Aphids are pests, known to cause significant economic damage to the agriculture industry. Aphids are sap-sucking insects which feed off plant phloem, removing nutrients as well facilitating the spread of viruses by acting as vectors. In light of a decrease in use of pesticides due to a decline in success and popularity, the need to understand the interactions that take place between aphids and plants is becoming more apparent. At the John Innes Centre in Norwich, increasing efforts are being made to determine the components involved in perception of aphid attack and subsequent defence responses, to help develop plants better suited to pest-filled environments. Additionally, their research has a focus on exploring how aphid effectors – molecules introduced into plant cells by an organism to bring about change to cell function or composition – can interfere with plant responses to promote colonisation of host plants. Within the John Innes centre, the Hogenhout lab is contributing to these research areas, with a particular focus on aphid perception in Arabidopsis thaliana and the mechanisms of aphid effectors such as Mp10. I joined this lab with the intention of assisting with this research, in particular helping to identify the signalling components involved in plant defence responses.
Currently, there are a lot of unknowns about the molecular components involved in plant perception and signalling as a result of aphid attack. As the Hogenhout lab begins to elucidate components of this response, such as calcium permeable GLR receptors and calcium signalling, it remains unclear which hormone signalling pathways are triggered by aphid feeding. Some authors suggest that wound-triggered hormone responses including Jasmonic acid signalling play a role. Others hypothesise that Jasmonic acid response occurs to a limited degree as a result of cell damage, but Salicylic acid signalling is also involved as part of an immune response, as a result of perception of aphid derived molecules. During my placement, I set out to answer the question of which hormone pathways are activated in Arabidopsis thaliana responses to attack from the aphid pest, Rhopalosiphum padi, which A. thaliana has a resistance towards.
To achieve this I utilised a range of nuclear-localised fluorescent reporters for Jasmonic acid, Salicylic acid and Ethylene. These reporters are genetically encoded in A. thaliana and are transcribed to produce a yellow fluorescent protein in the nucleus when hormone levels become elevated within the plant. If aphid feeding triggers defence signalling responses involving these hormones, we can identify these responses by a change in fluorescence visualised under an epifluorescence microscope with spatiotemporal resolution. Presence of Jasmonic acid would be in keeping with the wounding response pathway, and the presence of Salicylic acid and Ethylene could be attributed to involvement of an immune response. Leaves of the reporter plants were isolated at 14 days old, then treated with R. padi and visualised in a fluorescence microscopy time series.
Analysis of the microscope images revealed that both Jasmonic acid and Salicylic acid were produced locally in response to aphid feeding, however, interestingly, not every feeding event led to a recognisable hormone response. No recognisable localised Ethylene responses were observed in response to aphid feeding. In the future, trials using the aphid species M. persicae – which A. thaliana is not resistant to – would be interesting to determine if different hormone responses occur dependent upon aphid species, alongside the use of different hormone reporters to see if similar results can be observed across a range of reporters.
During my time at the John Innes Centre I also investigated the calcium responses of Nicotiana benthamiana to aphid elicitors: defence response inducing molecules introduced via aphid feeding, potentially recognised by plants. The aim of these experiments was to determine the level of calcium response brought about in N. benthamiana leaves upon exposure to aphid elicitors. In future this information will be utilised to identify how aphid effectors, like Mp10, suppress plant defence signalling.
I would like to express my gratitude to BSPP and the Hogenhout Lab for allowing me to undertake this fantastic opportunity, and to my supervisor Josh who helped me gain extremely valuable insight into the life of a researcher and the encouragement to pursue a research career of my own.
Rhopalosiphum padi aphids feeding on leaves of arabidopsis containing a Jasmonic Acid hormone reporter