Apoplast Metabolomics Profiling Reveals Nitrogen-Dependent Modulation of Plant-Pathogen Interactions

In the present study, we analysed the role played by the apoplast in the crosstalk between biotic and abiotic stress conditions. In particular, we studied the crosstalk between nitrogen (N) limitation and infection of the model plant Arabidopsis thaliana by Erwinia amylovora, an apoplastic bacterium. Our previous findings indicated that low N (LN) conditions increase E. amylovora in planta titres and expression of virulence factors. In this work, we extracted the apoplast wash fluids (AWF) from plants grown under low N or high N (HN) conditions and applied them to bacteria in vitro. We observed that LN-AWF induced stronger virulence gene expression than HN-AWF. Metabolomic analysis of both apoplast extracts revealed the presence of common metabolites; however, their proportions were distinct, indicating a direct effect of N availability on apoplast content. Interestingly, changes in the apoplast metabolite proportions were also observed early after bacterial infection, but only in plants grown under LN conditions. To evaluate the effect of single metabolites on virulence gene expression, we selected 43 metabolites and observed that 29 of them were activators, whereas two, GABA and citrate, acted as repressors. This study shows that environmental constraints, such as N availability, impact plant-pathogen interactions by altering the apoplastic content.