Interaction between the Bird Cherry-Oat Aphid (Rhopalosiphum padi) and Stagonospora Nodorum Blotch (Parastagonospora nodorum) on Wheat

Simple Summary

The bird cherry-oat aphid and the fungal plant pathogen causing stagonospora nodorum blotch (SNB) are common pests of wheat. Plants are under constant attack by multiple pests and diseases but there are limited studies on the interaction between several pests on wheat. We therefore conducted controlled greenhouse and laboratory experiments to determine how these pests affected each other on a wheat plant. We found that aphid feeding predisposed wheat to fungal disease, but that aphids preferred and reproduced better on leaves that had not been infected by the fungal pathogen. These results are important to understand the interactions between multiple pests on wheat and how to develop new control strategies in future integrated pest management (IPM).

Abstract

Wheat plants are under constant attack by multiple pests and diseases. Until now, there are no studies on the interaction between the aphid Rhopalosiphum padi and the plant pathogenic fungus Parastagonospora nodorum causal agent of septoria nodorum blotch (SNB) on wheat. Controlled experiments were conducted to determine: (i) The preference and reproduction of aphids on P. nodorum inoculated and non-inoculated wheat plants and (ii) the effect of prior aphid infestation of wheat plants on SNB development. The preference and reproduction of aphids was determined by releasing female aphids on P. nodorum inoculated (SNB+) and non-inoculated (SNB−) wheat leaves. The effect of prior aphid infestation of wheat plants on SNB development was determined by inoculating P. nodorum on aphid-infested (Aphid+) and aphid free (Aphid−) wheat plants. Higher numbers of aphids moved to and settled on the healthy (SNB−) leaves than inoculated (SNB+) leaves, and reproduction was significantly higher on SNB− leaves than on SNB+ leaves. Aphid infestation of wheat plants predisposed the plants to P. nodorum infection and colonization. These results are important to understand the interactions between multiple pests in wheat and hence how to develop new strategies in future integrated pest management (IPM).

Plant-insect-microbe interaction: A love triangle between enemies in ecosystem

In natural ecosystems, plants interact with biotic components such as microbes, insects, animals and other plants as well. Generally, researchers have focused on each interaction separately, which condenses the significance of the interaction. This limited presentation of the facts masks the collective role of constantly interacting organisms in complex communities disturbing not only plant responses but also the response of organisms for each other in natural ecological settings. Beneficial microorganisms interact with insect herbivores, their predators and pollinators in a bidirectional way through the plant. Fascinatingly, insects employ diverse tactics to protect themselves from parasites or predators. Influences of microbial and insects attack on plants can bring changes in info-chemical frameworks and play a role in the food chain also. After insect herbivory and microbial pathogenesis, plants exhibit intense morpho-physiological and chemical reprogramming that leads to repellence/attraction of attacking organism or its natural enemy. The characterization of such interactions in different ecosystems is receiving due consideration, and underlying molecular and physiological mechanisms must be the point of concentration to unveil the evolution of multifaceted multitrophic interactions. Therefore, we have focused this phenomenon in a more realistic setting by integrating ecology and physiology to portray these multidimensional interfaces. We have shown, in this article, physiological trajectories in plant-microbe and insect relationship and their ecological relevance in nature. We focus and discuss microbial pathogenesis in plants, induced defense and the corresponding behavior of herbivore insects and vice-versa. It is hoped that this review will stimulate interest and zeal in microbes mediated plant-insect interactions along with their ecological consequences and encourage scientists to accept the challenges in this field.

Fungi reduce preference and performance of insect herbivores on challenged plants

Although insect herbivores and fungal pathogens frequently share the same individual host plant, we lack general insights in how fungal infection affects insect preference and performance. We addressed this question in a meta-analysis of 1,113 case studies gathered from 101 primary papers that compared preference or performance of insect herbivores on control vs. fungus challenged plants. Generally, insects preferred, and performed better on, not challenged plants, regardless of experimental conditions. Insect response to fungus infection significantly differed according to fungus lifestyle, insect feeding guild, and the spatial scale of the interaction (local/distant). Insect performance was reduced on plants challenged by biotrophic pathogens or endophytes but not by necrotrophic pathogens. For both chewing and piercing-sucking insects, performance was reduced on challenged plants when interactions occurred locally but not distantly. In plants challenged by biotrophic pathogens, both preference and performance of herbivores were negatively impacted, whereas infection by necrotrophic pathogens reduced herbivore preference more than performance and endophyte infection reduced only herbivore performance. Our study demonstrates that fungi could be important but hitherto overlooked drivers of plant-herbivore interactions, suggesting both direct and plant-mediated effects of fungi on insect's behavior and development.

A Tripartite Interaction Between Spartina alterniflora, Fusarium palustre, and the Purple Marsh Crab (Sesarma reticulatum) Contributes to Sudden Vegetation Dieback of Salt Marshes in New England

Tripartite interactions are common and occur when one agent (an arthropod or pathogen) changes the host plant in a manner that alters the attack of the challenging agent. We examined herbivory from the purple marsh crab (Sesarma reticulatum) on Spartina alterniflora following exposure to drought or inoculation with Fusarium palustre in mecocosms in the greenhouse and in crab-infested creek banks along intertidal salt marshes. Initially, drought stress on S. alterniflora and disease from F. palustre were examined in the greenhouse. Then, a second challenger, the purple marsh crab, was introduced to determine how drought and disease from F. palustre affected the attraction and consumption of S. alterniflora. Plant height and shoot and root weights were reduced in plants subjected to severe drought treatment when compared with normally irrigated plants. When the drought treatment was combined with inoculation with F. palustre, plants were significantly more stunted and symptomatic, had less fresh weight, more diseased roots, and a greater number of Fusarium colonies growing from the roots (P < 0.001) than noninoculated plants. The effects were additive, and statistical interactions were not detected between drought and inoculation. Estimates of herbivory (number of grass blades cut or biomass consumption) by the purple marsh crab were significantly greater on drought-stressed, diseased plants than on healthy plants irrigated normally. Drought increased attraction to the purple marsh crab more than inoculation with F. palustre. However, when only mild drought conditions were imposed, plant consumption was greater on inoculated plants. Healthy, nonstressed transplants set into plots in crabinfested intertidal creek banks were grazed less each year than inoculated plants or plants that were exposed to drought. Several hypotheses relating to nutrition, chemotaxis, and visual attraction are presented to explain how stress from drought or disease might favor herbivory.

Phytophagous arthropods and a pathogen sharing a host plant: evidence for indirect plant-mediated interactions

In ecological systems, indirect interactions between plant pathogens and phytophagous arthropods can arise when infestation by a first attacker alters the common host plant so that although a second attacker could be spatially or temporally separated from the first one, the former could be affected. The induction of plant defense reactions leading to the production of secondary metabolites is thought to have an important role since it involves antagonistic and/or synergistic cross-talks that may determine the outcome of such interactions. We carried out experiments under controlled conditions on young rose plants in order to assess the impact of these indirect interactions on life history traits of three pests: the necrotrophic fungus Botrytis cinerea Pers.: Fr. (Helotiales: Sclerotiniaceae), the aphid Rhodobium porosum Sanderson (Hemiptera: Aphididae) and the thrips Frankliniella occidentalis Pergande (Thysanoptera: Thripidae). Our results indicated (i) a bi-directional negative interaction between B. cinerea and R. porosum, which is conveyed by decreased aphid growth rate and reduced fungal lesion area, as well as (ii) an indirect negative effect of B. cinerea on insect behavior. No indirect effect was observed between thrips and aphids. This research highlights several complex interactions that may be involved in structuring herbivore and plant pathogen communities within natural and managed ecosystems.

Responses of Helicoverpa armigera to tomato plants previously infected by ToMV or damaged by H. armigera

We report the comparative inducing effects of a phytopathogen and a herbivorous arthropod on the performance of an herbivore. Tomato, Lycopersicon esculentum Mill., was used as the test plant, and tomato mosaic virus (ToMV) and corn earworm, Helicoverpa armigera Hübner, were used as the phytopathogen and herbivore, respectively. There were decreases in the efficiency of conversion of ingested food and efficiency of conversion of digested food when H. armigera was reared on tomato plants that had been previously inoculated with ToMV. However, virus inoculation did not affect feeding or oviposition preferences by H. armigera. In contrast, approximate digestibility, total consumption, relative growth rate, and relative consumption rate were lower for fourth-instar H. armigera that fed on plants previously damaged by the same herbivore. Feeding and oviposition were both deterred for H. armigera that fed on previously damaged plants. The duration of development of H. armigera was also prolonged under this treatment. Infection by ToMV and feeding damage by H. armigera increased the host plant's peroxidase and polyphenol oxidase activity, respectively, suggesting that the performance of H. armigera may be affected by the induced phytochemistry of the host plant. Overall, this study indicated that, in general, insect damage has a stronger effect than ToMV infection on plant chemistry and, subsequently, on the performance of H. armigera.

Cross-induction of systemic induced resistance between an insect and a fungal pathogen in Austrian pine over a fertility gradient

Evidence for cross-induction of systemic resistance or susceptibility in plant-fungus-herbivore interactions is mostly derived from herbaceous model systems and not perennial woody plants. Furthermore, the effects of environmental variables such as soil fertility on these tripartite interactions are generally unknown. This study examined cross-induction of systemic resistance in Pinus nigra (Austrian pine) to infection by Sphaeropsis sapinea (a fungal pathogen), or feeding by Neodiprion sertifer (European pine sawfly), by prior induction with either S. sapinea or N. sertifer, over a fertility gradient. In a replicated 3-year study, cross-induction of systemic induced resistance (SIR) was found to be both asymmetric within a single year and variable between years. Prior induction with insect defoliation induced SIR to subsequent fungal challenge in 2006 but not in 2005. In 2005, a fertility-independent negative systemic effect of the fungal infection on herbivore growth was detected while herbivore survival was affected by a significant interaction between induction treatment and fertility level in 2006. Prior infection by the fungus induced SIR against the same fungus in both years regardless of fertility levels. This is the first report of whole-plant SIR against a defoliating insect induced by a fungal pathogen and vice versa, under variable nutrient availability, in a conifer or any other tree.

Multiple plant exploiters on a shared host: testing for nonadditive effects on plant performance

The combined impact of multiple plant parasites on plant performance can either be additive (the total damage equals the sum of the individual effects) or nonadditive (synergistic or antagonistic damage). Two statistical models are available for testing the independent (=additive) effects of two factors. Here we suggest that the natural history of the plant-parasite system should motivate the choice of a statistical model to test for additivity. Using in-field, manipulative experiments, we examined the interactions between the herbivorous mite Calacarus flagelliseta Fletchmann, De Moraes and Barbosa (Acari: Eriophyidae), the fungal pathogen Oidium caricae F. Noack (a powdery mildew), and their host plant Carica papaya L. in Hawaii. First, we found that herbivorous mites had a moderate negative effect on powdery mildew: when mites were absent, powdery mildew colonies were larger and more numerous. Second, we showed that each plant parasite, when evaluated alone, significantly reduced several measures of plant performance. Third, we found that the combined impact of mites and mildew on plant performance is mostly additive and, for a few variables, less than additive. Finally, we explored compensatory responses and found no evidence for nonlinearities in the relationship between plant performance and cumulative parasite impact. Plants are almost universally subject to attack by multiple herbivores and pathogens; thus a deeper understanding of how multiple plant parasites shape each other's population dynamics and plant performance is essential to understanding plant-parasite interactions.

Environmental variation mediates the deleterious effects of Coleosporium ipomoeae on Ipomoea purpurea

Variation in the environment is common within and between natural populations and may influence selection on plant resistance by altering the level of damage or the fitness consequences of damage from plant enemies. While much is known about how environmental variation influences the amount of damage a plant experiences, few studies have attempted to determine how variation in the environment may alter the fitness consequences of damage, particularly in plant-pathogen interactions. In this work we manipulated a rust pathogen, Coleosporium ipomoeae, in field experiments and showed that this pathogen reduced several components of fitness in its natural host plant, Ipomoea purpurea. Furthermore, we showed that the deleterious effects of C. ipomoeae were variable. We identified variation in the quality of a plant's microenvironment, the abundance of secondary enemy damage, and the length of a growing season as variable components of the environment that may influence the magnitude of damage and tolerance, causing the interaction between C. ipomoeae and I. purpurea to vary from parasitism to commensalism. Considering how environmental variation impacts the magnitude and negative fitness effects of pathogen damage is important to understanding spatially variable selection and coevolution in this and other plant-pathogen interactions.

Rapid accumulation of trihydroxy oxylipins and resistance to the bean rust pathogen Uromyces fabae following wounding in Vicia faba

BACKGROUND AND AIMS

Insect damage to plants leads to wound-activated responses directed to healing of damaged tissues, as well as activation of defences to prevent further insect damage. Negative cross-talk exists between the jasmonic acid-based signalling system that is activated upon insect attack and the salicylic acid-based system frequently activated following pathogen infection. Thus, insect attack may compromise the ability of the plant to defend itself against pathogens and vice versa. However, insect herbivory and mechanical wounding have been shown to reduce fungal infections on some plants, although the underlying mechanisms remain to be defined. This work examines the effects of mechanical wounding on rust infection both locally and systemically in the broad bean, Vicia faba and follows changes in oxylipins in wounded leaves and unwounded leaves on wounded plants.

METHODS

The lamina of first leaves was wounded by crushing with forceps, and first and second leaves were then inoculated, separately, with the rust Uromyces fabae at various times over a 24 h period. Wounded first leaves and unwounded second leaves were harvested at intervals over a 24 h period and used for analysis of oxylipin profiles. KEY RESULTS Mechanical wounding of first leaves of broad bean led to significantly reduced rust infection in the wounded first leaf as well as the unwounded second leaf. Increased resistance to infection was induced in plants inoculated with rust just 1 h after wounding and was accompanied by rapid and significant accumulation of jasmonic acid and two trihydroxy oxylipins in both wounded first leaves and unwounded second leaves. The two trihydroxy oxylipins were found to possess antifungal properties, reducing germination of rust spores.

CONCLUSIONS

These results demonstrate the rapidity with which resistance to pathogen infection can be induced following wounding and provides a possible mechanism by which pathogen infection might be halted.

Plant-mediated interactions between pathogenic microorganisms and herbivorous arthropods

Plant-mediated interactions between pathogenic microorganisms and arthropod herbivores occur when arthropod infestation or pathogen infection changes the shared host plant in ways that affect a subsequent attacker of the opposite type. Interest in such "tripartite" interactions has increased as the ecological and plant physiological framework for understanding and contextualizing them has developed. The outcomes of plant-mediated interactions are variable, and only a few provisional patterns can be identified at present. However, these interactions can have important consequences not only for individual pathogens and herbivores, but also for the population dynamics of both types of organisms in managed and natural ecosystems. Research has focused on the role of two plant response pathways in mediating tripartite interactions, one involving jasmonic acid and the other salicylic acid. Further studies of plant-mediated interactions will facilitate an understanding of how plants coordinate and integrate their defenses against multiple biotic threats.

Coping with multiple enemies: an integration of molecular and ecological perspectives

How plants respond to attack by the range of herbivores and pathogens that confront them in the field is the subject of considerable research by both molecular biologists and ecologists. However, in spite of the shared focus of these two bodies of research, there has been little integration between them. We consider the scope for such integration, and how greater dialogue between molecular biologists and ecologists could advance understanding of plant responses to multiple enemies.

The effect of an insect herbivore and a rust fungus individually, and combined in sequence, on the growth of two Rumex species

The chrysomelid beetle Gastrophysa viridula and the rust fungus Urmnyces rumicis both occur on leaves of Rumex crispus and R. obtusifolius. We investigated the effect of beetle grazing or rust infection individually and when combined in sequence on the growth of their hosts in the field. Singly, beetle or rust reduced leaf area and plant biomass; the effect was greater on R. crispus, and rust caused greater damage than the beetle. Beetle grazing with subsequent rust infection caused damage no greater than that caused by rust alone, although on R. obtusifolius damage was greater than that from beetle grazing alone. Rust infection of R. obtusifolius with subsequent beetle grazing produced damage similar to that from other treatments; involving rust infection. In R. crispus this treatment produced the greatest reduction in biomass, The reductions in root and total plant weight from rust infection with subsequent beetle grazing were accurately predicted by a model including the damage produced by beetle and rust alone and the length of time each was present on the plant. This model also predicted accurately the damage to R. obtusifolius from the beetle followed by rust treatment, but over-estimated by up to 40% the damage to R. crispus. This can be explained mainly by an inhibition of rust infection by beetle grazing.