Flowers prepare thyselves: leaf and root herbivores induce specific changes in floral phytochemistry with consequences for plant interactions with florivores.

The phenotypic plasticity of flowering plants in response to herbivore damage to vegetative tissues can affect plant interactions with flower-feeding organisms. Such induced systemic responses are probably regulated by defence-related phytohormones that signal flowers to alter secondary chemistry that affects resistance to florivores. Current knowledge on the effects of damage to vegetative tissues on plant interactions with florivores and the underlying mechanisms is limited. We compared the preference and performance of two florivores on flowering Brassica nigra plants damaged by one of three herbivores feeding from roots or leaves. To investigate the underlying mechanisms, we quantified expression patterns of marker genes for defence-related phytohormonal pathways, and concentrations of phytohormones and glucosinolates in buds and flowers. Florivores displayed contrasting preferences for plants damaged by herbivores feeding on roots and leaves. Chewing florivores performed better on plants damaged by folivores, but worse on plants damaged by the root herbivore. Chewing root and foliar herbivory led to specific induced changes in the phytohormone profile of buds and flowers. This resulted in increased glucosinolate concentrations for leaf-damaged plants, and decreased glucosinolate concentrations for root-damaged plants. The outcome of herbivore-herbivore interactions spanning from vegetative tissues to floral tissues is unique for the inducing root/leaf herbivore and receiving florivore combination.

Plant defence to sequential attack is adapted to prevalent herbivores.

Plants have evolved plastic defence strategies to deal with the uncertainty of when, by which species and in which order attack by herbivores will take place1-3. However, the responses to current herbivore attack may come with a cost of compromising resistance to other, later arriving herbivores. Due to antagonistic cross-talk between physiological regulation of plant resistance to phloem-feeding and leaf-chewing herbivores4-8, the feeding guild of the initial herbivore is considered to be the primary factor determining whether resistance to subsequent attack is compromised. We show that, by investigating 90 pairwise insect-herbivore interactions among ten different herbivore species, resistance of the annual plant Brassica nigra to a later arriving herbivore species is not explained by feeding guild of the initial attacker. Instead, the prevalence of herbivore species that arrive on induced plants as approximated by three years of season-long insect community assessments in the field explained cross-resistance. Plants maintained resistance to prevalent herbivores in common patterns of herbivore arrival and compromises in resistance especially occurred for rare patterns of herbivore attack. We conclude that plants tailor induced defence strategies to deal with common patterns of sequential herbivore attack and anticipate arrival of the most prevalent herbivores.

Insect species richness affects plant responses to multi-herbivore attack.

Plants are often attacked by multiple insect herbivores. How plants deal with an increasing richness of attackers from a single or multiple feeding guilds is poorly understood. We subjected black mustard (Brassica nigra) plants to 51 treatments representing attack by an increasing species richness (one, two or four species) of either phloem feeders, leaf chewers, or a mix of both feeding guilds when keeping total density of attackers constant and studied how this affects plant resistance to subsequent attack by caterpillars of the diamondback moth (Plutella xylostella). Increased richness in phloem-feeding attackers compromised resistance to P. xylostella. By contrast, leaf chewers induced a stronger resistance to subsequent attack by caterpillars of P. xylostella while species richness did not play a significant role for chewing herbivore induced responses. Attack by a mix of herbivores from different feeding guilds resulted in plant resistance similar to resistance levels of plants that were not previously exposed to herbivory. We conclude that B. nigra plants channel their defence responses stronger towards a feeding-guild specific response when under multi-species attack by herbivores of the same feeding guild, but integrate responses when simultaneously confronted with a mix of herbivores from different feeding guilds.

Community-level interactions between plants and soil biota during range expansion.

Plant species that expand their range in response to current climate change will encounter soil communities that may hinder, allow or even facilitate plant performance. It has been shown repeatedly for plant species originating from other continents that these plants are less hampered by soil communities from the new than from the original range. However, information about the interactions between intra-continental range expanders and soil communities is sparse, especially at community level.Here we used a plant-soil feedback experiment approach to examine if the interactions between range expanders and soil communities change during range expansion. We grew communities of range-expanding and native plant species with soil communities originating from the original and new range of range expanders. In these conditioned soils, we determined the composition of fungi and bacteria by high-throughput amplicon sequencing of the ITS region and the 16S rRNA gene respectively. Nematode community composition was determined by microscopy-based morphological identification. Then we tested how these soil communities influence the growth of subsequent communities of range expanders and natives.We found that after the conditioning phase soil bacterial, fungal and nematode communities differed by origin and by conditioning plant communities. Despite differences in bacterial, fungal and nematode communities between original and new range, soil origin did not influence the biomass production of plant communities. Both native and range expanding plant communities produced most above-ground biomass in soils that were conditioned by plant communities distantly related to them. Synthesis. Communities of range-expanding plant species shape specific soil communities in both original and new range soil. Plant-soil interactions of range expanders in communities can be similar to the ones of their closely related native plant species.

Latitudinal variation in soil nematode communities under climate warming-related range-expanding and native plants.

Current climate change has led to latitudinal and altitudinal range expansions of numerous species. During such range expansions, plant species are expected to experience changes in interactions with other organisms, especially with belowground biota that have a limited dispersal capacity. Nematodes form a key component of the belowground food web as they include bacterivores, fungivores, omnivores and root herbivores. However, their community composition under climate change-driven intracontinental range-expanding plants has been studied almost exclusively under controlled conditions, whereas little is known about actual patterns in the field. Here, we use novel molecular sequencing techniques combined with morphological quantification in order to examine nematode communities in the rhizospheres of four range-expanding and four congeneric native species along a 2,000 km latitudinal transect from South-Eastern to North-Western Europe. We tested the hypotheses that latitudinal shifts in nematode community composition are stronger in range-expanding plant species than in congeneric natives and that in their new range, range-expanding plant species accumulate fewest root-feeding nematodes. Our results show latitudinal variation in nematode community composition of both range expanders and native plant species, while operational taxonomic unit richness remained the same across ranges. Therefore, range-expanding plant species face different nematode communities at higher latitudes, but this is also the case for widespread native plant species. Only one of the four range-expanding plant species showed a stronger shift in nematode community composition than its congeneric native and accumulated fewer root-feeding nematodes in its new range. We conclude that variation in nematode community composition with increasing latitude occurs for both range-expanding and native plant species and that some range-expanding plant species may become released from root-feeding nematodes in the new range.

Symbiotic polydnavirus and venom reveal parasitoid to its hyperparasitoids.

Symbiotic relationships may provide organisms with key innovations that aid in the establishment of new niches. For example, during oviposition, some species of parasitoid wasps, whose larvae develop inside the bodies of other insects, inject polydnaviruses into their hosts. These symbiotic viruses disrupt host immune responses, allowing the parasitoid's progeny to survive. Here we show that symbiotic polydnaviruses also have a downside to the parasitoid's progeny by initiating a multitrophic chain of interactions that reveals the parasitoid larvae to their enemies. These enemies are hyperparasitoids that use the parasitoid progeny as host for their own offspring. We found that the virus and venom injected by the parasitoid during oviposition, but not the parasitoid progeny itself, affected hyperparasitoid attraction toward plant volatiles induced by feeding of parasitized caterpillars. We identified activity of virus-related genes in the caterpillar salivary gland. Moreover, the virus affected the activity of elicitors of salivary origin that induce plant responses to caterpillar feeding. The changes in caterpillar saliva were critical in inducing plant volatiles that are used by hyperparasitoids to locate parasitized caterpillars. Our results show that symbiotic organisms may be key drivers of multitrophic ecological interactions. We anticipate that this phenomenon is widespread in nature, because of the abundance of symbiotic microorganisms across trophic levels in ecological communities. Their role should be more prominently integrated in community ecology to understand organization of natural and managed ecosystems, as well as adaptations of individual organisms that are part of these communities.

Parasitic wasp-associated symbiont affects plant-mediated species interactions between herbivores.

Microbial mutualistic symbiosis is increasingly recognised as a hidden driving force in the ecology of plant-insect interactions. Although plant-associated and herbivore-associated symbionts clearly affect interactions between plants and herbivores, the effects of symbionts associated with higher trophic levels has been largely overlooked. At the third-trophic level, parasitic wasps are a common group of insects that can inject symbiotic viruses (polydnaviruses) and venom into their herbivorous hosts to support parasitoid offspring development. Here, we show that such third-trophic level symbionts act in combination with venom to affect plant-mediated interactions by reducing colonisation of subsequent herbivore species. This ecological effect correlated with changes induced by polydnaviruses and venom in caterpillar salivary glands and in plant defence responses to herbivory. Because thousands of parasitoid species are associated with mutualistic symbiotic viruses in an intimate, specific relationship, our findings may represent a novel and widespread ecological phenomenon in plant-insect interactions.