Imaging elemental defences in metal hyperaccumulator plants

Metal hyperaccumulation, defined as the uptake and storage of exceptionally high concentrations of a metal in the aerial tissues of a plant, is an unusual trait that has been observed in around 400 plant species. Our recent work has provided strong support for the hypothesis that metal ions can provide a direct defence against pathogens through their toxicity to invading microorganisms, a hypothesis sometimes referred to as the "elemental defence" hypothesis. However, the mechanistic contribution of metal accumulation to disease resistance has yet to be fully demonstrated.
This project will investigate how metal hyperaccumulation limits plant disease using the interaction of the metal hyperaccumulating plant Noccaea caerulescens with the bacterial pathogen Pseudomonas syringae as a model system. N. caerulescens can accumulate zinc, nickel and cadmium to many times higher than normal physiological concentrations, and is widely studied as a model metal hyperaccumulating plant.
All accessions of N. caerulescens we have studied to date acquire increased protection against P. syringae through metal accumulation, and the degree of protection correlates with the abundance of metal in plant tissues. However, the distribution of metals inside its leaves is not uniform. Therefore metal toxicity will depend on the local concentration of metal at the specific site of pathogen colonization and on host responses to infection that alter metal localisation or speciation. Hence, merely measuring the total amount of foliar metal is unlikely to be indicative of the concentration, speciation and metal complexes to which pathogens are exposed.
An important step forward would be imaging of the cellular and subcellular concentration, localization and chemical speciation of metal within healthy and infected plants, in relation to pathogen location, activity, viability, and other plant defence mechanisms, including the production of defensive metabolites and localized programmed cell death.
In this project we will use synchrotron-based imaging techniques including micro-X-ray fluorescence spectroscopy and micro X-ray absorption near-edge structure (mu-XANES) to examine the distribution and chemical speciation of elements, including zinc, cadmium and nickel, in healthy and infected N. caerulescens leaves. We also aim to further develop approaches to study elemental distributions in 3D using cryo-XRF-tomography and to apply them to this biological system.
We will combine insights from these analyses with super-resolution correlative cryo-fluorescence microscopy and electron microscopy, Raman microspectroscopy, analyses of pathogen gene expression and viability, and of plant defence responses, to investigate how metal hyperaccumulation limits pathogen growth. Raman micro-spectroscopy is able to detect bacterial metabolic activity at the single cell level in-situ. We will use analytical techniques such as GC-MS or MALDI imaging mass spectrometry to observe the changes that occur in the abundance and distribution of metabolites in healthy and infected plants.
The student appointed to this project will have the opportunity to work within three world-leading research organisations and to develop expertise in a range of advanced imaging techniques, as well as addressing important research questions about plant biology and plant-microbe interactions.