The ability to manipulate ROS metabolism in pepper may affect aphid virulence

Formica fusca ants use aphid supplemented foods to alleviate effects during the acute phase of a fungal infection

The modulation of nutritional intake by animals to combat pathogens is a behaviour that is receiving increasing attention. Ant studies using isolated compounds or nutrients in artificial diets have revealed a lot of the dynamics of the behaviour, but natural sources of medicine are yet to be confirmed. Here we explored whether Formica fusca ants exposed to a fungal pathogen can use an artificial diet containing foods spiked with different concentrations of crushed aphids for a medicinal benefit. We show that pathogen exposed colonies adjusted their diet to include more aphid supplemented foods during the acute phase of the infection, reducing the mortality caused by the disease. However, the benefit was only attained when having access to a varied diet, suggesting that while aphids contain nutrients or compounds beneficial against infection, it is a part of a complex nutritional system where costs and benefits of compounds and nutrients need to be moderated.

Molecular Mechanisms Underlying Host Plant Specificity in Aphids

Aphids are serious pests of agricultural and ornamental plants and important model systems for hemipteran-plant interactions. The long evolutionary history of aphids with their host plants has resulted in a variety of systems that provide insight into the different adaptation strategies of aphids to plants and vice versa. In the past, various plant-aphid interactions have been documented, but lack of functional tools has limited molecular studies on the mechanisms of plant-aphid interactions. Recent technological advances have begun to reveal plant-aphid interactions at the molecular level and to increase our knowledge of the mechanisms of aphid adaptation or specialization to different host plants. In this article, we compile and analyze available information on plant-aphid interactions, discuss the limitations of current knowledge, and argue for new research directions. We advocate for more work that takes advantage of natural systems and recently established molecular techniques to obtain a comprehensive view of plant-aphid interaction mechanisms.

Oligogalacturonides Enhance Resistance against Aphids through Pattern-Triggered Immunity and Activation of Salicylic Acid Signaling

The remarkable capacity of the generalist aphid Myzus persicae to resist most classes of pesticides, along with the environmental and human health risks associated with these agrochemicals, has necessitated the development of safer and greener solutions to control this agricultural pest. Oligogalacturonides (OGs) are pectin-derived molecules that can be isolated from fruit industry waste. OGs have been shown to efficiently stimulate plant defenses against pathogens such as Pseudomonas syringae and Botrytis cinerea. However, whether OGs confer resistance against phytophagous insects such as aphids remains unknown. Here, we treated Arabidopsis plants with OGs and recorded their effects on the feeding performance and population of M. persicae aphids. We also identified the defense mechanism triggered by OGs in plants through the analysis of gene expression and histological approaches. We found that OG treatments increased their resistance to M. persicae infestation by reducing the offspring number and feeding performance. Furthermore, this enhanced resistance was related to a substantial accumulation of callose and reactive oxygen species and activation of the salicylic acid signaling pathway.

Rapid systemic responses to herbivory

Rapid systemic signals travel within the first seconds and minutes after herbivore infestation to mount defense responses in distal tissues. Recent studies have revealed that wound-induced hydraulic pressure changes play an important role in systemic electrical signaling and subsequent calcium and reactive oxygen species waves. These insights raise new questions about signal specificity, the role of insect feeding guild and feeding style and the impact on longer term plant defenses. Here, we integrate the current molecular understanding of wound-induced rapid systemic signaling in the framework of insect-plant interactions.

Evolved transcriptional responses and their trade-offs after long-term adaptation of Bemisia tabaci to a marginally-suitable host

Although generalist insect herbivores can migrate and rapidly adapt to a broad range of host plants, they can face significant difficulties when accidentally migrating to novel and marginally-suitable hosts. What happens, both in performance and gene expression regulation, if these marginally-suitable hosts must be used for multiple generations before migration to a suitable host can take place, largely remains unknown. In this study, we established multigenerational colonies of the whitefly Bemisia tabaci, a generalist phloem-feeding species, adapted to a marginally-suitable host (habanero pepper) or an optimal host (cotton). We used reciprocal host tests to estimate the differences in performance of the populations on both hosts under optimal (30 oC) and mild-stressful (24 oC) temperature conditions, and documented the associated transcriptomic changes. The habanero pepper-adapted population greatly improved its performance on habanero pepper but did not reach its performance level on cotton, the original host. It also showed reduced performance on cotton, relative to the non-adapted population, and an antagonistic effect of the lower-temperature stressor. The transcriptomic data revealed that most of the expression changes, associated with long-term adaptation to habanero pepper, can be categorized as "evolved" with no initial plastic response. Three molecular functions dominated: enhanced formation of cuticle structural constituents, enhanced activity of oxidation-reduction processes involved in neutralization of phytotoxins and reduced production of proteins from the cathepsin B family. Taken together, these findings indicate that generalist insects can adapt to novel host plants by modifying the expression of a relatively small set of specific molecular functions.

Redox responses of Arabidopsis thaliana to the green peach aphid, Myzus persicae

The green peach aphid (Myzus persicae) is a phloem-feeding insect that causes economic damage on a wide array of crops. Using a luminol-based assay, a superoxide-responsive reporter gene (Zat12::luciferase), and a probe specific to hydrogen peroxide (HyPer), we demonstrated that this aphid induces accumulation of reactive oxygen species (ROS) in Arabidopsis thaliana. Similar to the apoplastic oxidative burst induced by pathogens, this response to aphids was rapid and transient, with two peaks occurring within 1 and 4 hr after infestation. Aphid infestation also induced an oxidative response in the cytosol and peroxisomes, as measured using a redox-sensitive variant of green fluorescent protein (roGFP2). This intracellular response began within minutes of infestation but persisted 20 hr or more after inoculation, and the response of the peroxisomes appeared stronger than the response in the cytosol. Our results suggest that the oxidative response to aphids involves both apoplastic and intracellular sources of ROS, including ROS generation in the peroxisomes, and these different sources of ROS may potentially differ in their impacts on host suitability for aphids.

Phytohormone responses in pepper (Capsicum annuum L.) leaves under a high density of aphid infestation

The time course response of selected phytohormones has been evaluated in sweet pepper plants (Capsicum annuum L.) submitted to a high density (200 aphids/plant) of aphid (Myzus persicae Sulzer) infestation. Abscisic acid (ABA), salicylic acid (SA), indole-3-acetic acid (IAA), and jasmonates (JAs), including jasmonic acid (JA), jasmonoyl-l-isoleucine (JA-Ile), and cis-OPDA have been simultaneously identified and quantitated by UHPLC-MS/MS in pepper leaf tissue harvested at 3, 8 hours post-infestation (hpi), 1, 2, 4 and 7 days post-infestation (dpi). Infested plants showed a reduction in stem length at 7 dpi and in the number of leaves and leaf width from 4 dpi onwards. JA and JA-Ile significantly increased very early (from 3 hpi) while SA only accumulated at 7 dpi. Despite the high density of infestation, the aphid-induced accumulation of JAs was much lower than the burst typically induced by chewing herbivores. On the other side, ABA peaked in aphid-infested plants at 2 and 4 dpi, while IAA content did not change significantly at any time point. Growth inhibition may be partially explained by the high levels of JAs found in aphid-infested plants. The possibility that the obtained results support the hypothesis of the aphid manipulation of plant metabolism is discussed.

An aphid facultative symbiont suppresses plant defence by manipulating aphid gene expression in salivary glands

Aphids often carry facultative symbionts to achieve diverse advantages. Serratia symbiotica, one of facultative endosymbionts, increases aphid tolerance to heat. However, whether it benefits aphid colonization on host plants is yet to be determined. In the current study, we found that Acyrthosiphon pisum harbouring S. symbiotica had longer feeding duration on Medicago truncatula than Serratia-free aphids. Contrastingly, Serratia-free aphids triggered higher accumulation of reactive oxygen species (ROS), jasmonic acid and salicylic acid responsive genes and cytosolic Ca²⁺ elevations than Serratia-infected aphids. Transcriptomic analysis of salivary glands indicated that a histidine-rich Ca²⁺ -binding protein-like gene (ApHRC) was expressed more highly in the salivary gland of Serratia-infected aphids than that of Serratia-free aphids. Once ApHRC was silenced, Serratia-infected aphids also displayed shorter phloem-feeding duration and caused Ca²⁺ elevation and ROS accumulation in plants. Our results suggest that ApHRC, a potential effector up-regulated by S. symbiotica in the salivary glands, impairs plant defence response by suppressing Ca²⁺ elevation and ROS accumulation, allowing colonization of aphids. This study has provided an insight into how facultative symbionts facilitate aphid colonization and adaptation to host plants.