Epichloë Fungal Endophytes Influence Seed-Associated Bacterial Communities

Seeds commonly harbour diverse bacterial communities that can enhance the fitness of future plants. The bacterial microbiota associated with mother plant’s foliar tissues is one of the main sources of bacteria for seeds. Therefore, any ecological factor influencing the mother plant’s microbiota may also affect the diversity of the seed’s bacterial community. Grasses form associations with beneficial vertically transmitted fungal endophytes of genus Epichloë. The interaction of plants with Epichloë endophytes and insect herbivores can influence the plant foliar microbiota. However, it is unknown whether these interactions (alone or in concert) can affect the assembly of bacterial communities in the produced seed. We subjected Lolium multiflorum plants with and without its common endophyte Epichloë occultans (E+, E-, respectively) to an herbivory treatment with Rhopalosiphum padi aphids and assessed the diversity and composition of the bacterial communities in the produced seed. The presence of Epichloë endophytes influenced the seed bacterial microbiota by increasing the diversity and affecting the composition of the communities. The relative abundances of the bacterial taxa were more similarly distributed in communities associated with E+ than E- seeds with the latter being dominated by just a few bacterial groups. Contrary to our expectations, seed bacterial communities were not affected by the aphid herbivory experienced by mother plants. We speculate that the enhanced seed/seedling performance documented for Epichloë-host associations may be explained, at least in part, by the Epichloë-mediated increment in the seed-bacterial diversity, and that this phenomenon may be applicable to other plant-endophyte associations.

Do foliar anthocyanin pigments in horopito (Pseudowintera colorata) function as visual signals to deter insect herbivores?

<p>Anthocyanin pigments are synthesised in the leaves of many plants, however the adaptive significance of these pigments is not entirely understood. It has been postulated that their red colours may function as visual signals through coevolution between herbivorous insects and their host tree species, though the hypothesis lacks solid empirical evidence. I investigated the leaf signalling hypothesis using Pseudowintera colorata, focusing on five areas: 1) I exploited the natural polymorphism in leaf colour of P. colorata to test the predictions that (i) bright leaf colour is a reliable signal of a plant’s defensive commitment; (ii) insects in the field avoid trees that are brightly coloured; and (iii) the trees with the brightest leaves will have higher fitness. Relative to green leaves, redder foliage contained higher concentrations of polygodial, a sesquiterpene dialdehyde known to have strong antifeedant properties, and incurred less insect feeding damage. Redder trees hosted fewer Ctenopseustis spp. leafroller larvae than neighbouring matched green trees. Contrary to the predictions of the leaf signalling hypothesis, there was no difference in any of the measured fitness parameters between red and green trees, indicating that the leaf colour polymorphism in P. colorata is stable. 2) Many insects are sensitive to volatile organic compounds (VOCs), however the role of VOCs in plant-herbivore signalling has not been investigated. I analysed VOCs released from undamaged, herbivore- and mechanically-damaged red and green leaves of P. colorata, and the olfactory preferences of brownheaded leafroller (C. obliquana) larvae. While the VOC profiles of browsed and unbrowsed leaves were statistically distinguishable, the VOC profiles released from intact, herbivore-, and mechanically-damaged P. colorata leaves did not reliably identify leaf colour. Moreover, naïve and experienced C. obliquana larvae displayed no preference for the volatiles from mechanically-damaged red or green leaves. Therefore, I concluded that VOC compounds are not likely to play a large role in mediating insect herbivore-plant interactions in P. colorata. 3) Studies of leaf signalling rarely consider the influence of the light-absorbing properties of non-green pigments upon photosynthesis. I compared the photosynthetic and photoinhibitory responses of red and green leaves from matched, neighbouring pairs of P. colorata of contrasting colour. Redder P. colorata leaves in the field had a lower maximum photosynthetic assimilation rate than matched green leaves from neighbouring trees. However, I was unable to detect any measurable advantage in terms of photoprotection in the red P. colorata leaves as indicated by chlorophyll fluorescence profiles. My results indicate that the presence of anthocyanin pigments within non-senescing leaves may impose a slight photosynthetic cost to the plant. 4) I used literature searches, field surveys and laboratory bioassays to identify which invertebrate herbivores are most likely to participate in leaf-signalling interactions with P. colorata. Feeding preference bioassays showed that brownheaded leafrollers (C. obliquana and C. herana) and Auckland tree weta (Hemideina thoracica) preferentially consumed leaf material from green than red P. colorata leaves. Results from these bioassays, combined with my field surveys suggest that Ctenopseustis spp. leafroller larvae are the most likely coevolution partners for P. colorata. 5) There is a well-established link between nitrogen deficiency and leaf reddening. Additionally, leaf nutrients can influence foraging behaviour and performance of insect herbivores. I measured N and C contents of leaves from neighbouring matched pairs of red and green P. colorata. There were no significant differences in the amounts of, or ratio between, N and C between matched red and green leaves. This result indicates that differences in colour and herbivory among P. colorata leaves are not attributable to differences in leaf nutrients. Taken together, my results suggest that foliar anthocyanins in P. colorata do function as visual signals, however their effect on herbivory is small. Additionally, interindividual variation in non-senescing leaf colour in P. colorata may be stable due to a trade off between signalling and photosynthesis. Discussions of leaf signalling need to follow the examples of other fields studying the interactions between plants and insects and move from overly simple models to those that incorporate more of the complexity that is observed in the natural world.</p>

Do foliar anthocyanin pigments in horopito (Pseudowintera colorata) function as visual signals to deter insect herbivores?

<p>Anthocyanin pigments are synthesised in the leaves of many plants, however the adaptive significance of these pigments is not entirely understood. It has been postulated that their red colours may function as visual signals through coevolution between herbivorous insects and their host tree species, though the hypothesis lacks solid empirical evidence. I investigated the leaf signalling hypothesis using Pseudowintera colorata, focusing on five areas: 1) I exploited the natural polymorphism in leaf colour of P. colorata to test the predictions that (i) bright leaf colour is a reliable signal of a plant’s defensive commitment; (ii) insects in the field avoid trees that are brightly coloured; and (iii) the trees with the brightest leaves will have higher fitness. Relative to green leaves, redder foliage contained higher concentrations of polygodial, a sesquiterpene dialdehyde known to have strong antifeedant properties, and incurred less insect feeding damage. Redder trees hosted fewer Ctenopseustis spp. leafroller larvae than neighbouring matched green trees. Contrary to the predictions of the leaf signalling hypothesis, there was no difference in any of the measured fitness parameters between red and green trees, indicating that the leaf colour polymorphism in P. colorata is stable. 2) Many insects are sensitive to volatile organic compounds (VOCs), however the role of VOCs in plant-herbivore signalling has not been investigated. I analysed VOCs released from undamaged, herbivore- and mechanically-damaged red and green leaves of P. colorata, and the olfactory preferences of brownheaded leafroller (C. obliquana) larvae. While the VOC profiles of browsed and unbrowsed leaves were statistically distinguishable, the VOC profiles released from intact, herbivore-, and mechanically-damaged P. colorata leaves did not reliably identify leaf colour. Moreover, naïve and experienced C. obliquana larvae displayed no preference for the volatiles from mechanically-damaged red or green leaves. Therefore, I concluded that VOC compounds are not likely to play a large role in mediating insect herbivore-plant interactions in P. colorata. 3) Studies of leaf signalling rarely consider the influence of the light-absorbing properties of non-green pigments upon photosynthesis. I compared the photosynthetic and photoinhibitory responses of red and green leaves from matched, neighbouring pairs of P. colorata of contrasting colour. Redder P. colorata leaves in the field had a lower maximum photosynthetic assimilation rate than matched green leaves from neighbouring trees. However, I was unable to detect any measurable advantage in terms of photoprotection in the red P. colorata leaves as indicated by chlorophyll fluorescence profiles. My results indicate that the presence of anthocyanin pigments within non-senescing leaves may impose a slight photosynthetic cost to the plant. 4) I used literature searches, field surveys and laboratory bioassays to identify which invertebrate herbivores are most likely to participate in leaf-signalling interactions with P. colorata. Feeding preference bioassays showed that brownheaded leafrollers (C. obliquana and C. herana) and Auckland tree weta (Hemideina thoracica) preferentially consumed leaf material from green than red P. colorata leaves. Results from these bioassays, combined with my field surveys suggest that Ctenopseustis spp. leafroller larvae are the most likely coevolution partners for P. colorata. 5) There is a well-established link between nitrogen deficiency and leaf reddening. Additionally, leaf nutrients can influence foraging behaviour and performance of insect herbivores. I measured N and C contents of leaves from neighbouring matched pairs of red and green P. colorata. There were no significant differences in the amounts of, or ratio between, N and C between matched red and green leaves. This result indicates that differences in colour and herbivory among P. colorata leaves are not attributable to differences in leaf nutrients. Taken together, my results suggest that foliar anthocyanins in P. colorata do function as visual signals, however their effect on herbivory is small. Additionally, interindividual variation in non-senescing leaf colour in P. colorata may be stable due to a trade off between signalling and photosynthesis. Discussions of leaf signalling need to follow the examples of other fields studying the interactions between plants and insects and move from overly simple models to those that incorporate more of the complexity that is observed in the natural world.</p>

Herbivory on the pedunculate oak along urbanization range in Europe: effects of local forest cover and insect feeding guild

Urbanization is recognized as an important driver of the diversity and abundance of tree associated insect herbivores, but its consequences for insect herbivory are controversial. A likely source of variability among studies is the insufficient consideration of intra-urban variability in forest cover. With the help of citizen scientists, we investigated the independent and interactive effect of urbanization and local canopy cover on insect herbivory in the pedunculate oak (Quercus robur) throughout most of its geographic range in Europe. The damage caused by chewing insect herbivores as well as the incidence of leaf-mining and gall-inducing herbivores consistently decreased with increasing urbanization around focal oaks. Herbivory by chewing herbivores increased with increasing forest cover, regardless of urbanization. In contrast, an increase in local canopy cover buffered the negative effect of urbanization on leaf-miners and strengthened its effect on gall-inducers. These results show the complexity of plant-herbivore interactions in urbanized areas, highlighting that the presence of local canopy cover within cities has the potential to attenuate or modify the effect of urbanization on biotic interactions.

Olfactory Responses of Trissolcus Mitsukurii to Plants Attacked by Target and Non-target Stink Bugs Suggest Low Risk for Biological Control

In crop systems, successful management of invasive insect herbivores can be achieved through the introduction of exotic biocontrol agents, i.e. parasitoids or predators, having a coevolutionary history with the pest. To avert threats to local biodiversity, recent legislations require a risk assessment for the organism to be released. Evaluation of its ability to exploit, for host location, odours associated with target and non-target species is crucial for a better definition of its ecological host range. Using Y-tube olfactometer in quarantine laboratory, we investigated the ability of the Asiatic egg parasitoid Trissolcus mitsukurii (Hymenoptera: Scelionidae) to exploit odours associated with the global invader Halyomorpha halys (Hemiptera: Pentatomidae) and with non-target stink bugs native to Southern Europe. We demonstrated that T. mitsukurii is attracted by plants exposed to feeding and egg deposition of the coevolved H. halys or the native Nezara viridula, while it is not attracted by physogastric females or eggs alone. Remarkably, T. mitsukurii is repelled by plants bearing eggs of the beneficial stink bug, Arma custos. Our results usefully contribute to a more thorough and nuanced assessment of the potential non-target risks in case of massive parasitoid release.

Risk of herbivory negatively correlates with the diversity of volatile emissions involved in plant communication

Plant-to-plant volatile-mediated communication and subsequent induced resistance to insect herbivores is common. Less clear is the adaptive significance of these interactions; what selective mechanisms favour plant communication and what conditions allow individuals to benefit by both emitting and responding to cues? We explored the predictions of two non-exclusive hypotheses to explain why plants might emit cues, the kin selection hypothesis (KSH) and the mutual benefit hypothesis (MBH). We examined 15 populations of sagebrush that experience a range of naturally occurring herbivory along a 300 km latitudinal transect. As predicted by the KSH, we found several uncommon chemotypes with some chemotypes occurring only within a single population. Consistent with the MBH, chemotypic diversity was negatively correlated with herbivore pressure; sites with higher levels of herbivory were associated with a few common cues broadly recognized by most individuals. These cues varied among different populations. Our results are similar to those reported for anti-predator signalling in vertebrates.