Predicted increases in atmospheric carbon dioxide (CO2) concentrations could modify crop resistance to insect herbivores by altering plant quality. The short generation times of aphids may allow them to exploit such changes and colonise previously resistant plant genotypes. Lucerne (Medicago sativa) has undergone global selective breeding against aphids, including the pea aphid, Acyrthosiphon pisum. The purpose of this study was to characterise how ambient CO2 (aCO2) and elevated (eCO2) (400 and 600 µmol mol−1, respectively) affected plant physiological traits potentially linked to aphid resistance, focussing on foliar amino acid concentrations, across five M. sativa genotypes with varying resistance to A. pisum. These included susceptible (Hunter River), low (Trifecta), moderate (Aurora and Genesis) and high resistance (Sequel). Under eCO2, root nodulation doubled and essential amino acid concentrations increased by 86% in resistant Sequel, whereas essential amino acid concentrations decreased by 53% in Genesis. Moreover, concentrations of lysine, an amino acid whose deficiency has been linked previously to A. pisum resistance in M. sativa, increased by 127% in Sequel at eCO2. Compared with aCO2, aphid colonisation of Sequel plants rose from 22% to 78% and reproduction rates increased from 1.1 to 4.3 nymphs week−1 under eCO2 conditions. In contrast, Genesis became more resistant at eCO2 compared with plants at aCO2; aphid colonisation rates fell from 78% to 44% of plants and reproductive rates decreased from 4.9 to 1.7 nymphs week−1. In conclusion, predicted changes in atmospheric CO2 concentrations could either reduce (Sequel) or enhance (Genesis) resistance to aphids, which might be linked to quantitative and qualitative changes in foliar amino acids.

Plants deploy an arsenal of chemical and physical defenses against arthropod herbivores, but it may be most cost efficient to produce these only when attacked. Herbivory activates complex signaling pathways involving several phytohormones, including jasmonic acid (JA), which regulate production of defensive compounds. The Poaceae also have the capacity to take up large amounts of silicon (Si) which accumulates in plant tissues. Si accumulation has anti-herbivore properties, but it is poorly understood how Si defenses relate to defense hormone signaling. Here we show that Si enrichment causes the model grass Brachypodium distachyon to show lower levels of JA induction when attacked by chewing herbivores. Triggering this hormone even at lower concentrations, however, prompts Si uptake and physical defenses (e.g. leaf hairs) which negatively impact chewing herbivores. Removal of leaf hairs restored performance. Crucially, activation of such Si-based defense is herbivore-specific and occurred only in response to chewing and not fluid-feeding (aphid) herbivores. This aligned with our meta-analysis of 88 studies that showed Si defenses were more effective against chewing herbivores than fluid-feeders. Our results suggest integration between herbivore defenses in a model Si-accumulating plant, which potentially allows it to avoid unnecessary activation of other costly defenses.

/0000-0002-5117-687X, Johnson, Scott N., Ryalls, James M. W. et al. (4 more authors) (2017) Silicon-induced root nodulation and synthesis of essential amino acids in a legume is associated with higher herbivore abundance. Functional Ecology.

1 Lucerne or alfalfa Medicago sativa is the most important temperate forage legume worldwide. Only one or two varieties of lucerne were grown in the U.S.A. and Australia (the two leading exporters of lucerne) before the late 1950s and late 1970s, respectively. These dates coincided with the arrival of aphid species, which devastated lucerne stands and prompted the development of aphid resistant cultivars. 2 Lucerne-feeding aphids, including bluegreen aphids Acyrthosiphon kondoi , pea aphids Acyrthosiphon pisum, spotted alfalfa aphids Therioaphis trifolii maculata and cowpea aphids Aphis craccivora, however, still present significant risks for the lucerne industry worldwide and account for 25% of global production losses. Moreover, increased production costs, negative environmental effects and emerging aphid resistance to insecticide applications have led to a narrowing of management options against lucerne aphids. 3 Understanding lucerne aphid biology and trophic ecology will be needed to develop future management practices, including biological control. We review and synthesize research on the four lucerne aphid species, focussing on cultivar resistance and their interactions with other organisms, including predators, parasitoids, entomopathogens and bacterial symbionts. The effects of global climate change are considered, with a particular emphasis on the potential for compromised aphid resistance in lucerne cultivars under future climates. 4 We conclude by identifying future research questions and perspectives for the sustainable management of lucerne aphids. These include the characterization of plant secondary metabolites associated with natural enemy recruitment, an understanding of the role of endosymbionts in cultivar resistance and a better comprehension of multi-trophic interactions of lucerne aphids, both with other herbivores and higher trophic groups.

HighlightMultiple abiotic factors can combine to alter crop quality and rates of herbivore attack. Aphids benefit from elevated CO2 and root damage, but these effects are neutralized by increased temperatures.

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Silicon (Si) has important functional roles in plants, including resistance against herbivores. Environmental change, such as increasing atmospheric concentrations of CO, may alter allocation to Si defences in grasses, potentially changing the feeding behaviour and performance of herbivores, which may in turn impact on higher trophic groups. Using Si-treated and untreated grasses () maintained under ambient (400 ppm) and elevated (640 and 800 ppm) CO concentrations, we show that Si reduced feeding by crickets (), resulting in smaller body mass. This, in turn, reduced predatory behaviour by praying mantids (), which consequently performed worse. Despite elevated CO decreasing Si concentrations in , this reduction was not large enough to affect the feeding behaviour of crickets or their predator. Our results suggest that Si-based defences in plants have adverse impacts on both primary and secondary trophic taxa, and these are not likely to decline under future climate change scenarios.

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