Compartmental, Selective and Reversible Redox Signalling in Plant Immunity

Like most organisms, plants are continuously exposed to many different disease causing agents, including pathogenic bacteria, fungi, viruses, and herbivorous insects. Each plant cell is thought to be capable of defending itself by launching an immune response that is specific to the intruder encountered (Spoel and Dong, 2012). Cellular oxidation-reduction (redox) reactions are an integral part of launching successful immune responses in all multicellular organisms. Redox reactions are underpinned by the production of different reactive oxygen- and nitrogen-containing species that drive the cell's content and environment into a more oxidized state, generating hostile conditions for the intruder. In addition, plant cells have evolved to utilize reactive oxygen and nitrogen species as signalling molecules in their immune response.

The reactive nitrogen and oxygen species, nitric oxide (NO) and hydrogen peroxide (H2O2), play especially important roles in plant immunity. NO and H2O2 are capable of post-translationally modifying proteins by forming amongst others S-nitrosothiols (protein-SNO), disulphide bonds, and S-hydroxyls that can alter the protein's function, localization, conformation, and activity. Genetic evidence indicates that overaccumulation of protein-SNO severely impairs plant immunity, suggesting that cellular mechanisms exist to remove excessive protein-SNO. Indeed, we recently reported that the evolutionary conserved redox enzyme, Thioredoxin-h5 (TRXh5), functions as a bona fide protein-SNO reductase during the immune response (Kneeshaw et al., 2014). Importantly, our findings indicated that TRXh5 discriminates between different protein-SNO substrates and reduces a specific subset to ensure establishment of disease resistance. Similarly, TRXh5 and other members of the Thioredoxin superfamily regulate the activity of redox-sensitive immune proteins by reducing protein disulphides and S-hydroxyls (Tada et al., 2008). However, the identities of many of these oxidised protein substrates remain elusive. Moreover, cellular compartments such as the mitochondria and chloroplasts, the powerhouses of the plant cell, may also exhibit protein-SNO, protein-disulphide and protein-S-hydroxyl reductase activity, but their nature and effect on the plant immune response are unknown.

In this PhD project the student will engage in innovative genomic and proteomic strategies to uncover the compartmental contributions of distinct Thioredoxin enzymes to cellular antioxidant activity and reveal the identities of their oxidised substrates. This will generate an in depth understanding of how signalling by reversibly oxidised proteins is controlled and employed by the plant cell to establish effective immune responses.