microspores and macrospores in the male and female flowers, respectively. Based on these structural and developmental characteristics, we discussed the ontogeny and phylogeny of the floral nectaries in L. chinensis and possible evolutional adaptation of nectaries for effective attraction of insect pollinators.

Colouring agents in yellow and white flowered papaver radicatum in Northern Greenland

In the extreme north, Papaver radicatum s.l. occurs in a yellow and in a white-flowered type. The more severe the weather conditions the higher the frequency of the white phenotype. The colouring agents in the petals are all flavonol glucosides; in the yellow flowers gossypitrin (1) and herbacitrin (2), in the white ones the kaempferol glucoside astragalin (3). Only astragalin shows UV reflection, which is valuable in attracting insect pollinators, so very likely the presence of astragalin may be responsible for the success of the white-flowered type under suboptimal weather conditions. We were unable to confirm the hypothesis that the yellow colouring agents, gossypitrin and herbacitrin, should be more costly to produce than the white one, astragalin, as an investment in order to obtain the higher temperature observed in yellow flowers. The distribution pattern with high frequency of the yellow-flowered poppies where the likelihood of sunshine is high, may have other reasons than carbon allocation to the colouring agents. Yellow flowers are more showy than white ones, and although this in general benefits pollination it may also increase the risk of herbivory. White flowers are at the same time believed to escape herbivory and obtain a better pollination under dull weather conditions, due to the UV-reflection by astragalin.

Acetyl-CoA:benzylalcohol acetyltransferase--an enzyme involved in floral scent production in Clarkia breweri.

Volatile esters impart distinct characteristics to the floral scent of many plants, and are important in attracting insect pollinators. They are also important flavor compounds in fruits. The ester benzylacetate is a major constituent of the floral scent of Clarkia breweri, an annual plant native to California. The enzyme acetyl-CoA:benzylalcohol acetyltransferase (BEAT), which catalyzes the formation of benzylacetate, has been purified from C. breweri petals, and a cDNA encoding this enzyme has been isolated and characterized. The sequence of the 433-residue BEAT protein does not show high similarity to any previously characterized protein, but a 35-residue region from position 135-163 has significant similarity (42-56% identity) to several proteins known or suspected to use an acyl-CoA substrate. E. coli cells expressing C. breweri BEAT produced enzymatically active protein, and also synthesized benzylacetate and secreted it into the medium. Of the different parts of the C. breweri flower, petals contained the majority of BEAT transcripts, and no BEAT mRNA was detected in leaves. The levels of BEAT mRNA in the petals increased as the bud matured, and peaked at anthesis, paralleling changes in BEAT activity. However, three days after anthesis, mRNA levels began a steep decline, whereas BEAT activity remained high for the next two days, suggesting that the BEAT protein is relatively stable.

Are olfactory signals sufficient to attract fig pollinators?

Flower fragrances are important stimuli in attracting insect pollinators. Different odours attract different pollinators and can, in some cases, achieve specific relationships. Among highly specific pollination systems, the genus Ficus (Moraceae) represents one extreme example, as each Ficus is pollinated by its specific wasp species. The identification of a receptive fig occurs in two steps and chemical information appears to be important in both stages. First, all figs of a tree develop synchronously, so that when the wasps emerge, their natal tree only bears mature figs. Therefore the wasps must locate another tree, bearing receptive figs. This long-distance attraction suggests olfactory signals. Second, once on the fig, fig wasps explore its surface by tapping their antennae on the surface (antennating) as they walk over it. This behaviour suggests contact chemostimulation. The fragrance of receptive Ficus carica figs was extracted using pentane, and the extracts tested on its pollinator wasp (Blastophaga psenes). The results from both distance attraction and contact stimulation experiments indicate that chemical compounds are sufficient to mediate these two steps. Visual cues are not sufficient to attract and stimulate fig pollinators and are probably only minor stimuli compared to the chemical signature of the fig. In this way, Ficus pollination is strikingly different from other specific pollination systems such as Trollius and Yucca, where visual cues are important in attraction to flowers.

A cladistic analysis of Asarum (Aristolochiaceae) and implications for the evolution of herkogamy

A cladistic analysis of Asarum was conducted to examine relationships among species within the genus and to test the monophyly of several groups of taxa that have often been treated as segregate genera. Thirty-two species were drawn from throughout the range of the genus, representing a broad sample of sections and all segregate genera. The data matrix included 37 characters derived from various aspects of vegetative and floral morphology. A strict consensus of all most parsimonious trees suggests that Asarum s.l. is monophyletic and consists of two main clades: an Asarum clade, which is characterized by connate styles and inferior ovaries, and an Asiasarum-Hexastylis-Heterotropa clade, which is characterized by ridges on the inner perianth surface, dorsal stigmas, and bifid style extensions. The latter is a large and morphologically diverse clade that includes the North American segregate Hexastylis and two Asiatic segregates. Examination of pollination mechanisms in the context of this phylogeny supports the conclusion that herkogamy, and thus obligate insect pollination, is derived from a plesiomorphic condition of autonomous self-pollination. Associated with herkogamy are characters such as glandular trichomes and other ornamentation of the surface of the calyx that probably represent increased specialization to attract insect pollinators. This study also indicates that chromosomal evolution has occurred via aneuploid decrease from an ancestral chromosome number of 2n = 26 to 2n = 24 in Heterotropa. The recognition of two subgenera, subgenus Asarum and subgenus Heterotropa, corresponding to the two clades in the cladistic analysis, is recommended.

Molecular Biology of the Alternative Oxidase

Possibly the most easily recognized component of mitochondrial respiration specific to plants is a second terminal oxidase that has been variously described as being responsible for cyanide-insensitive 'cyanide-resistant or pathway respiration.' This type of respiration has been best defined in thermogenic inflorescences of the Araceae, such as Sauromatum guttatum, the lily' (Meeuse, 1975). Alternative pathway respiration has also been identified in other types of organisms; for example, in protista such as trypanosomes, in fungi such as Neurospora crassa and Hansenula anomala, and in green algae such as Chlamydomonas (Moore and Siedow, 1991). Although some reports exist for such activity in animal tissues, they have never been reliably confirmed. What is the alternative pathway of respiration, how does it work, and, perhaps more importantly, what is its purpose? Alternative pathway respiration branches from the Cyt pathway at ubiquinone and donates electrons directly to oxygen to form water, apparently in a single four-electron step (Siedow and Moore, 1993). An important feature of this pathway is that it does not contribute to a transmembrane potential; electrons that flow down this pathway lose two of three potential coupling sites for proton transport, and thus ATP production (Fig. 1). The only confirmed function for alternative pathway respiration is to supply heat in thermogenic blooms of the Araceae, such as S. guttatum, the voodoo lily (Meeuse, 1975). In this organism heat is produced in ephemeral inflorescences during a single day of flowering to better volatilize scents to attract insect pollinators.

Birds as Possible Pollinators of Acacia pycnantha

Acacia pycnantha, like many members of its genus, produces nectar from a gland at the base of the petiole. Nectar is apparently produced only when the plant is flowering and only on petioles close to the inflorescences. The nectary seems poorly placed to attract insect pollinators, but honeyeaters and silvereyes take the nectar, brush against the flowers and could effect pollination. Insects also visit the nectaries and flowers and may act as an additional attractant to the birds.