Belowground Plant-Herbivore Interactions Vary among Climate-Driven Range-Expanding Plant Species with Different Degrees of Novel Chemistry

Biotechnological Tools to Elucidate the Mechanism of Plant and Nematode Interactions

Plant-parasitic nematodes (PPNs) pose a threat to global food security in both the developed and developing worlds. PPNs cause crop losses worth a total of more than USD 150 billion worldwide. The sedentary root-knot nematodes (RKNs) also cause severe damage to various agricultural crops and establish compatible relationships with a broad range of host plants. This review aims to provide a broad overview of the strategies used to identify the morpho-physiological and molecular events that occur during RKN parasitism. It describes the most current developments in the transcriptomic, proteomic, and metabolomic strategies of nematodes, which are important for understanding compatible interactions of plants and nematodes, and several strategies for enhancing plant resistance against RKNs. We will highlight recent rapid advances in molecular strategies, such as gene-silencing technologies, RNA interference (RNAi), and small interfering RNA (siRNA) effector proteins, that are leading to considerable progress in understanding the mechanism of plant-nematode interactions. We also take into account genetic engineering strategies, such as targeted genome editing techniques, the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR associated protein 9 (Cas9) (CRISPR/Cas-9) system, and quantitative trait loci (QTL), to enhance the resistance of plants against nematodes.

Soil legacy effects of plants and drought on aboveground insects in native and range-expanding plant communities

Soils contain biotic and abiotic legacies of previous conditions that may influence plant community biomass and associated aboveground biodiversity. However, little is known about the relative strengths and interactions of the various belowground legacies on aboveground plant-insect interactions. We used an outdoor mesocosm experiment to investigate the belowground legacy effects of range-expanding versus native plants, extreme drought and their interactions on plants, aphids and pollinators. We show that plant biomass was influenced more strongly by the previous plant community than by the previous summer drought. Plant communities consisted of four congeneric pairs of natives and range expanders, and their responses were not unanimous. Legacy effects affected the abundance of aphids more strongly than pollinators. We conclude that legacies can be contained as soil 'memories' that influence aboveground plant community interactions in the next growing season. These soil-borne 'memories' can be altered by climate warming-induced plant range shifts and extreme drought.

Temporal dynamics of range expander and congeneric native plant responses during and after extreme drought events

Climate change is causing range shifts of many species to higher latitudes and altitudes and increasing their exposure to extreme weather events. It has been shown that range-shifting plant species may perform differently in new soil than related natives; however, little is known about how extreme weather events affect range-expanding plants compared to related natives. In this study we used outdoor mesocosms to study how range-expanding plant species responded to extreme drought in live soil from a habitat in a new range with and without live soil from a habitat in the original range (Hungary). During summer drought, the shoot biomass of the range-expanding plant community declined. In spite of this, in the mixed community, range expanders produced more shoot biomass than congeneric natives. In mesocosms with a history of range expanders in the previous year, native plants produced less biomass. Plant legacy or soil origin effects did not change the response of natives or range expanders to summer drought. During rewetting, range expanders had less biomass than congeneric natives but higher drought resilience (survival) in soils from the new range where in the previous year native plant species had grown. The biomass patterns of the mixed plant communities were dominated by Centaurea spp.; however, not all plant species within the groups of natives and of range expanders showed the general pattern. Drought reduced the litter decomposition, microbial biomass, and abundances of bacterivorous, fungivorous, and carnivorous nematodes. Their abundances recovered during rewetting. There was less microbial and fungal biomass, and there were fewer fungivorous nematodes in soils from the original range where range expanders had grown in the previous year. We concluded that in mixed plant communities of range expanders and congeneric natives, range expanders performed better, under both ambient and drought conditions, than congeneric natives. However, when considering the responses of individual species, we observed variations among pairs of congenerics, so that under the present mixed-community conditions there was no uniformity in responses to drought of range expanders versus congeneric natives. Range-expanding plant species reduced soil fungal biomass and the numbers of soil fungivorous nematodes, suggesting that the effects of range-expanding plant species can trickle up in the soil food web.

Nematodes as Drivers of Plant Performance in Natural Systems

Nematodes form an important part of soil biodiversity as the most abundant and functionally diverse animals affecting plant performance. Most studies on plant-nematode interactions are focused on agriculture, while plant-nematode interactions in nature are less known. Here we highlight that nematodes can contribute to vegetation dynamics through direct negative effects on plants, and indirect positive effects through top-down predation on plant-associated organisms. Global change alters these interactions, of which better understanding is rapidly needed to better predict functional consequences. By expanding the knowledge of plant-nematode interactions in natural systems, an increase in basic understanding of key ecological topics such as plant-soil interactions and plant invasion dynamics will be obtained, while also increasing the insights and potential biotic repertoire to be applicable in sustainable plant management.

Plant population and soil origin effects on rhizosphere nematode community composition of a range-expanding plant species and a native congener

Climate change causes species range expansions to higher latitudes and altitudes. It is expected that, due to differences in dispersal abilities between plants and soil biota, range-expanding plant species will become associated with a partly new belowground community in their expanded range. Theory on biological invasions predicts that outside their native range, range-expanding plant species may be released from specialist natural enemies, leading to the evolution of enhanced defence against generalist enemies. Here we tested the hypothesis that expanded range populations of the range-expanding plant species Centaurea stoebe accumulate fewer root-feeding nematodes than populations from the original range. Moreover, we examined whether Centaurea stoebe accumulates fewer root-feeding nematodes in expanded range soil than in original range soil. We grew plants from three expanded range and three original range populations of C. stoebe in soil from the original and from the new range. We compared nematode communities of C. stoebe with those of C. jacea, a congeneric species native to both ranges. Our results show that expanded range populations of C. stoebe did not accumulate fewer root-feeding nematodes than populations from the original range, but that C. stoebe, unlike C. jacea, accumulated fewest root-feeding nematodes in expanded range soil. Moreover, when we examined other nematode feeding groups, we found intra-specific plant population effects on all these groups. We conclude that range-expanding plant populations from the expanded range were not better defended against root-feeding nematodes than populations from the original range, but that C. stoebe might experience partial belowground enemy release.

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.

Dihydroxyacetone of wheat root exudates serves as an attractant for Heterodera avenae

Heterodera avenae, as an obligate endoparasite, causes severe yield loss in wheat (Triticum aestivum). Investigation on the mechanisms how H. avenae perceives wheat roots is limited. Here, the attractiveness of root exudates from eight plant genotypes to H. avenae were evaluated on agar plates. Results showed that the attraction of H. avenae to the root exudates from the non-host Brachypodium distachyon variety Bd21-3 was the highest, approximately 50 infective second-stage juveniles (J2s) per plate, followed by that from three H. avenae-susceptible wheat varieties, Zhengmai9023, Yanmai84 and Xiangmai25, as well as the resistant one of Xinyuan958, whereas the lowest attractive activity was observed in the two H. avenae-resistant wheat varieties, Xianmai20 (approximately 12 J2s/plate) and Liangxing66 (approximately 11 J2s/plate). Then Bd21-3, Zhengmai9023 and Heng4399 were selected for further assays as their different attractiveness and resistance to H. avenae, and attractants for H. avenae in their root exudates were characterized to be heat-labile and low-molecular compounds (LM) by behavioral bioassay. Based on these properties of the attractants, a principle of identifying attractants for H. avenae was set up. Then LM of six root exudates from the three plants with and without heating were separated and analyzed by HPLC-MS. Finally, dihydroxyacetone (DHA), methylprednisolone succinate, embelin and diethylpropionin in the root exudates were identified to be putative attractants for H. avenae according to the principle, and the attraction of DHA to H. avenae was validated by behavioral bioassay on agar. Our study enhances the recognition to the orientation mechanism of H. avenae towards wheat roots.

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.

Contrasting responses of soil nematode communities to native and non-native woody plant expansion

Woody plant expansion into grasslands is widespread, driven by both successions to dominance by native woody species or invasion by non-native woody species. These shifts from grass- to woody-dominated systems also have profound effects on both above- and belowground communities and ecosystem processes. Woody-plant expansion should also alter the functional composition of the soil biota, including that of nematodes, which are major drivers of soil food-web structure and belowground processes, but such belowground impacts are poorly understood. We determined whether succession by a widespread native (Kunzea ericoides) and invasion by a non-native woody species (Pinus nigra) into tussock grasslands affect the composition of nematode functional guilds and the structure of nematode-based food webs. Although increasing dominance by woody species in both systems altered the functional guild composition of the nematode community, we found contrasting responses of nematode functional guilds to the different dominant plant species. Specifically, nematode communities reflected conditions of resource enrichment with increasing K. ericoides tree cover, whereas communities became structurally simplified and dominated by stress-tolerant nematode families with increasing P. nigra tree cover. Because nematodes regulate both bacterial- and fungal-dominated food webs in soils, these shifts could in turn alter multiple ecosystem processes belowground such as nutrient cycling. Incorporating species' functional traits into the assessment of habitat-change impacts on communities can greatly improve our understanding of species responses to environmental changes and their consequences in ecosystems.

Root traits and belowground herbivores relate to plant-soil feedback variation among congeners

Plant–soil feedbacks contribute to vegetation dynamics by species-specific interactions between plants and soil biota. Variation in plant–soil feedbacks can be predicted by root traits, successional position, and plant nativeness. However, it is unknown whether closely related plant species develop more similar plant–soil feedbacks than more distantly related species. Where previous comparisons included plant species from distant phylogenetic positions, we studied plant–soil feedbacks of congeneric species. Using eight intra-continentally range-expanding and native Geranium species, we tested relations between phylogenetic distances, chemical and structural root traits, root microbiomes, and plant–soil feedbacks. We show that root chemistry and specific root length better predict bacterial and fungal community composition than phylogenetic distance. Negative plant–soil feedback strength correlates with root-feeding nematode numbers, whereas microbiome dissimilarity, nativeness, or phylogeny does not predict plant–soil feedbacks. We conclude that root microbiome variation among congeners is best explained by root traits, and that root-feeding nematode abundances predict plant–soil feedbacks.

Most studies of plant–soil feedbacks and associated traits look at remotely-related species. Here the authors look at congeners, and show that nematode-driven plant–soil feedbacks depend on root chemical and morphological traits, independent of phylogenetic distance.

Range-expansion effects on the belowground plant microbiome

Plant range expansion is occurring at a rapid pace, largely in response to human-induced climate warming. Although the movement of plants along latitudinal and altitudinal gradients is well-documented, effects on belowground microbial communities remain largely unknown. Furthermore, for range expansion, not all plant species are equal: in a new range, the relatedness between range-expanding plant species and native flora can influence plant-microorganism interactions. Here we use a latitudinal gradient spanning 3,000 km across Europe to examine bacterial and fungal communities in the rhizosphere and surrounding soils of range-expanding plant species. We selected range-expanding plants with and without congeneric native species in the new range and, as a control, the congeneric native species, totalling 382 plant individuals collected across Europe. In general, the status of a plant as a range-expanding plant was a weak predictor of the composition of bacterial and fungal communities. However, microbial communities of range-expanding plant species became more similar to each other further from their original range. Range-expanding plants that were unrelated to the native community also experienced a decrease in the ratio of plant pathogens to symbionts, giving weak support to the enemy release hypothesis. Even at a continental scale, the effects of plant range expansion on the belowground microbiome are detectable, although changes to specific taxa remain difficult to decipher.

LAESI mass spectrometry imaging as a tool to differentiate the root metabolome of native and range-expanding plant species

LAESI-MSI, an innovative high-throughput technique holds a unique potential for untargeted detection, profiling and spatial localization of metabolites from intact plant samples without need for extraction or extensive sample preparation. Our understanding of chemical diversity in biological samples has greatly improved through recent advances in mass spectrometry (MS). MS-based-imaging (MSI) techniques have further enhanced this by providing spatial information on the distribution of metabolites and their relative abundance. This study aims to employ laser-ablation electrospray ionization (LAESI) MSI as a tool to profile and compare the root metabolome of two pairs of native and range-expanding plant species. It has been proposed that successful range-expanding plant species, like introduced exotic invaders, have a novel, or a more diverse secondary chemistry. Although some tests have been made using aboveground plant materials, tests using root materials are rare. We tested the hypothesis that range-expanding plants possess more diverse root chemistries than native plant species. To examine the root chemistry of the selected plant species, LAESI-MSI was performed in positive ion mode and data were acquired in a mass range of m/z 50-1200 with a spatial resolution of 100 µm. The acquired data were analyzed using in-house scripts, and differences in the spatial profiles were studied for discriminatory mass features. The results revealed clear differences in the metabolite profiles amongst and within both pairs of congeneric plant species, in the form of distinct metabolic fingerprints. The use of ambient conditions and the fact that no sample preparation was required, established LAESI-MSI as an ideal technique for untargeted metabolomics and for direct correlation of the acquired data to the underlying metabolomic complexity present in intact plant samples.

Nematode community responses to range-expanding and native plant communities in original and new range soils

Many plant species expand their range to higher latitudes in response to climate change. However, it is poorly understood how biotic interactions in the new range differ from interactions in the original range. Here, in a mesocosm experiment, we analyze nematode community responses in original and new range soils to plant communities with either (a) species native in both the original and new range, (b) range-expanding species related to these natives (related range expanders), or (c) range expanders without native congeneric species in the new range (unrelated range expanders). We hypothesized that nematode community shifts between ranges are strongest for unrelated range expanders and minimal for plant species that are native in both ranges. As a part of these community shifts, we hypothesized that range expanders, but not natives, would accumulate fewer root-feeding nematodes in their new range compared to their original range. Analyses of responses of nematodes from both original and new ranges and comparison between range expanders with and without close relatives have not been made before. Our study reveals that none of the plant communities experienced evident nematode community shifts between the original and new range. However, in soils from the new range, root-feeding nematode communities of natives and related range expanders were more similar than in soils from the original range, whereas the nematode community of unrelated range expanders was distinct from the communities of natives and related range expanders in soils from both ranges. The abundances of root-feeding nematodes were comparable between the original and new range for all plant communities. Unexpectedly, unrelated range expanders overall accumulated most root-feeding nematodes, whereas related range expanders accumulated fewest. We conclude that nematode communities associated with native and range-expanding plant species differ between the original and the new range, but that range-expanding plant species do not accumulate fewer root-feeding nematodes in their new than in their original range.

Identification of Key Root Volatiles Signaling Preference of Tomato over Spinach by the Root Knot Nematode Meloidogyne incognita

The root knot nematode, Meloidogyne incognita (Kofoid and White) Chitwood, is a serious pest of tomato ( Solanum lycopersicum) and spinach ( Spinacea oleracea) in sub-Saharan Africa. In East Africa these two crops are economically important and are commonly intercropped by smallholder farmers. The role of host plant volatiles in M. incognita interactions with these two commodities is currently unknown. Here, we investigate the olfactory basis of attraction of tomato and spinach roots by the infective second stage juveniles (J2s) of M. incognita. In olfactometer assays, J2s were attracted to root volatiles from both crops over moist sand (control), but in choice tests using the two host plants, volatiles of tomato roots were more attractive than those released by spinach. Root volatiles sampled by solid phase microextraction (SPME) fiber and analyzed by gas chromatography/mass spectrometry (GC/MS) identified a total of eight components, of which five (2-isopropyl-3-methoxypyrazine, 2-(methoxy)-3-(1-methylpropyl)pyrazine, tridecane, and α- and β-cedrene) occurred in the root-emitted volatiles of both plants, with three (δ-3-carene, sabinene, and methyl salicylate) being specific to tomato root volatiles. In a series of bioassays, methyl salicylate contributed strongly to the attractiveness of tomato, whereas 2-isopropyl-3-methoxypyrazine and tridecane contributed to the attractiveness of spinach. M. incognita J2s were also more attracted to natural spinach root volatiles when methyl salicylate was combined than to spinach volatiles alone, indicating that the presence of methyl salicylate in tomato volatiles strongly contributes to its preference over spinach. Our results indicate that since both tomato and spinach roots are attractive to M. incognita, identifying cultivars of these two plant species that are chemically less attractive can be helpful in the management of root knot nematodes.