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.

An experimental investigation of costs of tolerance against leaf and floral herbivory in the herbaceous weed horsenettle (Solanum carolinense, Solanaceae)

Background and aims – A plant’s tolerance of herbivory depends on its ability to endure and compensate for damage so as to lessen the impact that herbivores have on the plant’s performance (e.g. its growth, reproduction, or fitness). While tolerance of herbivory is beneficial to plants, it is rarely complete, and individuals in plant populations tend to vary in their levels of tolerance. The goal of this study was to investigate potential costs associated with tolerance of leaf and floral herbivory in horsenettle (Solanum carolinense), a perennial herbaceous weed that is often subjected to high levels of damage from a diversity of herbivores. Material and methods – We exposed 96 potted individuals across eight genets of horsenettle to factorial treatments of leaf herbivory by lace bugs and simulated floral herbivory by weevils. We quantified tolerance for each plant genet for both types of herbivory in terms of the impact of damage on the number of flowers opened, number of seeds produced, and root biomass (i.e. paternal, maternal, and vegetative tolerance, respectively).Key results – Plant genets ranged widely in their ability to compensate for leaf and flower damage. While there was little evidence for tradeoffs in tolerance through the different routes, there was strong evidence of tradeoffs in genets’ abilities to tolerate herbivore damage to leaves and damage to flowers.Conclusion – Tolerance is a useful defence strategy to cope with damage caused by herbivores, but its evolution may be constrained by concomitant costs and tradeoffs. The evolutionary role of the tradeoffs identified in this study are likely to be greater the more species of herbivores a plant hosts, and the more that herbivore levels vary both spatially and temporally.

Plant spines deter herbivory by restricting caterpillar movement

The spines of flowering plants are thought to function primarily in defence against mammalian herbivores; however, we previously reported that feeding by Manduca sexta caterpillars on the leaves of horsenettle plants ( Solanum carolinense ) induces increased development of internode spines on new growth. To determine whether and how spines impact caterpillar feeding, we conducted assays with three Solanaceous plant species that vary in spine numbers ( S. carolinense , S. atropurpureum and S. aethiopicum ) and also manipulated spine numbers within each species. We found that M. sexta caterpillars located experimentally isolated target leaves much more quickly on plants with experimentally removed spines compared with plants with intact spines. Moreover, it took caterpillars longer to defoliate species with relatively high spine numbers ( S. carolinense and particularly S . atropurpureum ) compared with S. aethiopicum , which has fewer spines . These findings suggest that spines may play a significant role in defence against insect herbivores by restricting herbivore movement and increasing the time taken to access feeding sites, with possible consequences including longer developmental periods and increased vulnerability or apparency to predators.

Non-glandular trichomes of Solanum carolinense deter feeding by Manduca sexta caterpillars and cause damage to the gut peritrophic matrix

Plant trichomes constitute a first line of defence against insect herbivores. The pre- and post-ingestive defensive functions of glandular trichomes are well documented and include direct toxicity, adhesion, antinutrition and defence gene induction. By contrast, the defensive functions of non-glandular trichomes are less well characterized, although these structures are thought to serve as physical barriers that impede herbivore feeding and movement. We experimentally varied the density of stellate non-glandular trichomes in several ways to explore their pre- and post-ingestive effects on herbivores. Larvae of Manduca sexta (Sphingidae) initiated feeding faster and gained more weight on Solanum carolinense (Solanaceae) leaves having lower trichome densities (or experimentally removed trichomes) than on leaves having higher trichome densities. Adding trichomes to artificial diet also deterred feeding and adversely affected caterpillar growth relative to controls. Scanning electron and light microscopy revealed that the ingestion of stellate trichomes by M. sexta caterpillars caused extensive damage to the peritrophic membrane, a gut lining that is essential to digestion and pathogen isolation. These findings suggest that, in addition to acting as a physical barrier to deter feeding, trichomes can inhibit caterpillar growth and development via post-ingestive effects.