Floral organs act as environmental filters and interact with pollinators to structure the yellow monkeyflower ( Mimulus guttatus ) floral microbiome

Distinct Communities and Differing Dispersal Routes in Bacteria and Fungi of Honey Bees, Honey, and Flowers

Microbiota, the communities of microbes on and in organisms or organic matter, are essential for the functioning of ecosystems. How microbes are shared and transmitted delineates the formation of a microbiota. As pollinators forage, they offer a route to transfer microbes among the flowering plants, themselves, and their nests. To assess how the two components of the microbiota, bacteria and fungi, in pollination communities are shared and transferred, we focused on the honey bee Apis mellifera and collected honey bee, honey (representing the hive microbiota), and flower samples three times during the summer in Finland. We identified the bacteria and fungi by DNA metabarcoding. To determine the impact of honey bees' flower choices on the honey bee and hive microbiota, we identified also plant DNA in honey. The bacterial communities of honey bees, honey, and flowers all differ greatly from each other, while the fungal communities of honey bees and honey are very similar, yet different from flowers. The time of the summer and the sampling area influence all these microbiota. For flowers, the plant identity impacts both bacterial and fungal communities' composition the most. For the dispersal pathways of bacteria to honey bees, they are acquired directly from the honey and indirectly from flowers through the honey, while fungi are directly transmitted to honey bees from flowers. Overall, the distinctiveness of the microbiota of honey bees, honey, and the surrounding flowers suggests the sharing of microbes among them occurs but plays a minor role for the established microbiota.

Discussion: Harnessing microbiome-mediated adaptations in insect pollinators to mitigate climate change impact on crop pollination

Insect pollinators, vital for agriculture and biodiversity, face escalating threats from climate change. We argue and explore the pivotal role of the microbiomes in shaping adaptations of insect pollinator resilience amid climate-induced challenges (climate change and habitat alteration). Examining diverse taxonomic groups, we unravel the interplay between insect physiology, microbiomes, and adaptive mechanisms. Climate-driven alterations in microbiomes impact insect health, behavior, and plant interactions, posing significant effects on agricultural ecosystems. We propose harnessing microbiome-mediated adaptations as a strategic approach to mitigate climate change impacts on crop pollination. Insights into insect-pollinator microbiomes offer transformative avenues for sustainable agriculture, including probiotic interventions (use of EM PROBIOTIC) and microbiome engineering (such as engineering gut bacteria) to induce immune responses and enhanced pollination services. Integrating microbiome insights into conservation practices elucidates strategies for preserving pollinator habitats, optimizing agricultural landscapes, and developing policies to safeguard pollinator health in the face of environmental changes. Finally, we stress interdisciplinary collaboration and the urgency of understanding pollinator microbiome dynamics under climate change in future research.

Floral Volatile Organic Compounds Change the Composition and Function of the Endophytic Fungal Community in the Flowers of Osmanthus fragrans

Endophytic fungi in flowers influence plant health and reproduction. However, whether floral volatile organic compounds (VOCs) affect the composition and function of the endophytic fungal community remains unclear. Here, gas chromatography-mass spectrometry (GC-MS) and high-throughput sequencing were used to explore the relationship between floral VOCs and the endophytic fungal community during different flower development stages in Osmanthus fragrans 'Rixiang Gui'. The results showed that the composition of the endophytic fungal community and floral VOCs shifted along with flowering development. The highest and lowest α diversity of the endophytic fungal community occurred in the flower fading stage and full blooming stage, respectively. The dominant fungi, including Dothideomycetes (class), Pleosporales (order), and Neocladophialophora, Alternaria, and Setophoma (genera), were enriched in the flower fading stage and decreased in the full blooming stage, demonstrating the enrichment of the Pathotroph, Saprotroph, and Pathotroph-Saprotroph functions in the flower fading stage and their depletion in the full blooming stage. However, the total VOC and terpene contents were highest in the full blooming stage and lowest in the flower fading stage, which was opposite to the α diversity of the endophytic fungal community and the dominant fungi during flowering development. Linalool, dihydro-β-ionone, and trans-linalool oxide(furan) were key factors affecting the endophytic fungal community composition. Furthermore, dihydro-β-ionone played an extremely important role in inhibiting endophytic fungi in the full blooming stage. Based on the above results, it is believed that VOCs, especially terpenes, changed the endophytic fungal community composition in the flowers of O. fragrans 'Rixiang Gui'. These findings improve the understanding of the interaction between endophytic fungi and VOCs in flowers and provide new insight into the mechanism of flower development.

Blueberry floral probiotics: nectar microbes inhibit the growth of Colletotrichum pathogens

AIMS

To identify whether microorganisms isolated from blueberry flowers can inhibit the growth of Colletotrichum, an opportunistic plant pathogen that infects flowers and threatens yields, and to assess the impacts of floral microbes and Colletotrichum pathogens on artificial nectar sugars and honey bee consumption.

METHODS AND RESULTS

The growth inhibition of Colletotrichum (Colletotrichum acutatum, Colletotrichum fioriniae, and Colletotrichum gloeosporioides) was screened using both artificial nectar co-culture and dual culture plate assays. All candidate nectar microbes were screened for antagonism against a single C. acutatum isolate. Then, the top four candidate nectar microbes showing the strongest inhibition of C. acutatum (Neokomagataea thailandica, Neokomagataea tanensis, Metschnikowia rancensis, and Symmetrospora symmetrica) were evaluated for antagonism against three additional C. acutatum isolates, and single isolates of both C. fioriniae and C. gloeosporioides. In artificial nectar assays, single and three-species cultures inhibited the growth of two of four C. acutatum isolates by ca. 60%, but growth of other Colletotrichum species was not affected. In dual culture plate assays, inhibition was observed for all Colletotrichum species for at least three of four selected microbial antagonists (13%‒53%). Neither honey bee consumption of nectar nor nectar sugar concentrations were affected by any microbe or pathogen tested.

CONCLUSIONS

Selected floral microbes inhibited growth of all Colletotrichum species in vitro, although the degree of inhibition was specific to the assay and pathogen examined. In all microbial treatments, nectar sugars were preserved, and honey bee preference was not affected.

Intraspecific variation in realized dispersal probability and host quality shape nectar microbiomes

Epiphytic microbes frequently affect plant phenotype and fitness, but their effects depend on microbe abundance and community composition. Filtering by plant traits and deterministic dispersal-mediated processes can affect microbiome assembly, yet their relative contribution to predictable variation in microbiome is poorly understood. We compared the effects of host-plant filtering and dispersal on nectar microbiome presence, abundance, and composition. We inoculated representative bacteria and yeast into 30 plants across four phenotypically distinct cultivars of Epilobium canum. We compared the growth of inoculated communities to openly visited flowers from a subset of the same plants. There was clear evidence of host selection when we inoculated flowers with synthetic communities. However, plants with the highest microbial densities when inoculated did not have the highest microbial densities when openly visited. Instead, plants predictably varied in the presence of bacteria, which was correlated with pollen receipt and floral traits, suggesting a role for deterministic dispersal. These findings suggest that host filtering could drive plant microbiome assembly in tissues where species pools are large and dispersal is high. However, deterministic differences in microbial dispersal to hosts may be equally or more important when microbes rely on an animal vector, dispersal is low, or arrival order is important.

Inter- and intraspecific phytochemical variation correlate with epiphytic flower and leaf bacterial communities

Microbes associated with flowers and leaves affect plant health and fitness and modify the chemical phenotypes of plants with consequences for interactions of plants with their environment. However, the drivers of bacterial communities colonizing above-ground parts of grassland plants in the field remain largely unknown. We therefore examined the relationships between phytochemistry and the epiphytic bacterial community composition of flowers and leaves of Ranunculus acris and Trifolium pratense. On 252 plant individuals, we characterized primary and specialized metabolites, that is, surface sugars, volatile organic compounds (VOCs), and metabolic fingerprints, as well as epiphytic flower and leaf bacterial communities. The genomic potential of bacterial colonizers concerning metabolic capacities was assessed using bacterial reference genomes. Phytochemical composition displayed pronounced variation within and between plant species and organs, which explained part of the variation in bacterial community composition. Correlation network analysis suggests strain-specific correlations with metabolites. Analysis of bacterial reference genomes revealed taxon-specific metabolic capabilities that corresponded with genes involved in glycolysis and adaptation to osmotic stress. Our results show relationships between phytochemistry and the flower and leaf bacterial microbiomes suggesting that plants provide chemical niches for distinct bacterial communities. In turn, bacteria may induce alterations in the plants' chemical phenotype. Thus, our study may stimulate further research on the mechanisms of trait-based community assembly in epiphytic bacteria.

Hidden worlds within flowers

There is a growing realization that ecological interactions take place at many scales, from acorns to forests, and that formerly overlooked community members, particularly microbes, can play outsized ecological roles. Beyond their primary function as the reproductive organs of angiosperms, flowers constitute resource-rich, ephemeral habitats teeming with flower-loving symbionts, or 'anthophiles'. The physical, chemical, and structural properties of flowers combine to create a habitat filter, selectively determining which anthophiles can reside there, and how, and when they interact. The microhabitats within flowers can provide shelter from predators or inclement weather, places to eat, sleep, thermoregulate, hunt, mate or reproduce. In turn, floral microhabitats contain the full range of mutualists, antagonists and apparent commensals, whose complex interactions impact how flowers look and smell, how profitable they are to foraging pollinators, and how selection feeds back upon the traits shaping those interactions. Recent studies suggest coevolutionary paths by which floral symbionts might be co-opted as mutualists and provide compelling examples in which ambush predators or florivores are recruited as floral allies. Unbiased studies that include the full roster of floral symbionts are likely to reveal novel links and additional nuance in the rich ecological communities hidden within flowers.

The pollen virome: A review of pollen-associated viruses and consequences for plants and their interactions with pollinators

The movement of pollen grains from anthers to stigmas, often by insect pollinator vectors, is essential for plant reproduction. However, pollen is also a unique vehicle for viral spread. Pollen-associated plant viruses reside on the outside or inside of pollen grains, infect susceptible individuals through vertical or horizontal infection pathways, and can decrease plant fitness. These viruses are transferred with pollen between plants by pollinator vectors as they forage for floral resources; thus, pollen-associated viral spread is mediated by floral and pollen grain phenotypes and pollinator traits, much like pollination. Most of what is currently known about pollen-associated viruses was discovered through infection and transmission experiments in controlled settings, usually involving one virus and one plant species of agricultural or horticultural interest. In this review, we first provide an updated, comprehensive list of the recognized pollen-associated viruses. Then, we summarize virus, plant, pollinator vector, and landscape traits that can affect pollen-associated virus transmission, infection, and distribution. Next, we highlight the consequences of plant-pollinator-virus interactions that emerge in complex communities of co-flowering plants and pollinator vectors, such as pollen-associated virus spread between plant species and viral jumps from plant to pollinator hosts. We conclude by emphasizing the need for collaborative research that bridges pollen biology, virology, and pollination biology.

Honeybees affect floral microbiome composition in a central food source for wild pollinators in boreal ecosystems

Basic knowledge on dispersal of microbes in pollinator networks is essential for plant, insect, and microbial ecology. Thorough understanding of the ecological consequences of honeybee farming on these complex plant-pollinator-microbe interactions is a prerequisite for sustainable honeybee keeping. Most research on plant-pollinator-microbe interactions have focused on temperate agricultural systems. Therefore, information on a wild plant that is a seasonal bottleneck for pollinators in cold climate such as Salix phylicifolia is of specific importance. We investigated how floral visitation by insects influences the community structure of bacteria and fungi in Salix phylicifolia inflorescences under natural conditions. Insect visitors were experimentally excluded with net bags. We analyzed the microbiome and measured pollen removal in open and bagged inflorescences in sites where honeybees were foraging and in sites without honeybees. Site and plant individual explained most of the variation in floral microbial communities. Insect visitation and honeybees had a smaller but significant effect on the community composition of microbes. Honeybees had a specific effect on the inflorescence microbiome and, e.g., increased the relative abundance of operational taxonomic units (OTUs) from the bacterial order Lactobacillales. Site had a significant effect on the amount of pollen removed from inflorescences but this was not due to honeybees. Insect visitors increased bacterial and especially fungal OTU richness in the inflorescences. Pollinator visits explained 38% variation in fungal richness, but only 10% in bacterial richness. Our work shows that honeybee farming affects the floral microbiome in a wild plant in rural boreal ecosystems.

The secret life of plant-beneficial rhizosphere bacteria: insects as alternative hosts

Root-colonizing bacteria have been intensively investigated for their intimate relationship with plants and their manifold plant-beneficial activities. They can inhibit growth and activity of pathogens or induce defence responses. In recent years, evidence has emerged that several plant-beneficial rhizosphere bacteria do not only associate with plants but also with insects. Their relationships with insects range from pathogenic to mutualistic and some rhizobacteria can use insects as vectors for dispersal to new host plants. Thus, the interactions of these bacteria with their environment are even more complex than previously thought and can extend far beyond the rhizosphere. The discovery of this secret life of rhizobacteria represents an exciting new field of research that should link the fields of plant-microbe and insect-microbe interactions. In this review, we provide examples of plant-beneficial rhizosphere bacteria that use insects as alternative hosts, and of potentially rhizosphere-competent insect symbionts. We discuss the bacterial traits that may enable a host-switch between plants and insects and further set the multi-host lifestyle of rhizobacteria into an evolutionary and ecological context. Finally, we identify important open research questions and discuss perspectives on the use of these rhizobacteria in agriculture.

Contrasting effects of nectar yeasts on the reproduction of Mediterranean plant species

PREMISE

Yeasts are often present in floral nectar and can influence plant fitness directly (independently of pollinators) or indirectly by influencing pollinator visitation and behavior. However, few studies have assessed the effect of nectar yeasts on plant reproductive success or compared effects across different plant species, limiting our understanding of the relative impact of direct vs. indirect effects.

METHODS

We inoculated the nectar of six plant species in the field with the cosmopolitan yeast Metschnikowia reukaufii to analyze the direct and indirect effects on female reproductive success over 2 years. The pollinator assemblage for each species was recorded during both flowering years.

RESULTS

Direct yeast effects on female fecundity were statistically nonsignificant for all plant species. There were significant indirect, pollinator-mediated effects on fruit production and seed mass for the two species pollinated almost exclusively by bumblebees or hawkmoths, with the direction of the effects differing for the quantity- and quality-related fitness components. There were no consistent effects of the yeast on maternal fecundity for any of the species with diverse pollinator assemblages.

CONCLUSIONS

Effects of M. reukaufii on plant reproduction ranged from negative to neutral or positive depending on the plant species. The among-species variation in the indirect effects of nectar yeasts on plant pollination could reflect variation in the pollinator community, the specific microbes colonizing the nectar, and the order of microbial infection (priority effects), determining potential species interactions. Elucidating the nature of these multitrophic plant-pollinator-microbe interactions is important to understand complex processes underlying plant pollination.

Horsenettle (Solanum carolinense) fruit bacterial communities are not variable across fine spatial scales

Fruit house microbial communities that are unique from the rest of the plant. While symbiotic microbial communities complete important functions for their hosts, the fruit microbiome is often understudied compared to other plant organs. Fruits are reproductive tissues that house, protect, and facilitate the dispersal of seeds, and thus they are directly tied to plant fitness. Fruit microbial communities may, therefore, also impact plant fitness. In this study, we assessed how bacterial communities associated with fruit of Solanum carolinense, a native herbaceous perennial weed, vary at fine spatial scales (<0.5 km). A majority of the studies conducted on plant microbial communities have been done at large spatial scales and have observed microbial community variation across these large spatial scales. However, both the environment and pollinators play a role in shaping plant microbial communities and likely have impacts on the plant microbiome at fine scales. We collected fruit samples from eight sampling locations, ranging from 2 to 450 m apart, and assessed the fruit bacterial communities using 16S rRNA gene amplicon sequencing. Overall, we found no differences in observed richness or microbial community composition among sampling locations. Bacterial community structure of fruits collected near one another were not more different than those that were farther apart at the scales we examined. These fine spatial scales are important to obligate out-crossing plant species such as S. carolinense because they are ecologically relevant to pollinators. Thus, our results could imply that pollinators serve to homogenize fruit bacterial communities across these smaller scales.

Nectar bacteria stimulate pollen germination and bursting to enhance microbial fitness

Many organisms consume pollen, yet mechanisms of its digestion remain a fundamental enigma in pollination biology,^1-3 as pollen is protected by a recalcitrant outer shell.^4-8 Pollen is commonly found in floral nectar,^9,10 as are nectar microbes, which are nearly ubiquitous among flowers.^11-13 Nectar specialist bacteria, like Acinetobacter, can reach high densities (up to 10⁹ cells/mL), despite the fact that floral nectar is nitrogen poor.^14-17 Here, we show evidence that the genus Acinetobacter, prevalent nectar- and bee-associated bacteria,^12,18-20 can induce pollen germination and bursting, gain access to protoplasm nutrients, and thereby grow to higher densities. Although induced germination had been suggested as a potential method in macroscopic pollen consumers,^2,21-23 and fungal inhibition of pollen germination has been shown,^24-27 direct biological induction of germination has not been empirically documented outside of plants.^28-32Acinetobacter pollinis SCC477¹⁹ induced over 5× greater pollen germination and 20× greater pollen bursting than that of uninoculated pollen by 45 min. When provided with germinable pollen, A. pollinis stimulates protein release and grows to nearly twice the density compared to growth with ungerminable pollen, indicating that stimulation of germination benefits bacterial fitness. In contrast, a common nectar-inhabiting yeast (Metschnikowia)³³ neither induced nor benefited from pollen germination. We conclude that Acinetobacter both specifically causes and benefits from inducing pollen germination and bursting. Further study of microbe-pollen interactions may inform many aspects of pollination ecology, including floral microbial ecology,^34,35 pollinator nutrient acquisition from pollen,^2,3,21,36 and cues of pollen germination for plant reproduction.^37-39.

Invertebrate decline reduces bacterial diversity associated with leaves and flowers

Defaunation including invertebrate decline is one of the major consequences of anthropogenic alterations of the environment. Despite recent reports of ubiquitous invertebrate decline, the ecosystem consequences have been rarely documented. We exposed standardized plant communities grown in the iDiv Ecotron to different levels of invertebrate numbers and biomass and tracked effects on the diversity and composition of bacterial communities associated with flowers and leaves of Scorzoneroides autumnalis and Trifolium pratense using next-generation 16S rRNA gene amplicon sequencing. Our data indicate that invertebrate decline reduces bacterial richness and β-diversity and alters community composition. These effects may result from direct effects of invertebrates that may serve as dispersal agents of bacteria; or from indirect effects where animal-induced changes in the plant's phenotype shape the niches plants provide for bacterial colonizers. Because bacteria are usually not dispersal limited and because species sorting, i.e. niche-based processes, has been shown to be a dominant process in bacterial community assembly, indirect effects may be more likely. Given that a healthy microbiome is of fundamental importance for the well-being of plants, animals (including humans) and ecosystems, a loss of bacterial diversity may be a dramatic yet previously unknown consequence of current invertebrate decline.

Transmitting silks of maize have a complex and dynamic microbiome

In corn/maize, silks emerging from cobs capture pollen, and transmit resident sperm nuclei to eggs. There are > 20 million silks per U.S. maize acre. Fungal pathogens invade developing grain using silk channels, including Fusarium graminearum (Fg, temperate environments) and devastating carcinogen-producers (Africa/tropics). Fg contaminates cereal grains with mycotoxins, in particular Deoxynivalenol (DON), known for adverse health effects on humans and livestock. Fitness selection should promote defensive/healthy silks. Here, we report that maize silks, known as styles in other plants, possess complex and dynamic microbiomes at the critical pollen-fungal transmission interval (henceforth: transmitting style microbiome, TSM). Diverse maize genotypes were field-grown in two trial years. MiSeq 16S rRNA gene sequencing of 328 open-pollinated silk samples (healthy/Fg-infected) revealed that the TSM contains > 5000 taxa spanning the prokaryotic tree of life (47 phyla/1300 genera), including nitrogen-fixers. The TSM of silk tip tissue displayed seasonal responsiveness, but possessed a reproducible core of 7–11 MiSeq-amplicon sequence variants (ASVs) dominated by a single Pantoea MiSeq-taxon (15–26% of sequence-counts). Fg-infection collapsed TSM diversity and disturbed predicted metabolic functionality, but doubled overall microbiome size/counts, primarily by elevating 7–25 MiSeq-ASVs, suggestive of a selective microbiome response against infection. This study establishes the maize silk as a model for fundamental/applied research of plant reproductive microbiomes.

Pollinators mediate floral microbial diversity and microbial network under agrochemical disturbance

How pollinators mediate microbiome assembly in the anthosphere is a major unresolved question of theoretical and applied importance in the face of anthropogenic disturbance. We addressed this question by linking visitation of diverse pollinator functional groups (bees, wasps, flies, butterflies, beetles, true bugs and other taxa) to the key properties of the floral microbiome (microbial α- and β-diversity and microbial network) under agrochemical disturbance, using a field experiment of bactericide and fungicide treatments on cultivated strawberries that differ in flower abundance. Structural equation modelling was used to link agrochemical disturbance and flower abundance to pollinator visitation to floral microbiome properties. Our results revealed that (i) pollinator visitation influenced the α- and β-diversity and network centrality of the floral microbiome, with different pollinator functional groups affecting different microbiome properties; (ii) flower abundance influenced the floral microbiome both directly by governing the source pool of microbes and indirectly by enhancing pollinator visitation; and (iii) agrochemical disturbance affected the floral microbiome primarily directly by fungicide, and less so indirectly via pollinator visitation. These findings improve the mechanistic understanding of floral microbiome assembly, and may be generalizable to many other plants that are visited by diverse insect pollinators in natural and managed ecosystems.

Microbes and pollinator behavior in the floral marketplace

Pollinator foraging decisions shape microbial dispersal, and microbes change floral phenotypes in ways perceivable by pollinators. Yet, the role microbes play in the cognitive ecology of pollination is relatively unexplored. Reviewing recent literature on floral microbial ecology and pollinator behavior, we advocate for further integration between these two fields. Insights into pollinator learning, memory, and decision-making can help explain their responses to microbially-altered floral phenotypes. Specifically, considering how pollinators forage for multiple nutrients, cope with uncertainty, structure foraging bouts, and move through their environment could inform predictions about microbial dispersal within plant communities. We highlight how behavior connects microbial changes in floral phenotype to downstream effects on both microbial dispersal and plant fitness.

Volatile microbial semiochemicals and insect perception at flowers

Many plant-associated microbial communities produce volatile signals that influence insect responses, yet the impact of floral microorganisms has received less attention than other plant microbiomes. Floral microorganisms alter plant and floral odors by adding their own emissions or modifying plant volatiles. These contextual and microbe species-specific changes in floral signaling are detectable by insects and can modify their behavior. Opportunities for future work in floral systems include identifying specific microbial semiochemicals that underlie insect behavioral responses and examining if insect species vary in their responses to microbial volatiles. Examining if documented patterns are consistent across diverse plant-microbe-insect interactions and in realistic plant-based studies will improve our understanding of how microbes mediate pollination interactions in complex system.

Integrating microbes into pollination

Microbes (fungi, bacteria and viruses) living within flowers are hypothesized to affect pollination. We evaluate current support for this idea at each stage of the pollination process. Evidence to date is convincing that microbes influence pollinator attraction, but data are heavily weighted toward bumblebees and the effects of nectar yeasts. Effects of microbes on the efficacy of pollinator visits is understudied and variable outcomes from field studies suggest quality of pollinator visits, not only quantity, are likely involved. The effect of microbes on pollen performance is underappreciated. Beyond the effect of pathogenic viruses, the impacts of pollen-transmitted endophytic microbes on pollen viability or tube growth are unknown but could affect the outcome of pollen receipt. Future research integrating microbes into pollination should broaden taxonomic diversity of microbes, pollinators and plants and the processes under study.

Fertilization alters protistan consumers and parasites in crop-associated microbiomes

Crop plants carry an enormous diversity of microbiota that provide massive benefits to hosts. Protists, as the main microbial consumers and a pivotal driver of biogeochemical cycling processes, remain largely understudied in the plant microbiome. Here, we characterized the diversity and composition of protists in sorghum leaf phyllosphere, and rhizosphere and bulk soils, collected from an 8-year field experiment with multiple fertilization regimes. Phyllosphere was an important habitat for protists, dominated by Rhizaria, Alveolata and Amoebozoa. Rhizosphere and bulk soils had a significantly higher diversity of protists than the phyllosphere, and the protistan community structure significantly differed among the three plant-soil compartments. Fertilization significantly altered specific functional groups of protistan consumers and parasites. Variation partitioning models revealed that soil properties, bacteria and fungi predicted a significant proportion of the variation in the protistan communities. Changes in protists may in turn significantly alter the compositions of bacterial and fungal communities from the top-down control in food webs. Altogether, we provide novel evidence that fertilization significantly affects the functional groups of protistan consumers and parasites in crop-associated microbiomes, which have implications for the potential changes in their ecological functions under intensive agricultural managements.

Spatially explicit depiction of a floral epiphytic bacterial community reveals role for environmental filtering within petals

The microbiome of flowers (anthosphere) is an understudied compartment of the plant microbiome. Within the flower, petals represent a heterogeneous environment for microbes in terms of resources and environmental stress. Yet, little is known of drivers of structure and function of the epiphytic microbial community at the within-petal scale. We characterized the petal microbiome in two co-flowering plants that differ in the pattern of ultraviolet (UV) absorption along their petals. Bacterial communities were similar between plant hosts, with only rare phylogenetically distant species contributing to differences. The epiphyte community was highly culturable (75% of families) lending confidence in the spatially explicit isolation and characterization of bacteria. In one host, petals were heterogeneous in UV absorption along their length, and in these, there was a negative relationship between growth rate and position on the petal, as well as lower UV tolerance in strains isolated from the UV-absorbing base than from UV reflecting tip. A similar pattern was not seen in microbes isolated from a second host whose petals had uniform patterning along their length. Across strains, the variation in carbon usage and chemical tolerance followed common phylogenetic patterns. This work highlights the value of petals for spatially explicit explorations of bacteria of the anthosphere.

The Floral Microbiome: Plant, Pollinator, and Microbial Perspectives

Flowers at times host abundant and specialized communities of bacteria and fungi that influence floral phenotypes and interactions with pollinators. Ecological processes drive variation in microbial abundance and composition at multiple scales, including among plant species, among flower tissues, and among flowers on the same plant. Variation in microbial effects on floral phenotype suggests that microbial metabolites could cue the presence or quality of rewards for pollinators, but most plants are unlikely to rely on microbes for pollinator attraction or reproduction. From a microbial perspective, flowers offer opportunities to disperse between habitats, but microbial species differ in requirements for and benefits received from such dispersal. The extent to which floral microbes shape the evolution of floral traits, influence fitness of floral visitors, and respond to anthropogenic change is unclear. A deeper understanding of these phenomena could illuminate the ecological and evolutionary importance of floral microbiomes and their role in the conservation of plant–pollinator interactions.

Climate Disruption of Plant-Microbe Interactions

Interactions between plants and microbes have important influences on evolutionary processes, population dynamics, community structure, and ecosystem function. We review the literature to document how climate change may disrupt these ecological interactions and develop a conceptual framework to integrate the pathways of plant-microbe responses to climate over different scales in space and time. We then create a blueprint to aid generalization that categorizes climate effects into changes in the context dependency of plant-microbe pairs, temporal mismatches and altered feedbacks over time, or spatial mismatches that accompany species range shifts. We pair a new graphical model of how plant-microbe interactions influence resistance to climate change with a statistical approach to predictthe consequences of increasing variability in climate. Finally, we suggest pathways through which plant-microbe interactions can affect resilience during recovery from climate disruption. Throughout, we take a forward-looking perspective, highlighting knowledge gaps and directions for future research.

Floral fungal-bacterial community structure and co-occurrence patterns in four sympatric island plant species

Flowers' fungal and bacterial communities can exert great impacts on host plant wellness and reproductive success-both directly and indirectly through species interactions. However, information about community structure and co-occurrence patterns in floral microbiome remains scarce. Here, using culture-independent methods, we investigated fungal and bacterial communities associated with stamens and pistils of four plant species (Scaevola taccada, Ipomoea cairica, Ipomoea pes-caprae, and Mussaenda kwangtungensis) growing together under the same environment conditions in an island located in South China. Plant species identity significantly influenced community composition of floral fungi but not bacteria. Stamen and pistil microbiomes did not differ in community composition, but differed in co-occurrence network topological features. Compared with the stamen network, pistil counterpart had fewer links between bacteria and fungi and showed more modular but less concentrated and connected structure. In addition, degree distribution of microbial network in each host species and each microhabitat (stamen or pistil) followed a significant power-law pattern. These results enhance our understanding in the assembly principles and ecological interactions of floral microbial communities.

The Plant Microbiome: From Ecology to Reductionism and Beyond

Methodological advances over the past two decades have propelled plant microbiome research, allowing the field to comprehensively test ideas proposed over a century ago and generate many new hypotheses. Studying the distribution of microbial taxa and genes across plant habitats has revealed the importance of various ecological and evolutionary forces shaping plant microbiota. In particular, selection imposed by plant habitats strongly shapes the diversity and composition of microbiota and leads to microbial adaptation associated with navigating the plant immune system and utilizing plant-derived resources. Reductionist approaches have demonstrated that the interaction between plant immunity and the plant microbiome is, in fact, bidirectional and that plants, microbiota, and the environment shape a complex chemical dialogue that collectively orchestrates the plantmicrobiome. The next stage in plant microbiome research will require the integration of ecological and reductionist approaches to establish a general understanding of the assembly and function in both natural and managed environments.