Both heavy metal-amendment of soil and aphid-infestation increase Cd and Zn concentrations in phloem exudates of a metal-hyperaccumulating plant.

Plants that are able to hyperaccumulate heavy metals show increased concentrations of these metals in their leaf tissue. However, little is known about the concentrations of heavy metals and of organic defence metabolites in the phloem sap of these plants in response to either heavy metal-amendment of the soil or biotic challenges such as aphid-infestation. In this study, we investigated the effects of heavy metal-exposure and of aphid-infestation on phloem exudate composition of the metal hyperaccumulator species Arabidopsis halleri L. O'Kane & Al-Shehbaz (Brassicaceae). The concentrations of elements and of organic defence compounds, namely glucosinolates, were measured in phloem exudates of young and old (mature) leaves of plants challenged either by amendment of the soil with cadmium and zinc and/or by an infestation with the generalist aphid Myzus persicae. Metal-amendment of the soil led to increased concentrations of Cd and Zn, but also of two other elements and one indole glucosinolate, in phloem exudates. This enhanced defence in the phloem sap of heavy metal-hyperaccumulating plants can thus potentially act as effective protection against aphids, as predicted by the elemental defence hypothesis. Aphid-infestation also caused enhanced Cd and Zn concentrations in phloem exudates. This result provides first evidence that metal-hyperaccumulating plants can increase heavy metal concentrations tissue-specifically in response to an attack by phloem-sucking herbivores. Overall, the concentrations of most elements, including the heavy metals, and glucosinolates were higher in phloem exudates of young leaves than in those of old leaves. This defence distribution highlights that the optimal defence theory, which predicts more valuable tissue to be better defended, is applicable for both inorganic and organic defences.

Effects of single and combined heavy metals and their chelators on aphid performance and preferences.

When present at elevated levels in the environment, heavy metals are toxic for most organisms. However, so-called hyperaccumulator plants tolerate heavy metals and use chelators for their internal long-distance transport. Thus, phloem-sucking insects may come in contact with the chelated metals. In the present study, the effects of individual and combined heavy metals, zinc (Zn) and cadmium (Cd), as well as of common chelators, nicotianamine and phytochelatin, were investigated on the performance, preferences, and metal accumulation of the generalist aphid Myzus persicae, using artificial diets. Added Zn increased aphid growth, whereas Cd reduced the survival of aphids. Chelators had neither protective nor negative effects on aphids. The combination of the 2 heavy metals in chelated or nonchelated form caused a potentiation effect that led to an extinction of the aphids within less than 2 wk, before they could reproduce. Both Cd and Zn accumulated in the aphids, indicating a possible biomagnification. In choice assays, aphids preferred diets amended with Zn with or without nicotianamine compared to a control diet. In contrast, a Cd-containing diet led to neither attraction nor aversion. The present study provides insight into how mixtures of heavy metals and their chelators influence the life history of a generalist aphid. The results have implications for the use of phytoremediation to remove heavy metals from contaminated soils. Environ Toxicol Chem 2016;35:3023-3030. © 2016 SETAC.

The red flour beetle as a model for bacterial oral infections.

Experimental infection systems are important for studying antagonistic interactions and coevolution between hosts and their pathogens. The red flour beetle Tribolium castaneum and the spore-forming bacterial insect pathogen Bacillus thuringiensis (Bt) are widely used and tractable model organisms. However, they have not been employed yet as an efficient experimental system to study host-pathogen interactions. We used a high throughput oral infection protocol to infect T. castaneum insects with coleopteran specific B. thuringiensis bv. tenebrionis (Btt) bacteria. We found that larval mortality depends on the dietary spore concentration and on the duration of exposure to the spores. Furthermore, differential susceptibility of larvae from different T. castaneum populations indicates that the host genetic background influences infection success. The recovery of high numbers of infectious spores from the cadavers indicates successful replication of bacteria in the host and suggests that Btt could establish infectious cycles in T. castaneum in nature. We were able to transfer plasmids from Btt to a non-pathogenic but genetically well-characterised Bt strain, which was thereafter able to successfully infect T. castaneum, suggesting that factors residing on the plasmids are important for the virulence of Btt. The availability of a genetically accessible strain will provide an ideal model for more in-depth analyses of pathogenicity factors during oral infections. Combined with the availability of the full genome sequence of T. castaneum, this system will enable analyses of host responses during infection, as well as addressing basic questions concerning host-parasite coevolution.