Sustainable approaches to crop protection: silicon supplementation and its effects on symbiont-mediated multi-trophic interactions

Herbivorous pest outbreaks decrease the productivity of crops and are predicted to increase under climate change. Controlling these pests with synthetic pesticides is environmentally damaging and therefore sustainable alternatives need to be developed. Supplementing soils with silicon (Si) offers such a sustainable approach: in cereals, it increases crop yield and resistance to pests, but there is little information whether such benefits also occur in other important crop families such as legumes, or whether there are effects on other species in the crop ecosystem, such as the natural enemies of herbivores. One study has demonstrated a positive effect of Si in legumes, due to an increase in nodules containing nitrogen-fixing Rhizobia. This led to elevated foliar nitrogen but also to increased performance of aphids. Si addition could thus be counter-productive for legumes, but we do not know the generality of this effect, nor do we understand the underpinning mechanisms including the role of bacterial symbionts.

Aphids carry two types of bacterial symbionts: the obligate Buchnera aphidicola, which transforms plant-derived amino acids into essential ones, and a number of facultative symbionts that provide protection from natural enemies and other environmental threats. The increased aphid performance after Si addition is probably modulated by the obligate symbiont, but this has not yet been tested. Increased aphid densities might attract more natural enemies that could potentially control the aphids, but this would in turn select for aphids that carry certain facultative symbionts. Resistance to natural enemies might therefore increase over time. Again, this has not been tested and the net effect on the plant is unknown.

This project aims to gain a mechanistic understanding of Si supplementation on lucerne, Medicago sativa, one of its dominant pests, the pea aphid and its obligate and facultative symbionts. It also aims to understand whether Si supplementation provides a net benefit to the plant and whether this depends on the genotypes of the aphid or its symbiotic partners. To address this, the student will use a combination of entomological approaches with molecular (PCR, qPCR) and biochemical (X-ray fluorescence) methods to measure aphid and symbiont performance, plant nutrients and the deposition of Si defences.

This project is of fundamental and applied importance. It takes a multitrophic approach to test, for the first time, the plant-mediated effects of Si on the on the interactions between herbivores, their symbionts and natural enemies.