Identification of a New Class of SUMO Proteases in Plants,
and Investigation Into The Role of SUMOylation in Pathogen Perception

Abstract

Plants, as primary produces of energy and the main source of food for
many organisms, are both ecologically and economically important to
sustaining life. Understanding the molecular mechanisms by which
plants respond to stress is key to optimizing food production and
maintaining biodiversity in an increasingly uniform global ecosystem.
Arabidopsis thaliana is the major model plant species in which primary
research provides insights into the workings of more important plant
species. The research presented here advances our understanding of the role of SUMOylation and deSUMOylation in plants. The discovery of a previously undescribed group of SUMO proteases in plants, called
deSUMOylating Isopeptidases (DeSis) has provided evidence for the
first time of SUMOylation cycles outside the nucleus. Using proteomic
comparisons from animal species, this group of SUMO proteases has
been classified into 3 distinct sub-classes; DeSi1, DeSi2 and the plant
specific DeSi3. Characterisation of the SUMO isopeptidase activity was
achieved through biochemical assays in which SUMO1 conjugation
chains were reduced in the presence of the DeSi proteins. Furthermore,
investigation into the role of the DeSi3 protein At1g47740 (DeSi3a), has
revealed key regulation of the immune response pathway. DeSi3a
knockout plants were more resistant to pathogen attack, showed a
higher response to the detection of pathogenic elicitor molecules and
appeared to be primed for immune response. Together this provided
evidence that DeSi3a is a major negative regulator of plant immunity. In
addition to this, investigation into potential DeSi3a target substrates has
revealed that the activation of the key receptor for the perception of
bacterial pathogens, FLS2, is SUMOylation dependent. Further evidence
suggests that the fungal pathogen receptor, CERK1, may also be
regulated by SUMO. This thesis provides evidence that the regulation of
pathogen perception is SUMOylation dependent and that DeSi3a is a key negative regulator of defence.