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

Hyperaccumulation of nickel but not selenium drives floral microbiome differentiation: A study with six species of Brassicaceae

PREMISE

Intraspecific variation in flower microbiome composition can mediate pollination and reproduction, and so understanding the community assembly processes driving this variation is critical. Yet the relative importance of trait-based host filtering and dispersal in shaping among-species variation in floral microbiomes remains unknown.

METHODS

Within two clades of Brassicaceae, we compared diversity and composition of floral microbiomes in natural populations of focal nickel and selenium hyperaccumulator species and two of their non-accumulating relatives. We assessed the relative strengths of floral elemental composition, plant phylogenetic distance (host filtering), and geography (dispersal) in driving floral microbiome composition.

RESULTS

Species in the nickel hyperaccumulator clade had strongly divergent floral microbiomes, the most of that variation driven by floral elemental composition, followed by geographic distance between plant populations and, lastly, phylogenetic distance. Conversely, within the selenium hyperaccumulator clade, floral microbiome divergence was much lower among the species and elemental composition, geography, and plant phylogeny were far weaker determinants of microbiome variation.

CONCLUSIONS

Our results show that the strength of elemental hyperaccumulation's effect on floral microbiomes differs substantially among plant clades, possibly due to variation in elements as selective filters or in long-distance dispersal probability in different habitats.

Do flower-colonizing microbes influence floral evolution? A test with fast-cycling Brassica

Pollinators are thought to be the main drivers of floral evolution. Flowers are also colonized by abundant communities of microbes that can affect the interaction between plants and their pollinators. Very little is known, however, about how flower-colonizing microbes influence floral evolution. Here we performed a 6-generation experimental evolution study using fast-cycling Brassica rapa, in which we factorially manipulated the presence of pollinators and flower microbes to determine how pollinators and microbes interact in driving floral evolution. We measured the evolution of 6 morphological traits, as well as the plant mating system and flower attractiveness. Only one of the 6 traits (flower number) evolved in response to pollinators, while microbes did not drive the evolution of any trait, nor did they interact with pollinators in driving the evolution of morphological traits. Moreover, we did not find evidence that pollinators or microbes affected the evolution of flower attractiveness to pollinators. However, we found an interactive effect of pollinators and microbes on the evolution of autonomous selfing, a trait that is expected to evolve in response to pollinator limitations. Overall, we found only weak evidence that microbes mediate floral evolution. However, our ability to detect an interactive effect of pollinators and microbes might have been limited by weak pollinator-mediated selection in our experimental setting. Our results contrast with previous (similar) experimental evolution studies, highlighting the susceptibility of such experiments to drift and to experimental artefacts.

A quantitative survey of the blueberry (Vaccinium spp.) culturable nectar microbiome: variation between cultivars, locations, and farm management approaches

Microbes in floral nectar can impact both their host plants and floral visitors, yet little is known about the nectar microbiome of most pollinator-dependent crops. In this study, we examined the abundance and composition of the fungi and bacteria inhabiting Vaccinium spp. nectar, as well as nectar volume and sugar concentrations. We compared wild V. myrsinites with two field-grown V. corymbosum cultivars collected from two organic and two conventional farms. Differences in nectar traits and microbiomes were identified between V. corymbosum cultivars but not Vaccinium species. The microbiome of cultivated plants also varied greatly between farms, whereas management regime had only subtle effects, with higher fungal populations detected under organic management. Nectars were hexose-dominant, and high cell densities were correlated with reduced nectar sugar concentrations. Bacteria were more common than fungi in blueberry nectar, although both were frequently detected and co-occurred more often than would be predicted by chance. "Cosmopolitan" blueberry nectar microbes that were isolated in all plants, including Rosenbergiella sp. and Symmetrospora symmetrica, were identified. This study provides the first systematic report of the blueberry nectar microbiome, which may have important implications for pollinator and crop health.

Floral nectar and honeydew microbial diversity and their role in biocontrol of insect pests and pollination

Sugar-rich plant-related secretions, such as floral nectar and honeydew, that are commonly used as nutrient sources by insects and other animals, are also the ecological niche for diverse microbial communities. Recent research has highlighted the great potential of nectar and honeydew microbiomes in biological pest control and improved pollination, but the exploitation of these microbiomes requires a deep understanding of their community dynamics and plant-microbe-insect interactions. Additionally, the successful application of microbes in crop fields is conditioned by diverse ecological, legal, and ethical challenges that should be taken into account. In this article, we provide an overview of the nectar and honeydew microbiomes and discuss their potential applications in sustainable agricultural practices.

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.

Nature's Most Fruitful Threesome: The Relationship between Yeasts, Insects, and Angiosperms

The importance of insects for angiosperm pollination is widely recognized. In fact, approximately 90% of all plant species benefit from animal-mediated pollination. However, only recently, a third part player in this story has been properly acknowledged. Microorganisms inhabiting floral nectar, among which yeasts have a prominent role, can ferment glucose, fructose, sucrose, and/or other carbon sources in this habitat. As a result of their metabolism, nectar yeasts produce diverse volatile organic compounds (VOCs) and other valuable metabolites. Notably, some VOCs of yeast origin can influence insects' foraging behavior, e.g., by attracting them to flowers (although repelling effects have also been reported). Moreover, when insects feed on nectar, they also ingest yeast cells, which provide them with nutrients and protect them from pathogenic microorganisms. In return, insects serve yeasts as transportation and a safer habitat during winter when floral nectar is absent. From the plant's point of view, the result is flowers being pollinated. From humanity's perspective, this ecological relationship may also be highly profitable. Therefore, prospecting nectar-inhabiting yeasts for VOC production is of major biotechnological interest. Substances such as acetaldehyde, ethyl acetate, ethyl butyrate, and isobutanol have been reported in yeast volatomes, and they account for a global market of approximately USD 15 billion. In this scenario, the present review addresses the ecological, environmental, and biotechnological outlooks of this three-party mutualism, aiming to encourage researchers worldwide to dig into this field.