A Herbivore Tag-and-Trace System Reveals Contact- and Density-Dependent Repellence of a Root Toxin

Natural rubber reduces herbivory and alters the microbiome below ground

Laticifers are hypothesized to mediate both plant-herbivore and plant-microbe interactions. However, there is little evidence for this dual function. We investigated whether the major constituent of natural rubber, cis-1,4-polyisoprene, a phylogenetically widespread and economically important latex polymer, alters plant resistance and the root microbiome of the Russian dandelion (Taraxacum koksaghyz) under attack of a root herbivore, the larva of the May cockchafer (Melolontha melolontha). Rubber-depleted transgenic plants lost more shoot and root biomass upon herbivory than normal rubber content near-isogenic lines. Melolontha melolontha preferred to feed on artificial diet supplemented with rubber-depleted rather than normal rubber content latex. Likewise, adding purified cis-1,4-polyisoprene in ecologically relevant concentrations to diet deterred larval feeding and reduced larval weight gain. Metagenomics and metabarcoding revealed that abolishing biosynthesis of natural rubber alters the structure but not the diversity of the rhizosphere and root microbiota (ecto- and endophytes) and that these changes depended on M. melolontha damage. However, the assumption that rubber reduces microbial colonization or pathogen load is contradicted by four lines of evidence. Taken together, our data demonstrate that natural rubber biosynthesis reduces herbivory and alters the plant microbiota, which highlights the role of plant-specialized metabolites and secretory structures in shaping multitrophic interactions.

The plant metabolome guides fitness-relevant foraging decisions of a specialist herbivore

Plants produce complex mixtures of primary and secondary metabolites. Herbivores use these metabolites as behavioral cues to increase their fitness. However, how herbivores combine and integrate different metabolite classes into fitness-relevant foraging decisions in planta is poorly understood. We developed a molecular manipulative approach to modulate the availability of sugars and benzoxazinoid secondary metabolites as foraging cues for a specialist maize herbivore, the western corn rootworm. By disrupting sugar perception in the western corn rootworm and benzoxazinoid production in maize, we show that sugars and benzoxazinoids act as distinct and dynamically combined mediators of short-distance host finding and acceptance. While sugars improve the capacity of rootworm larvae to find a host plant and to distinguish postembryonic from less nutritious embryonic roots, benzoxazinoids are specifically required for the latter. Host acceptance in the form of root damage is increased by benzoxazinoids and sugars in an additive manner. This pattern is driven by increasing damage to postembryonic roots in the presence of benzoxazinoids and sugars. Benzoxazinoid- and sugar-mediated foraging directly improves western corn rootworm growth and survival. Interestingly, western corn rootworm larvae retain a substantial fraction of their capacity to feed and survive on maize plants even when both classes of chemical cues are almost completely absent. This study unravels fine-grained differentiation and combination of primary and secondary metabolites into herbivore foraging and documents how the capacity to compensate for the lack of important chemical cues enables a specialist herbivore to survive within unpredictable metabolic landscapes.

Tracking wireworm burrowing behaviour in soil over time using 3D X-ray computed tomography

BACKGROUND

Wireworms (larvae of the click beetle, Elateridae) are a significant agricultural pest, causing crop damage and reducing yields globally. Owing to the complex nature and opacity of the soil environment, research to investigate wireworm behaviour in situ has been scarce. X-ray computed tomography (CT) has previously been demonstrated as a powerful tool to independently visualise the 3D root system architecture, macroinvertebrate movement and distribution of burrow systems in soil, but not simultaneously within the same sample. In this study, we apply X-ray CT to visualise and quantify wireworms, their burrow systems and the root architecture of two contrasting crop species (Hordeum vulgare and Zea mays) in a soil pot experiment scanned at different time intervals.

RESULTS

The majority of wireworm burrows were produced within the first 20 h post inoculation, suggesting that burrow systems are established quickly and persist at a similar volume. There was a significant difference in the volume of burrow systems produced by wireworms between the two crop species suggesting differences in wireworm behaviour elicited by crop species. There was no significant correlation between burrow volume and either root volume or surface area, indicating this behavioural difference is caused by factor(s) other than the mass of root systems.

CONCLUSION

X-ray CT shows potential as a non-destructive technique to quantify the interaction of wireworms in the natural soil environment with crop roots, and aid the development of effective pest management strategies to minimise their negative impact on crop production. © 2020 Society of Chemical Industry.

Adapted dandelions trade dispersal for germination upon root herbivore attack

A plant's offspring may escape unfavourable local conditions through seed dispersal. Whether plants use this strategy to escape insect herbivores is not well understood. Here, we explore how different dandelion (Taraxacum officinale agg.) populations, including diploid outcrossers and triploid apomicts, modify seed dispersal in response to root herbivore attack by their main root-feeding natural enemy, the larvae of the common cockchafer Melolontha melolontha. In a manipulative field experiment, root herbivore attack increased seed dispersal potential through a reduction in seed weight in populations that evolved under high root herbivore pressure, but not in populations that evolved under low pressure. This increase in dispersal potential was independent of plant cytotype, but associated with a reduction in germination rate, suggesting that adapted dandelions trade dispersal for establishment upon attack by root herbivores. Analysis of vegetative growth parameters suggested that the increased dispersal capacity was not the result of stress flowering. In summary, these results suggest that root herbivory selects for an induced increase in dispersal ability in response to herbivore attack. Induced seed dispersal may be a strategy that allows adapted plants to escape from herbivores.

Impact of Seasonal and Temperature-Dependent Variation in Root Defense Metabolites on Herbivore Preference in Taraxacum officinale

Plants experience seasonal fluctuations in abiotic and biotic factors such as herbivore attack rates. If and how root defense expression co-varies with seasonal fluctuations in abiotic factors and root herbivore attack rates is not well understood. Here, we evaluated seasonal changes in defensive root latex chemistry of Taraxacum officinale plants in the field and correlated the changes with seasonal fluctuations in abiotic factors and damage potential by Melolontha melolontha, a major natural enemy of T. officinale. We then explored the causality and consequences of these relationships under controlled conditions. The concentration of the defensive sesquiterpene lactone taraxinic acid β-D glucopyranosyl ester (TA-G) varied substantially over the year and was most strongly correlated to mean monthly temperature. Both temperature and TA-G levels were correlated with annual fluctuations in potential M. melolontha damage. Under controlled conditions, plants grown under high temperature produced more TA-G and were less attractive for M. melolontha. However, temperature-dependent M. melolontha feeding preferences were not significantly altered in TA-G deficient transgenic lines. Our results suggest that fluctuations in temperature leads to variation in the production of a root defensive metabolites that co-varies with expected attack of a major root herbivore. Temperature-dependent herbivore preference, however, is likely to be modulated by other phenotypic alterations.

Distinct defense strategies allow different grassland species to cope with root herbivore attack

Root-feeding insect herbivores are of substantial evolutionary, ecological and economical importance. Plants defend themselves against insect herbivores through a variety of tolerance and resistance strategies. To date, few studies have systematically assessed the prevalence and importance of these strategies for root-herbivore interactions across different plant species. Here, we characterize the defense strategies used by three different grassland species to cope with a generalist root herbivore, the larvae of the European cockchafer Melolontha melolontha. Our results reveal that the different plant species rely on distinct sets of defense strategies. The spotted knapweed (Centaurea stoebe) resists attack by dissuading the larvae through the release of repellent chemicals. White clover (Trifolium repens) does not repel the herbivore, but reduces feeding, most likely through structural defenses and low nutritional quality. Finally, the common dandelion (Taraxacum officinale) allows M. melolontha to feed abundantly but compensates for tissue loss through induced regrowth. Thus, three co-occurring plant species have evolved different solutions to defend themselves against attack by a generalist root herbivore. The different root defense strategies may reflect distinct defense syndromes.

Root volatiles in plant-plant interactions II: Root volatiles alter root chemistry and plant-herbivore interactions of neighbouring plants

Volatile organic compounds (VOCs) emitted by plant roots can influence the germination and growth of neighbouring plants. However, little is known about the effects of root VOCs on plant-herbivore interactions of neighbouring plants. The spotted knapweed (Centaurea stoebe) constitutively releases high amounts of sesquiterpenes into the rhizosphere. Here, we examine the impact of C. stoebe root VOCs on the primary and secondary metabolites of sympatric Taraxacum officinale plants and the resulting plant-mediated effects on a generalist root herbivore, the white grub Melolontha melolontha. We show that exposure of T. officinale to C.stoebe root VOCs does not affect the accumulation of defensive secondary metabolites but modulates carbohydrate and total protein levels in T. officinale roots. Furthermore, VOC exposure increases M. melolontha growth on T. officinale plants. Exposure of T. officinale to a major C. stoebe root VOC, the sesquiterpene (E)-β-caryophyllene, partially mimics the effect of the full root VOC blend on M. melolontha growth. Thus, releasing root VOCs can modify plant-herbivore interactions of neighbouring plants. The release of VOCs to increase the susceptibility of other plants may be a form of plant offense.

Molecular Interactions Between Plants and Insect Herbivores

Diverse molecular processes regulate the interactions between plants and insect herbivores. Here, we review genes and proteins that are involved in plant-herbivore interactions and discuss how their discovery has structured the current standard model of plant-herbivore interactions. Plants perceive damage-associated and, possibly, herbivore-associated molecular patterns via receptors that activate early signaling components such as Ca²⁺, reactive oxygen species, and MAP kinases. Specific defense reprogramming proceeds via signaling networks that include phytohormones, secondary metabolites, and transcription factors. Local and systemic regulation of toxins, defense proteins, physical barriers, and tolerance traits protect plants against herbivores. Herbivores counteract plant defenses through biochemical defense deactivation, effector-mediated suppression of defense signaling, and chemically controlled behavioral changes. The molecular basis of plant-herbivore interactions is now well established for model systems. Expanding molecular approaches to unexplored dimensions of plant-insect interactions should be a future priority.